Editor's Picks

Round-up of RF Quick Connect/Quick Disconnect Connectors

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From Threaded to Quick

The technology for RF threaded connectors began in the early days of UHF communications and, since that time, other RF connectors have been developed to meet the needs of advancing technologies in the communications industry. In the early 1990’s, RF manufactures needed RF cable assemblies with interconnects that could manage the higher power and higher voltage of emerging technologies. The trend in the RF Microwave industry has evolved over the years from threaded coupling connectors to quick disconnect style coupling with push-on, blindmate, and quick disconnect interfaces.

Advantages of Quick Connect/Quick Disconnect Connectors

Advantages of using quick connect connectors are the speed of installation, flexibility of connection, and efficient use of space. This means less downtime during installation and maintenance, no special tooling when making RF connections,

And ease of mating of RF connections in challenging installation systems. Consider that many RF connections are in difficult to reach areas and mating and unmating these connections is often performed with limited line of sight or by feel. Using a torque wrench in these difficult to reach areas can be problematic. Quick connect/quick disconnect connectors eliminates many of the logistical difficulties by providing a mechanism for an easy and safe connection without the use of threads or assembly tools. Other installation advantages of quick connect/quick disconnect connectors include:

<  replaces traditional threaded connection of SMA and N with a snap-fastening to allow for faster mating and demating

< eliminates the need for a torque wrench

< decreases the overall size of the connector while maintaining the performance and reliability

< allows for 360° cable rotation after installation.

Types of Quick Connect/Quick Disconnect Connectors

QMA and QN connectors are quick connect RF connectors that were designed to replace the common SMA and Type N connectors. RF connectors are built in male, female, plug, jack, receptacle or sexless gender with 50 Ohm or 75 Ohm Impedance and in standard polarity, reverse polarity or reverse thread designs supporting an operating frequency range of from DC to 40 GHz. Designs for RF quick connect/disconnect connectors are available with QMA, QN, 4.3-10, SMP, SMP-M and BMA connectors with rapid snap-on or push-on mating. Many of these modern connectors provide a VSWR as low as 1.1:1 with an Input power up to 2W maximum.

 

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Basics of Surge Protectors and Lightning Arrestors for Telecommunications and RF Applications

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Who dares thwart the power of Zeus?

Surge protectors and lightning arrestors are devices that protect electrical components from a temporary rise in voltage and current on an electrical circuit.  Surge protectors are designed to protect electronic equipment from power surges and voltage spikes by diverting the excess current from the transient event or surge into a grounding wire. Surge protectors are also referred to as Transient Voltage Surge Suppressors (TVSS), Surge Protection Devices (SPD), or Surge Suppression Equipment (SSE). Lightning arresters, aka lightning diverter or surge arrestors, are protective devices for limiting surge voltages caused by lightning strikes to prevent damage to equipment and disruption of service.

What Causes a Power Surge?

The obvious answer is an angry Hellenic deity and, in truth, much of the damage to electronic systems equipment from a random power surge is due to lightning strikes. However, the most damaging aspect about lightning to a telecommunication system or transmission line is not the power wielded in a spear of lightning, or a direct lightning strike, but is instead caused by power surges via the strong electromagnetic fields created during a lightning strike. Lightning bolts carry from 5 kA to 200 kA and voltages vary from 40 kV to 120 kV. A typical cloud-to-ground lightning bolt moves downward from the bottom of a storm cloud toward the ground at about 200k mph and just as it nears ground, a surge of positively charged electricity moves up to meet the negatively charged bolt and a visible flash of lightning streaks upward conducting electricity as lightning. Lightning strikes, even several miles from a structure, can generate a power surge that travels through aerial or buried cable lines to sensitive electronic equipment. Direct lightning strikes to equipment and cables are too powerful to protect against, however, protection against transient surges can be accommodated by equipment and system design. Other causes of power surges can be attributed to equipment fluctuations or failures, faulty wiring or system design, or environmental hazards.

Performance Specifications for Surge Protectors

There are two types of surge protectors on low voltage AC systems and the difference between them is in the ability to divert energy in the form of a current:

> Class 1 (Lightning Protection)

Diverts energy with a current waveform of 10/350ms with a current ratings from 10Ka to 35Ka. Includes high energy metal oxide varistor (MOV) and gas discharge tube/air gap components.

> Class 2 (Surge Suppression)

Diverts energy with a waveform of 8/20ms with a current ratings from 5Ka to 200 Ka. Includes silicon avalanche diode (SAD) and metal oxide varistor (MOV).

Underwriters Limited issued UL 1449 4th Edition as the safety standard for all AC surge protection devices (SPDs) which covers SPDs designed for repeated limiting of transient voltage surges on 50 or 60 Hz power circuits not in excess of 1000 V. There are several kinds of surge protective devices (SPDs) and the following performance characteristics are the most important to consider in system design.

Maximum continuous operating voltage (MCOV)

MCOV is the maximum steady state voltage the SPD can tolerate before becoming a safety hazard. Current safety requirement specify that, in the case of an overvoltage of 110% of nominal voltage, the device must remain functional and safe and should an abnormal overvoltage of 125% occur, the device is will safely but permanently cease to function.

Voltage protection level (VPL)

VPL is the residual voltage or clamping level of the arrester which is indicative of the reaction time of the arrester such that the faster the reaction time, the lower the VPL. The arrester does not detect the transient surge or the transient damages the equipment commonly generated by inductive loads switches on air conditioners, lift motors, or standby generators.

Surge rating

Measured by a short-duration, high-current impulse with an 8µsec rise time and a 20µsec decay time. The selection of a suitable surge rating for the intended application is key to ensuring longer service life of the product. Ratings include:

> Clamping voltage – indicates at what voltage the MOVs will conduct electricity to the ground line. A lower clamping voltage usually indicates better protection.

> Energy absorption/dissipation – given in joules, this indicator measures how much energy the SPD can absorb before it fails. A higher number indicates greater protection.

> Response time – SPDs have a slight delay when triggered by a power surge. A longer response time indicates equipment will be exposed to a surge for a longer time, and where high voltage is concerned, every nanosecond matters.

Short Circuit Current Rating SCCR)

SCCR indicates the suitability of an SPD for use on an AC power circuit capable of delivering not more than a declared rms symmetrical current at a specified voltage during a short circuit condition. It is the amount of current that the SPD can tolerate safely and safely disconnect from the power source under short circuit conditions.

For RF and Microwave applications, RF surge protectors must also be made to limit degradation of wanted signals, while still preventing surges. This means that a low inline attenuation to signals above surge frequencies is desired.

 

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Overview of Military Standards for RF Cables and Connectors

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Many coaxial cable and connector types are available in the RF and microwave industry designed for specific purposes and applications; compatibility with other RF microwave components is achieved with universally accepted cable and connector standards. The US Department of Defense (DOD) develops standards for materials, facilities, engineering, and testing practices in order to improve military operational readiness and reduce costs and production time. DOD standards have adopted non-government standards and practices that meet DoD performance requirements. This process is managed by the Defense Standardization Program who provides the standards and relevant specifications. RF cables and connectors used in military applications have demanding requirements for environmental specifications, such as temperature, shock, and vibration; these requirements are specified in detail the US Department of Defense military standards in MIL-DTL-17J for coaxial cables and MIL-PRF-39012 RF for coaxial connectors.

Department of Defense Standards (DoD)

The Acquisition Streamlining and Standardization Information System (ASSIST) database is a list of defense and federal standardization documents as well as adopted non-government standards (NGS), and Materiel International Standardization Agreements (ISAs). The approved standardization documents in the database include:

> Defense specifications, standards, and handbooks

> Commercial item descriptions (CIDs) and federal specifications and standards

> NGS created by consensus procedures in non-government standards organizations

> Materiel ISAs within NATO countries which have been ratified by the US.

When developing defense standards and specifications, one must be authorized by a military defense agency or department as a Standardization Management Activities (SMA) organization. Military standardization documents in the DoD Index of Specifications and Standards can be found at the DoD Single Stock Point in Philadelphia while the Industry standards, for example, the Institute of Electrical and Electronics Engineers and American National Standards Institute, are available from the specific association that authored the standards and specifications. It is important to note that military specifications can change at any time and it’s a good practice to check to make sure that the latest revision is being referenced. These standards are often referred to by their acronyms such as, “MIL-STD” Military Standard, “MIL-SPEC” Military Specifications or “MilSpecs.” According to the Government Accountability Office (GAO), military specifications serve to “describe the physical and/or operational characteristics of a product” while military standards “detail the processes and materials to be used to make the product.”

MIL-DTL-17

Coaxial cables for military and aerospace applications must meet the standards and specifications in MIL-DTL-17 which details specifications for flexible and semi-rigid coaxial cables with solid and semisolid dielectric cores as well as single, dual, twin, and triaxial conductors intended for use in radio frequency applications. The most current version of MIL-DTL-17 is revision “J” produced in 2014. To locate a regulation for a specific part or its specifications, a Part or Identifying Number (PIN) is assigned which begins with the letter “M” for Military followed by the specification number, the corresponding slash sheet number, and the assigned dash number or “RG” number.

MIL-PRF-39012

Coaxial connectors for military and aerospace applications must meet the standards and specifications in MIL-PRF-39012 which provides general requirements and test methods for RF connectors used with flexible and semirigid coaxial cables. This standard gives the following classification standards:

> Class 1 is a connector with “superior RF performance at specified frequencies” and

> Class 2 is  a connector that provides a  mechanical connection within an RF circuit.

This standard also describes categories by which connectors are classified:

  • A  = field serviceable
  • B  = non-field replaceable
  • C  = field replaceable solder center contact
  • D  = field replaceable crimp center contact
  • E  = field replaceable and
  • F  = field replaceable crimp, for semi-rigid cable

 

MIL-PRF-39012’s latest amendment “E” in 2014 (section 3.3.5) adds that recycled, recovered, or environmentally friendly materials “should be used to the maximum extent possible, provided that the material meets or exceeds the operational and maintenance requirements, and promotes economically advantageous life cycle costs.” MIL-PRF-39012 Rev. F specification sheet, produced in 2016, gives the PIN for connectors. For example, MIL-PRF-39012/55 is the PIN referring to an RF coaxial SMA connector plug with a pin contact where the letter “M” is followed by the basic specification sheet number, and a sequentially assigned four-digit dash number. The first digit in the dash number designates the material and plating of the connector body as in “0” for silver plated brass, “3” for passivated corrosion resistant steel, “4” for gold plated copper beryllium, or “7” for nickel plated brass. More specifically, M39012/55 – 3028 refers to an SMA connector pin made of passivated corrosion resistant steel.

 

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Hi-Rel Cable Overview

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Increasing reliance on electronic devices for industrial, aerospace, and military applications compels manufacturers of RF/microwave cable assemblies to offer cables that exhibit high reliability in many types of environments and operating conditions. Some areas of applications that require Hi-Rel components include space, military and defense, test and measurement, wireless mobile communications, automotive, medical, and other industrial applications.

What is High Reliability?

Hi-Rel means “high reliability” which, one would assume, is based upon a reliability measure and it might, depending upon the device or application, but for Hi-Rel coaxial cables reliability can also be gaged by how the cable measures up to standards.  As reliability is thought of as a measure of consistency or repeatability, for coaxial cable, not only do you want your assembly to perform as intended, or accurately, you would want it to perform this way every time. Unfortunately, Hi-Rel coaxial cable is rated differently depending on its application in a particular industry.  Commercial, automotive, U.S. military, or European Union industries have different standards with ranges of acceptable conditions which makes comparison of cables or devices in terms of “Hi-Rel” somewhat inadequate.

Keep in mind, standards are written as “minimum specifications” for components and systems providing a minimum performance level for cable assemblies. Competition in the industry compels manufacturers to make high quality cables that outperform the standards in order to have an advantage over competitors. In other words, these standards allow manufacturers to make cables that are compatible with other cables but have other advantages, either in performance, installation or cost.  Many think industry standards are mandatory but standard compliance is a voluntary, rational approach to coaxial cable design and manufacturing that allows interoperability, upgradability, and cost savings for communication systems.

In the RF industry, reliability refers to an accepted level of performance based on typical usage, without degradation or loss in performance for the lifetime of the cable. In reality, the reliability of a cable assembly may related to one or several performance characteristics which can differ from one application to another when one parameter or the other is performing well for one application but not another. Yet consumers rely on reliability – it is crucial for maintaining service in cellular communications network, in protecting national interests, and in emergency services.  Hi-Rel cable assemblies are used in test-and-measurement facilities where the accuracy and reliability is part of the quality assurance to validate the performance of DUT.

What Factors Constitute Hi-Rel?

Operating Environment

The operating environment of an RF cable assembly has a tremendous impact on its optimum reliability. Unwanted interference from a cable assembly lacking proper EMI shielding would not be acceptable in a surveillance or radar application. High quality materials like low-loss dielectrics or precious metal platings do not necessarily ensure high reliability.  Design for application is key and design testing is a must in the R&D phase of product development. Hi-rel cable assemblies are designed for extreme temperatures, harsh environments, and excessive stress where performance is paramount. The production process of a hi-rel component involves detailed handling procedures, additional conformance testing and inspection to ensure that the product has superior performance and quality to ensure optimum performance and a high survival rate under extreme environments.  These cable assemblies are subject to thermal and mechanical shock tests, exposed to moisture and humidity, and high temperatures to test whether they can withstand the harsh conditions where they are expected to perform.

Testing

Testing and measurements are critical for assessing and determining product reliability. Manufacturers of high-reliability cable assemblies, to be competitive, must embrace new concepts and test these designs to bring innovation to the market.

Measurement parameters include insertion loss, return loss (VSWR), phase stability, and thermal cycling, to name a few. Regular and repeatable measurements and analysis of test results are crucial in the product development process. Computer simulation software can provide a new basis for development of new products and high-performance test systems with reliable (accurate and repeatable) test data feedback to refine the development process. On a final note, be mindful that, depending upon the testing process, a cable assembly or part of the assembly may be referred to as Hi-Rel without being tested against the quality standards.

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RF Interconnect Insights for High Speed Digital Signals and Data Communications Part 2

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Introduction

In the previous blog, Part 1 of “RF Interconnect Insights for High Speed Digital Signals and Data Communications” two part series, the concept of RF interconnect used in high speed digital applications was introduced. In this section, more insights will be provided on intermodulation distortion concerns and other challenges with trends in high speed digital data transfer and near-future wireless communications that rely on digital signals.

Interconnect Intermodulation

When it comes to reducing interference. some RF connectors are better than others and, as passive intermodulation, or PIM, is a major cause of signal degradation. Especially considering the latest high speed digital data communications for cellular and wireless backhaul, the high peak power ratios make PIM a substantial consideration. PIM is detected as unwanted signals created by the mixing of two or more strong RF signals in a nonlinear device, as a loose or corroded connector, or by rust on the interconnect device.

Unfortunately, the source of PIM isn’t always so easily explained, and for high speed digital data communications, can be a product of material use, cable design, connector design, connector material matchup, connector tightness, and other factors. PIM becomes a problem when, for example, PIM is generated, it increases the noise floor and interferes with wireless device signals leading to access failures, slower data rates, and dropped calls. Conditions that lead to PIM:

<  poor cable termination

< damaged or loose connector

< over-torqued or broken connector

< metal flakes or debris inside the connector or inside the cable

Proper termination on the cable and the connector and proper torquing techniques when installing an interconnect to the interface can greatly reduce the amount of PIM.

Interconnect Challenges

As  RF/microwave technologies are increasingly used in the high speed digital data exchange world, the mix of connector and cable technologies will necessarily become more complex. In the next generation of wireless technology, higher data rates and greater demand for high integrity interconnect will require higher performance capabilities in RF interconnection–with proposed 5G and wireless communication frequencies reaching almost 80 GHz, this means that the digital signal speed and density that drives these communications will also be unprecedented speeds. To meet these high frequency and performance needs, RF interconnects product design and manufacturing is expanding to improve interface compatible designs that are tested and qualified per EIA-364 standards with high GHz performance, high durability cycles.

With high speed digital signals, challenges associated with digital systems such as clock skew, jitter, power consumption, bit error rate (BER), and signal integrity using conventional copper interconnect technology may not meet the needs for current and future Very Large Scale Integration (VLSI) circuits. In order to address these concerns and improve performance to meet high speed demands, adopting RF/wireless interconnects in future on-chip and board-level clock distribution networks may offer desired performances for future high speed clock distribution network operating in multi GHz frequencies.

To learn more about Pasternack’s diverse line of coaxial RF cables, visit:

https://www.pasternack.com/cable-assemblies-category.aspx

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RF Interconnect Insights for High Speed Digital Signals and Data Communications Part 1

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RF high speed interconnection is used in many electronic products where signal transmission quality is a critical factor in applications using high speed digital signals. An RF interconnect is the complete path that connects one device to another; RF-interconnect structures include coaxial cable assemblies, microstrip transmission lines, and waveguides with RF connectors, adapters, and attenuators. Many wireless communication systems require numerous RF interconnect paths such as in the case of sensitive sensor modules, RF modules to antennas, and between networked devices. As RF hardware is increasingly digitized, there are more applications that require high speed digital signals for data transmission between RF hardware. Also, the latest high speed digital technologies also leverage RF coaxial transmission lines and microstrip (stripline) transmission lines for connector interfaces, cabling, and chip fan-out.

Achieving maximum performance from RF communication systems and high speed digital data transmission requires close attention to interconnect technology, circuit-to-circuit interconnections, and circuit design. RF interconnectors can perform to hundreds of gigahertz (Gigabits per second) and are used in a variety of high speed digital applications, such as communication devices, high performance computing, and sensors. There are of numerous types of RF interconnect used with high speed digital signals, such as BNC, CX/MMCX, SMT/SSMT, SMA/SSMA, SMB/SSMB, SMC/SSMC, and TNC Connectors, and RF Adapters.

Timing is Everything

Signal timing and the quality of the signal are important concepts in high speed digital designs. A primary concern when designing a digital communication system involves isolating high speed signals, which are more likely to impact or be impacted by other signals, and maintaining signal integrity to ensure a signal reaches its destination. In a communication system, digital signals travel through various interconnections from chip to package, package to RF board trace, and trace to high speed connectors; any electrical discontinuity at the source end, on the transmission path, or at the receiving end, will affect the signal timing and quality.

RF interconnect for high speed digital applications require reasonable isolation and protection from electromagnetic interference. Hence, shielding and connector quality are of great importance. Moreover, many high speed digital applications have high interconnect densities, requiring small pitch distances. Hence, push type connectors and connector assemblies with a multitude of RF connectors, such as pogo-pins and snap-connectors, are common. An important factor to note is that reducing the delay between separate parallel digital signals requires very closely matched transmission line lengths, which is a design challenge considering the complexity of dense digital signal pathways.

Next Blog Preview

In Part 2 of “RF Interconnect Insights for High Speed Digital Signals and Data Communications” we will review concepts, such as intermodulation distortion with high speed digital communications and interconnect challenges with high speed digital applications.

To learn more about Pasternack’s diverse line of coaxial RF cables, visit:

https://www.pasternack.com/cable-assemblies-category.aspx

The post RF Interconnect Insights for High Speed Digital Signals and Data Communications Part 1 appeared first on Pasternack Blog.

RF Coaxial Probes and Pins 101

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What is an RF Coaxial Probe?

In RF applications, transmission lines generally are coaxial cables linked to circuit boards and microstrips within circuit boards. An RF coaxial probe is a measurement device used with electronic test equipment in the measurement of radio frequency (RF) signals in an electronic circuit as found in silicon wafers, dies, and open microchips. RF coaxial probes are also used with narrow pitch or high density RF interconnect applications in connector assemblies.

Prior to the development of RF probes in the 1980’s, there was no convenient way to test Monolithic Microwave Integrated Circuit (MMIC) devices without mounting or bonding, which often damaged the integrity of the circuit, contributed to interference in the system, or reduced the power load. First generation RF probes used coplanar ceramic feeds and covered up to 18 GHz. Development in RF probes have led to RF coaxial probes having spring-loaded inner and outer conductors that are used in modern communication electronics. Today, RF coaxial probes continue to be useful in the testing of RF switches, RF traces in printed circuit boards, terminations, and other RF components. Consumer products in the 60-80 GHz mmwave band include use in automotive radar systems. WiGig standards testing and compliance, wireless HDMI, and high performance-LAN.

Why Do We Need Probes?

RF circuit measurement is tricky due to the sensitive and often delicate nature and composition of the devices under testing or DUT. Concerns in reliable RF measurement often occurs when the frequency is too high for available test equipment to measure the RF energy or when a circuit sensitive to small changes in the electrical environment needs to be measured without disrupting the frequency or amplitude. The solution is a measurement probe that disrupts the energy from the circuit as little as possible where high impedance probes are used which may include an amplifier to level the amount of energy harnessed from the circuit, For RF circuit systems testing, matching probe impedance with the testing device is crucial for efficient power transfer, which becomes increasingly challenging as frequency increases and tolerances become more strict. In RF testing, various types of RF test probes are available with suitability based on consideration of the test point to be contacted, the frequency or data rate, the space available for probe installation, and environmental conditions. In the near future, RF probes will need to have the design capability to probe smaller pads with the ability to probe multiple channels and possess the measure range capacity to include multiple mm-wave, RF, logic, and power channels simultaneously.

Important Probe Parameters

When considering high-frequency testing, high-frequency product components require sophisticated test equipment which may include a vector network analyzer (VNA), wafer probe system, high-frequency probes, semi-rigid or flexible coaxial RF cables, and calibration substrates. Perhaps the most critical part of the measurement system is the probe which must physically connect with the DUT. A reliable RF probe should demonstrate impedance repeatability with no degradation from its characteristic (50 Ohms commonly) impedance with no visible or physical wear on mating connectors after repeated insertions.

Frequency Matching

Based on the application of the circuit, a probe that operates up to the operation frequency is recommended; some probes are designed to support frequencies up to 110 GHz. Most RF probes are 50 ohm impedance although high-impedance probes, differential probes, and dual-signal probes (SGS) are available for high-frequency testing purposes.

Configuration

In order to probe a circuit, a signal is sent through a transmission line to the DUT. RF probes require at least two conductors, a signal and a ground, and how these conductors are configured determines the type of probe required for testing the circuit.  Configurations include GS (Ground and Signal), GSG (Ground, Signal, Ground), and GSSG (Ground, Signal, Signal, Ground). The most common type of RF probe configuration is the GSG which is similar to coplanar waveguide.

Probe Pitch

Probe pitch is the distance between the tips of the probe and its center. Typical values are 150/250 microns, but a range of 50 to 1000 microns is possible however, larger probe pitch is not practical for millimeter-wave frequencies.

Touch-down

A touchdown is the motion of the probe tip as it comes in contact with the surface of a wafer, component, or printed-circuit board in which the probe exerts enough force to measure the DUT, but not enough force to damage the DUT’s surface.

Probe stations

Probe stations are designed to allow the positioning of RF probes on a silicon wafer so that testing from basic continuity or isolation checks to full functional testing of microcircuits can be run either before or after the wafer has been adapted to individual dies.

Probe skate

When the Z-axis of a probe adjusts as it comes in contact with the DUT and travels along the Z-Y plane as it flexes. A desirable skate probe is one mil or 25 um. The more overtravel (OT) or downward pressure is applied to the probe, the more the probe will skate across the pad. To tolerate excessive OT, a lower skate ratio is required.

 

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Upper Microwave and Millimeter-wave Frequency Coaxial Connector Overview

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Many coaxial connector types are available in the RF and microwave industry designed for specific purposes and applications with smaller connectors that perform into the GHz and millimeter wave range. Compatibility with other RF microwave components is achieved with universally accepted connector standards so that interconnecting coaxial modules within a system is possible and must retain the coaxial nature of the transmission line with which they are used. As with coaxial cable, impedance, frequency range, power handling, physical size, and cost are the parameters which determine the best type of connector for a given application.

In RF and microwave applications, there are generally three grades of connectors designed for use in production, instrumentation, and metrology. Production grade connectors are low cost simple devices used in components and cables for most common applications. Instrument grade connectors are precision or test connectors the high performance standards of low reflection and good repeatability used in testing and measurement equipment. Metrology grade connectors are high precision connectors with high accuracy and are typically more expensive. Recently, there are more upper microwave and millimeter-wave coaxial cables and connectors being used in prototype and production for military and aerospace applications, which are of a specifically designed nature to meet with the stringent reliability (Hi-rel) standards associated with those industries.

Usually, a connector is identified by its type or the coaxial cable it is connected to along with the term male or female based on design–becoming a connector pair when coupled. A typical connector pairing is reliable from 50 to several hundred cycles depending upon design features and, while two connectors can have identical specifications, a design feature like silver over nickel plating, can yield a measurable difference in performance.

Connector Families and Frequency Limitations

There are several types of RF microwave coaxial connector families. As with coaxial cable lines, the cutoff frequency is a key property of any coaxial cable connector above which the desired TEM mode will no longer be the only mode that propagates. The frequency range of any connector is limited by the propagation mode in the coaxial system. Millimeter-wave coaxial connectors are coaxial connectors for use above 18 GHz.

Connector Type N, BNC and TNC

Developed in the 1940’s, the Type N 50 ohm connector was designed for military systems operating below 5 GHz. The Type N uses an internal gasket with an air gap between center and outer conductor. Later improvements increased performance to 18 GHz but even modern designs begin to mode around 20 GHz producing unpredictable results if used at that frequency or higher. A 75 ohm versions is widely used in the cable-TV industry. The BNC, used in video and RF applications to 2 GHz, uses a slotted outer conductor with a plastic dielectric on each gender connector. At higher frequencies above 4 GHz, the slots may radiate signals up to about 10 GHz. Because the mating geometries are compatible with the N connector, it is possible to temporarily mate some gender combinations of BNC and N. A threaded version, the TNC, helps resolve leakage and stability problems allowing use in applications up to 12 GHz and 18 GHz. The TNC connector is in wide use in cellular telephone RF/antenna connections.

Connector type SMA and SMB Push-On

The SMA, subminiature A, connector uses a 4.2 millimeter diameter outer coax filled with PTFE dielectric with an upper frequency limit ranging from 18 to 26 GHz, depending upon the manufacturer. SMAs are sized to fit a 5/16 inch wrench and will mate with 3.5mm and 2.92mm connectors. The SMB, or subminiature B, is a push-on connectors typically specified for 4 GHz to 12.4 GHz. With frequency demands increasing, these connectors are too large and lack the bandwidth needed for high frequency applications.

Connector type 3.5mm and 2.92mm

These connector types use air dielectric and are compatible with one another and the SMA type. The 3.5 mm connector performs well up to 26 GHz. The 2.92 mm connector performs through 40 GHz.

Connector type 2.4mm and 1.85mm

The 2.4mm and 1.85mm connectors are compatible with each other but not the SMA, 3.5 or 2.92 mm connectors by design, as the less precise connectors can cause irreparable harm to the more expensive and more precise 2.4 and 1.85mm connector.

Connector type 1mm and .8mm

Used for millimeter-wave analysis, these connectors support transmission and repeatable interconnections from DC to 110 GHz.

 

Reference http://microwave.unipv.it/pages/microwave_measurements/appunti/01b_MM_connectors.pdf http://www.uniroma2.it/didattica/SMM/deposito/RF_Connectors.pdf https://www.microwaves101.com/encyclopedias/microwave-coaxial-connectors

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How High is a Coaxial Cables Max Frequency?

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Coaxial cable is the most commonly used transmission line for RF and microwave applications, because it provides reliable transmission with the benefits of wide bandwidth and low loss and high isolation. Major manufacturers of transmitting equipment, i.e. radio and TV, radar, GPS, emergency management systems, air and marine craft, use coaxial cables. The uses of coaxial cable apply to any system in which signal loss and attenuation must be minimized. Unlike waveguides, coaxial cable has no lower cutoff frequency but what about its upper frequency?

Frequency

Like other parts of the electromagnetic spectrum, radio frequency (RF) is identified by its frequency in Hertz (Hz) or wavelength in meters. An inverse relationship exists between these two concepts such that as frequency increases, wavelength decreases, with the reverse being true as well. The strength of a radio frequency signal is measured in Watts. A frequency band refers to a designated section of the RF spectrum like, for example, the AM and FM band used in radio broadcasting and, within this band, a section of spectrum is referred to as bandwidth. Frequency is identified as the number of reverses or cycles in the flow of alternating current (AC) per second. For example, broadcast stations operate at frequencies of thousands of cycles per second and their frequencies are called kilohertz (kHz); higher frequencies are in millions of cycles per second and are called megahertz (MHz). Radio frequency is the frequency band which is primarily used for transmission of radio and television signals and ranges from 3 MHz to 3 GHz. Microwave frequencies range from Ultra-High Frequency (UHF) 0.3 – 3 GHz, Super High Frequency (SHF) 3 – 30 GHz to Extremely High Frequency (EHF) 30 – 300 GHz.

Max Frequency

With some exceptions, most coaxial cables do not have an actual cut-off terms of a specific stop-band frequency but instead use the term cutoff to refer to the highest frequency tested by the manufacturer, or when the frequency reaches a point where the coaxial cable becomes a waveguide and other modes, aside from the transverse-electromagnetic mode (TEM), occur. Hence, a coaxial cable cutoff frequency could be where the coaxial cable remains within specification, or within a reasonable margin to avoid transverse-magnetic (TM) or transverse-electric (TE) propagation modes. Though coaxial cables can still carry signals with frequencies above the TEM mode cutoff, TM or TE transmission modes are much less efficient not desirable for most applications.

Cutoff Frequency and Skin-Depth

Two important concepts of note when discussing frequency in coaxial cable are skin-depth and cutoff frequency. Coaxial cable is made up of two conductors, an inner pin, and an outer grounded shield. Skin depth occurs along the coaxial line when high frequencies cause electrons to migrate towards the surface of the conductors. This skin effect leads to increased attenuation and dielectric heating and causes greater resistive loss along the coaxial line. To reduce the losses from the skin affect, a larger diameter coaxial cable can be used but increasing the coaxial cables dimensions will reduce the maximum frequency the coaxial cable can transmit. The problem is that when the size of the wavelength of electromagnetic energy exceeds the transverse electromagnetic (TEM) mode and begins to “bounce” along the coaxial line as a transverse electric 11 mode (TE11), the coaxial cable cut-off frequency is created. Because the new frequency mode travels at a different velocity than the TEM mode, it creates reflections and interference to the TEM mode signals traveling through the coaxial cable. This is referred to as the upper frequency limit or cutoff frequency.

A cutoff frequency is a point at which energy flowing through the EM system begins to be reduced, by attenuation or reflected, rather than passing through the line. TE and TM modes are the lowest order mode propagating on a coaxial line. In TEM mode, both the electric field and the magnetic field are transverse to the direction of travel and the desired TEM mode is allowed to propagate at all frequencies. Higher modes are excited at frequencies above the cutoff frequency when the first higher-order mode, called TE11, is also allowed to propagate. To be sure that only one mode propagates for a clear signal, the signals need to be below the cutoff frequency. Reducing the size of the coaxial cable increases the cut-off frequency. Coaxial cables and coaxial connectors can reach into the millimeter wave frequencies but as the physical dimensions shrink, power handling capabilities are reduced and losses increase.

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Analog vs. Digital Beamforming

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Beamforming as a Buzzword

Beamforming is used for directional signal transmission and reception with the versatility to change both amplitude and phase to help regulate power needs and steer the beam in the intended direction. Bandwidth from 6 to 100 GHz, or millimeter Wave (mmWave), is likely an integral part of future mobile broadband as 5G communication systems are introduced in the global market. Concepts like beamforming and analog vs. digital become part of the discussion when the topic turns to “What’s next?”  In high frequency mmWave transmission, large path loss during signal propagation limits the transmission range; to overcome this obstacle, directional antennas with beamforming abilities are used in transmission and reception. Beamforming directs the antenna beams at the transmitter and receiver so that the transmission rate is maximized with minimal loss.

Analog vs. Digital

When working with electronics, both analog and digital signals have to be understood and integrated in meaningful ways in order for our electronic systems have perform as intended. While analog signals may be limited to a range of maximum and minimum values, there are an infinite number of possible values within that range. The waves of a time-versus-voltage graph of an analog signal are smooth and continuous. Conversely, digital signals have a finite set of possible values and are one of two values such as either 0V or 5V, for example, and timing graphs of these signals look like square waves. To identify whether a signal is analog or digital, compare how the signal appears; a time-versus-voltage graph of an analog signal should be smooth and continuous while digital waves are stepping, square, and discrete. Most basic electronic components like resistors, capacitors, inductors, diodes, transistors, and amplifiers are analog. Digital circuits use digital, discrete signals using a combination of transistors, logic gates, and microcontrollers. An analog to digital converter (ADC) allows a microcontroller to connect to an analog sensor to read in an analog voltage. A digital to analog converter (DAC) allows a microcontroller to produce analog voltages. A digital down converter (DDC) preserves information in the original signal and is often used to convert analog radio frequency or intermediate frequency down to a complex baseband signal.

Analog

In analog beamforming, a single signal is fed to each antenna element in the array by passing through analog phase-shifters where the signal is amplified and directed to the desired receiver. The amplitude/phase variation is applied to the analog signal at transmit end where the signals from different antennas are added before the ADC conversion. At present, analogue beamforming is the most cost-effective way to build a beamforming array but it can manage and generate only one signal beam.

Digital

In digital beamforming, the conversion of the RF signal at each antenna element into two streams of binary baseband signals cos and sin, are used to recover both the amplitudes and phases of the signals received at each element of the array. The goal of this technology is the accurate translation of the analog signal into the digital realm. Matching receivers is a complex calibration process with each antenna having its own transceiver and data converters that generate multiple beams simultaneously from one array. The amplitude/phase variation is applied to digital signal before DAC conversion at transmit end. The received signals from antennas pass from ADC converters and DDC converters.

Digital Beamforming Challenges

 < Amount of data generated – The data rate out of the ADC effects the digital interface and processing power requirements and the question becomes how to handle the data when electronic systems want increased resolution and higher sampling rate for increased bandwidth.

 < Power consumption – Processors require lots and lots of power. Because of the limitation in data bandwidth, there is a practical limit on the number of elements in the array which requires waveform generators at each element.

 < Loss – The losses high frequency mmWave transmission incur include high free space path loss, absorption from atmospheric gases and rainfall, and non-line of sight propagation

 < Expense – The overall expensive of implementing digital beamforming systems includes but is not limited to the physical size of the electronics and the high cost of a large number of ADCs operating at high sampling frequencies.

Possible solutions for some of these challenges in 5G mobile communications are forthcoming in the research and tend to dominate discussions on the future of beamforming. It appears that, at present, digital beamforming is the future in communication systems but not without its challenges. To mitigate these challenges, it appears evident that the first 5G mobile systems will integrate a combination of analogue and digital beamforming systems.

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What is Beamforming?

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In array antennas, beamforming, also known as spatial filtering, is a signal processing technique used to transmit or receive radio or sound waves in a directional signal. Beamforming applications are found in radar and sonar systems, wireless communications, and in acoustics and biomedicine equipment. Beamforming and beam scanning are generally accomplished by phasing the feed to each element of an array so that signals received or transmitted from all elements will be in phase in a particular direction. When transmitting, a beamformer controls the phase and relative amplitude of the signal at each transmitter thus creating a pattern of constructive and destructive interference in the wavefront. When receiving, sensors are combined in a way where the expected pattern of radiation is preferentially received.

Beamforming Techniques

Beamforming techniques direct the beam radiation pattern in the desired direction with a fixed response. Array beams can be formed or scanned using either phase shift or time delay systems.

Phase shifting

In narrowband systems, a time delay is known as a phase shift. Beamforming by phase shifting can be accomplished using ferrite phase shifters at RF or IF. Phase shifting can also be done in digital signal processing at baseband. Systems using time delays are preferred for broadband operation because the direction of the main beam does not change with frequency.

Time delays

Time delays are introduced by varying the lengths of transmission lines. As with phase shifting, time delays can be introduced at RF or IF and works well over a broadband, but the bandwidth of a time scanning array is limited by the bandwidth and spacing of the elements. As the frequency of operation is increased, the electrical spacing between the elements increases so that the beams will be somewhat narrower at higher frequencies and, as the frequency is increased further, grating lobes appear. In a phased array, grating lobes are induced when the direction of the beamform extends beyond the maxima of the main beam and the main beam reappears on the wrong side. Elements must be spaced properly in order to avoid grating lobes.

Weights

The weight vector is a vector of complex weights where the amplitude components control the sidelobe level and main beam width and phase components control the angle of the main beam and nulls. Phase weights for narrowband arrays are applied by a phase shifter.

Beamforming Designs

Antennas that are designed to adapt and change their radiation pattern in order to adjust to the RF environment are called active phased array antennas. Examples of beamform designs include the Butler Matrix, the Blass Matrix, and the Wullenweber Array.

Butler Matrix

The Butler matrix uses a combination of 90° hybrids and phase shifters and can cover a sector of up to 360° depending on element design and patterns. Each beam can be used by a dedicated or single transmitter or receiver controlled by an RF switch. Thus controlled, a Butler matrix can be used to steer the beam of a circular array.

Blass Matrix

In broadband operations, the Blass matrix uses transmission lines and directional couplers to form beams by using time delays. A Blass matrix can be designed for use as a broadside beam but can be lossy because of the resistive terminations.

Wullenweber Array

A Wullenweber array is a circular array designed for direction-finding at HF frequencies. This array uses Omni-directional elements or directional elements usually designed with 30 to 100 elements, a third of which are used sequentially dedicated to form a high directional beam. A goniometer is used to connect the elements to the radio with amplitude weighting to control the array pattern and has the ability to scan over 360° with little deviation in pattern characteristics. Time delays are used to form beams radial to the array, enabling broadband operation.

 

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Waveguide Frequencies and Geometries

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Loss, whether due to radiation leakage or conduction resonance, is a common problem in RF microwave transmission lines, especially when high-powered frequency transmissions are involved. The solution? Waveguides.

Waveguide Basics

A waveguide is an electromagnetic feed line used for high frequency microwave signals in high-power transmitters and receivers and is used in radar equipment, in microwave ovens, in satellite dishes or in any RF microwave system where high-power transmission is needed. Waveguides are hollow metallic tubes or light carbon fiber composites constructed with high grade metals like copper, brass, or plated metals. Silver or other plating is used on the inside walls of a waveguide which acts to decrease the resistance loss by shielding and provides efficient isolation between adjacent signals. Transmission lines like microstrip, stripline, or coaxial cable may also be considered to be waveguides and they are usually referred to as dielectric waveguides with a solid center core. However, on high-powered microwave waveguides where the line may get too hot, air in the cavity may be pressurized, recirculated, or a Freon-like gas is used to keep the waveguide cool and also can prevent arcing.

While the disadvantages of using a waveguide include a high production cost, large size and mass of the guide, and the inability of running a DC current alongside the RF signal, the advantages in using a waveguide are that they are completely shielded, high-powered transmission lines that provide good isolation and very low loss that can bend without compromising performance.

Frequencies and Geometries

For the signal to propagate, waveguides need a minimum cross section relative to the wavelength of the signal; these cross sections can be either rectangular, circular, or elliptical. The dimensions of a waveguide determine the wavelengths it can support and in which modes. The lowest frequency range a waveguide will operate is where the cross section is large enough to fit one complete wavelength of the signal. In hollow waveguides, or waveguides using a single conductor, transverse-electromagnetic (TEM) mode of transmission waves are not possible, since Maxwell’s Equations demonstrates that an electric field must have zero divergence and zero curl and be equal to zero at boundaries, resulting in a zero field.

Comparatively, for two-conductor lower frequency transmission lines, like microstrip, stripline, or coaxial cable, TEM mode is possible. In rectangular and circular waveguides, the dominant modes are designated the TE10 mode and TE11 modes.

According to Maxwell’s equations, there are three rules that apply to waveguides:

1 )Electromagnetic waves are reflected by conductors,

2 )Electric field lines that make contact with a conductor must be perpendicular to it,

3 )Magnetic field lines close to a conductor must be parallel to it.

These rules allow for certain modes of propagation such that the TE10 (transverse electric) mode is the mode in which energy propagates in rectangular waveguide. The mode with the lowest cutoff frequency is noted as the dominant mode of the guide.

Because waveguides are the transmission lines for super high frequency (SHF) systems, they must operate with only one mode propagating through the waveguide. Waveguide propagation modes depend on the operating wavelength, polarization, shape, and size of the guide.  Waveguides standards are based on rectangular waveguides and are designed with these characteristics:

one band starts where another band ends, with a band overlapping the two bands in order to allow for applications near band edges,

> the lower edge of the band is approximately 30% higher than a waveguide cutoff frequency thus limiting dispersion and loss per unit length,

>In order to avoid evanescent-wave coupling by way of higher order modes, the upper edge of the band is approximately 5% lower than the cutoff frequency of the next higher order mode,

>the waveguide height is half the waveguide width which allows a 2:1 operation bandwidth – having the height exactly half the width maximizes the power inside the waveguide.

For convenience, our waveguide calculator provides the cutoff frequency, operating frequency range and closest waveguide size for a rectangular waveguide.

Variations on the Waveguide

>The double-ridge rectangular waveguide is a type of waveguide used in RF microwave systems. The ridges in this waveguide design serve to increase the bandwidth but, on the downside, creates higher attenuation and lower power-handling capability.

>The single ridge waveguide is similar to the rectangular waveguide but noted for its large capacitive loading centered on its broad wall.  Compared to a rectangular waveguide, the single ridge waveguide has a lower cut-off frequency with a smaller cross section. However, when compared with the double ridge waveguide, the single ridge yields increased loss and reduced power handling capabilities.

>The slotted waveguide, generally used for radar and similar applications, serves as a feed path with each slot acting as a separate radiator. This antenna structure can generate an electromagnetic wave transmission in a specified narrow and targeted direction.

 

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An Array of Antenna Arrays

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An antenna array, or phased array, is a set of two or more antennas whose signals are combined in order to improve performance over that of a single antenna. An antenna array is used to increase overall gain, provide diversity reception, cancel out interference, maneuver the array in a particular direction, gage the direction of arrival of incoming signals, and to maximize the Signal to Interference plus Noise Ratio (SINR). An array antenna is usually made up of more than one dipole but it may be composed of driven elements. As these antennas elements radiate individually and while in array, the radiation of all the elements sum up, to form the radiation beam, which has high gain, high directivity and better performance, with minimum losses. Similar to the dipole, a driven element can function as a transmitter or a receiver. When connected to the transmission line, a driven element gets power directly from the transmitter or, as a receiver, transfers the received energy directly to the receiver. Applications of array antennas include satellite communications, wireless communications, radar communications, and in the astronomical study.

Types of Arrays

Arrays can be described by their radiation patterns and the types of elements in the system. When placed close enough to the driven element to permit coupling, a parasitic element will produce the maximum transmission radiation from its associated driver. When a parasitic element reinforces power from the driver, it is referred to as a director. When a parasitic element causes maximum energy to radiate towards the driven element, the element is called a reflector. An array antenna is known as a driven or connected when all of the elements in an array are driven. Interestingly, if one or more elements in the array are parasitic, the entire system is said to be a parasitic array. Multi-element arrays are usually associated with their directivity, for example, a bidirectional array radiating in opposite directions or a unidirectional array radiating in one direction.

Driven arrays

Collinear array

Unidirectional, high-gain antennas designed with two or more half-wave dipoles placed end to end and seated on a common line or axis making them parallel or collinear. The main purpose of this array is to increase the power radiated and to provide high directional beam by avoiding power loss in other directions. Advantages of collinear array antennas include increasing directivity with a reduction in power losses.

Broadside array

Bidirectional array used to radiate electromagnetic waves in specific direction to enhance transmission. The design elements include two or more half-wave dipoles of equal size and equally spaced along a straight line or axis forming collinear points with all dipoles in the same phase from the same source. The broadside array antenna has a radiation pattern that is perpendicular to the axis with a narrow beam radiation pattern and high gain.

End-fire array

Similar to the broadside array and uses two half-wave dipoles spaced one-half wavelength apart with a bidirectional radiation pattern with narrower beam widths, lower gain, and higher directivity than the broadside array. The direction of radiation is along the plane of the array and perpendicular to the elements which radiates to the end of the array, hence the name.

Parasitic arrays

 Yagi-Uda array

The most common type of antenna for home TV reception with high gain and directivity. In this antenna, several directors are positioned to increase the directivity of the antenna. The disadvantages of Yagi-Uda antennas are that they can be prone to noise and atmospheric effects.

Log-periodic array

An array antenna whose impedance is a logarithmically periodic function of frequency. Similar to a Yagi-Uda, the advantage of this antenna is that it maintains constant characteristics over a desired frequency range of operation with the same radiation resistance, SWR, and gain and front-to-back ratio are also the same. Types of log-periodic antennas include the planar, trapezoidal, zig-zag, V-type, slot and the dipole or LPDA (log-periodic dipole array).

Turnstile array

Basic construction is two identical half-wave dipoles placed at right angles to each other and fed inphase. Several turnstiles can be stacked along a vertical axis for higher gain called a bay. The polarization of the turnstile antenna depends on their mode of operation, Normal and axial, where in normal mode, the antenna radiates horizontally polarized waves perpendicular to its axis and in axial mode, the antenna radiates circularly polarized waves along its axis.

Super-turnstile array

Also known as the Batwing antenna, the dipole elements in turnstile are replaced by four flat sheets where 1 to 8 bays can be constructed on a single mast. The advantages of this design are high-gain and better directivity than the regular turnstile but with some power losses.

 

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Types of Antennas at the RF and Microwave Frequency

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Monopole

The monopole is a resonant antenna and the length of the antenna is determined by the wavelength of the radio waves being received and transmitted. A monopole antenna is usually made of a single conductor mounted over the ground with one side of the feedline from the receiver or transmitter connected to the conductor and the other side to ground. Monopole antennas have an omnidirectional radiation pattern and are used for broad coverage transmission. Examples of monopole antennas include:

• Whip -used on mobile and portable radios in VHF/UHF bands and usually have a flexible, telescoping rod,

• Rubber Ducky – used on portable two way radios made with a short wire helix that adds inductance to cancel the capacitive reactance of the short radiator, making it resonant with low gain.

• Ground plane – a modified whip antenna with horizontal rods protruding from base of whip attached to the ground side of the feedline and is used as base station antennas for emergency services.

• Mast radiator – radio tower used for AM radio stations.

• Umbrella – large wire transmitting antennas used on VLF bands with a central mast radiator tower from which numerous wires extend radially from the mast to ground and is used for long range military communications.

Dipole

The dipole antenna is used in applications that require transmission over a range of frequencies and, in the basic form, consists of two poles, or two conductive elements, whereby current flows in these two conductive elements and the associated voltage causes an electromagnetic wave or radio signal to be radiated outwards from the antenna. A dipole antenna can be varied away from its resonant frequency and fed with a high impedance feeder thus enabling it to operate over a much wider bandwidth. Various types of dipole antennas used as include half wave, multiple, folded, and non-resonant. Examples of dipole antennas include:

• Yagi-Uda –  most common directional antennas at HF, VHF, and UHF frequencies as a unidirectional antenna with a narrowband and used as rooftop TV antennas and long distance shortwave communication

• Log-periodic dipole array – a directional antenna with a wide bandwidth used as rooftop TV antennas with less gain than the Yagi-Uda.

• Turnstile – used for receiving signals from satellites and is made of two dipole antennas mounted at right angles, radiating in all directions  with horizontal, circular, and elliptical polarization.

 Loop

Loop antennas are used in communication links with the frequency of around 3 GHz and as electromagnetic field probes in the microwave frequencies. The two types of loop antennas are electrically small and electrically large antennas based on the circumference of the loop. The large self-resonant loop antenna has a circumference close to one wavelength of the operating frequency and so is resonant at that frequency. Smaller loops, 5% to 30% of a wavelength in circumference, use a capacitor to make them resonant. These antennas are used for transmitting and receiving although small loop antennas less than 1% of a wavelength in size are inefficient radiators, and so are only used for reception. The larger resonant loop antennas are used at higher frequencies, such as VHF and UHF.

Aperture

Aperture antennas emit electromagnetic waves through an opening or aperture. Aperture antennas are the main type of directional antennas used at microwave frequencies. At radio and microwave frequencies, horns, waveguide apertures, reflectors and microstrip patches are examples of aperture antennas. Since the antenna structure itself is nonresonant, they can be used over a wide frequency range by replacing or tuning the feed antenna.

• Parabolic – common high gain antenna, up to 60 dBi, at microwave frequencies made of a dish-shaped metal parabolic reflector with a focal feed antenna and used for radar antennas, point-to-point data links, satellite communication, and radio telescopes.

• Planar Inverted-F Antennas – high gain antenna used in wireless communications where the radiating element is replaced by a plate to increase the bandwidth but small enough that they can be hidden into the housing of a mobile device.

• Horn – a flaring metal horn attached to a waveguide with moderate gains of 15 to 25 dBi and used as radar guns, radiometers, and as feed antennas for parabolic dishes.

• Slot – a waveguide with one or more slots to emit the microwaves and used as UHF broadcast antennas and marine radar antennas.

• Patch – made of metal sheets mounted over a ground plane and attached to surfaces in aircrafts and naval vessels low profile antennas are preferred.

Array

Array antennas are multiple antennas working as a single antenna, usually dipoles fed in phase. A few examples of these include:

• Collinear – a high gain omnidirectional antenna, made up of several dipoles in a vertical line and used as base station antennas for land mobile radio systems.

• Reflective array – multiple dipoles mounted in front of a flat reflecting screen and used for radar and UHF television transmitting and receiving.

• Phased array – transmitted at UHF and microwave frequencies, made up of multiple dipoles fed through an electronic phase shifter where the beam can be pointed in any direction over an angle in front of the antenna, and used for military radar and jamming systems.

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Antenna Performance Criteria Part 2

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Antennas are essential components of RF and microwave devices and are used in a wide variety of applications including radio and television broadcasting, radar, cellular transmission, and satellite communications to name a few. Antennas are designed to transmit and receive radio waves determined by the design of the application intended to receive the transmissions and can be in all horizontal directions equally as in omnidirectional antennas, or in a designated direction as in directional or high gain antennas.  An antenna in the receiving mode, in the form of a wire, horn, aperture, array, dielectric rod, for example, is used to collect electromagnetic waves and to extract power from them. Important properties related to the design of an antenna include gain and radiated efficiency, as discussed in an earlier article, aperture, directivity, bandwidth, polarization, radiation pattern, effective length, and resonance and are discussed here:

  • Aperture

Power received by the antenna is associated with a collective area known as the effective aperture measured as the area of a circle to the incoming signal as the power density (watts per square meter) x aperture (square meters) = available power from antenna in watts. Antenna gain is proportional to aperture and gain is increased by focusing waves in a single direction while reducing other directions. Thus, the larger the aperture, the higher gain and narrower the beam-width. In most cases, larger antennas tend to have a higher maximum effective area.

  • Directivity

Antenna directivity is the measure of concentrated energy radiated in a particular direction expressed as the ratio of radiation intensity in a given direction to the average radiation intensity. In other words, it is the ability of an antenna to focus energy in a specific direction when transmitting or receiving.

  • Bandwidth

The bandwidth of an antenna refers to the range of frequencies over which the antenna can operate and is conceived of in terms of percentage of the center frequency of the band. Bandwidth is constant relative to frequency and antennas of different types have different bandwidth limitations.

  • Polarization

Polarization is the orientation of the electric field of an electromagnetic wave, usually described as an ellipse. Electromagnetic waves emitted from an antenna can be polarized vertically and horizontally. The initial polarization of a radio wave is determined by the antenna. For example, if the wave is polarized in the vertical direction, then the E vector is vertical and it requires a vertical antenna. Circular polarization is a combination of both horizontal and vertical waves and, in the electric field vector, appear to be rotating with circular motion around the direction of propagation, making one full turn for each RF cycle.

  • Radiation Pattern

Because antennas do not radiate power equally in all directions, antenna radiation patterns or polar diagrams are important tools to quickly evaluate the overall picture of antenna response. The radiation pattern of a transmitting antenna is a plot that describes the strength of the power field radiated by the antenna in various degrees. Radiation plots are often shown in the plane of the axis of the antenna (E plane) or the plane perpendicular to the axis (H-plane) and are usually shown in relative dB (decibels).

  • Effective Length

The effective length describes the efficiency of an antennas in transmitting and receiving electromagnetic waves. It is used to determine the voltage induced on the open-circuit terminals of the antenna when a wave hits it. In a receiving antenna, the effective length is the length and orientation of a uniform current required to produce the same electric field as the transmitting antenna. It is a useful tool in determining the effect of polarization mismatch between the propagated waves of the transmitting antenna and the receiving antenna.

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