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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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Antenna Performance Criteria (Gain and Radiated Efficiency) – Part 1

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RF Antennas

Antennas are used to channel radio waves for the purpose of communicating information across distances without wires. Antennas are necessary for a radio receiver or transmitter to convey signals between, for example, radio and television broadcast, cellular networks, Wi-Fi devices, radar and GPS, and remote control devices. Antennas transmit and receive radio waves which can be often be polarized by adjusting the axis of the antenna. As there are many different types of transmitting and receiving devices, there are equally a variety of antenna types to meet this transmission need.

Performance Indicators

An antenna’s performance is generally characterized by basic RF indicators, such as antenna gain and efficiency. An antenna should cover all intended frequency ranges with a reasonable impedance matching and high radiation performance. Many antenna properties are the same for both transmitting and receiving which simplifies testing and measurements for the following performance indicators. For example, it may be more useful to calculate the gain of a transmitting antenna than the area of a receiving antenna; likewise, it may be more useful to measure the receiving power pattern than to measure the transmitting power pattern of a large radio telescope. Thus, this receiving/transmitting reciprocity simplifies antenna calculations and measurements.

Gain

Gain, measured in Decibels (dBi), is the performance indicator that refers to directivity and electrical efficiency. In an antenna, gain measures the degree of directivity of the radiation pattern such that a high gain antenna would radiate power in a directional manner while a low gain antenna would radiate over a wider angle. High gain antennas propagate the signal further in one direction allowing for longer range without an increase in signal strength, but requires precise aiming toward a receiver. Conversely, low gain antennas have a shorter range but do not necessarily require being aimed at the receiver.  Examples of these include a high gain satellite dish versus a low gain and Omni-directional cellular phone built-in antenna.

The isotropic antenna is a hypothetical model that radiates equal signal power in all directions and is used as the base of comparison to calculate the gain of real antennas. While no real antenna has an isotropic radiation pattern, several antenna types have a uniform radiation pattern on the horizontal plane. In this sense, antennas can be either directional or Omni-directional, depending on their application.

A directional antenna is used to maximize its coupling to the electromagnetic field in the direction of the other station. This type of antenna is preferred in small scale environments, as the system can be tuned for optimal use so that when focused, the smaller the percentage of 360 degrees that the signal radiates, the farther the reachable distance the signal travels.

Because an Omni-directional antenna receives and transmits at a 360 degrees radius, the signal radiates uniformly in all directions. Antennas that are, by design, quite small compared to the wavelength, cannot be highly directional. Gain, therefore, is not a measure the overall efficiency of an antenna and can only determine the efficiency of radiated output in one direction.

Radiated efficiency

The power required to achieve a certain performance level can be determined by radiated efficiency – a useful and informative measure of an antenna’s power efficiency and demonstrates the antenna’s capability to use the power fed to the terminals. The ratio of the power delivered to an antenna relative to the power radiated from the antenna is the radiated efficiency of the antenna.

With the ideal antenna, it would transform all of the power fed to its terminals to a radiating electromagnetic energy that propagates to the surrounding space. However, in real applications, some of the power fed to the antenna terminals is lost. Examples of loss include the mismatch between the antenna element and the feeding network and natural losses due to resistances of the conductors used to make the antenna.

In antenna design, radiated efficiency does not consider radiation direction and thus is a useful performance indicator for measuring the efficiency of, for example, cellular devices and other Omni-directional radiation patterned devices. Conversely, if the antenna is supposed to radiate in a specific direction such that the antenna is designed to have directive characteristics in its radiation pattern, then gain is a better performance indicator. Increasingly, there are more applications that require an Omni-directional signal and radiated efficiency, in these applications, is becoming the preferred test method. With radiated efficiency, the efficiency and performance are for all areas surrounding the antenna can be determined.

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Coaxial Cable Loss due to Loss Tangent

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Electrical losses in a coaxial cable create heat in the outer and center conductors and are the two main types of coaxial cable loss, skin-effect loss and dielectric loss, respectively. This heat, or loss, can be calculated with the understanding of the following concepts.

What is Skin Effect Loss?

In coaxial cable, skin effect is the movement of an alternating electric current (AC) whereby the current density is greater near the surface of the conductor and lessens within the conductor.  Skin effect is the decline in current density and the skin depth is a measure of the depth at which the current density falls to 1/e of its value near the surface. Over 98 percent of the current flows within a layer 4 times the skin depth from the surface. At high frequencies, the skin depth becomes much smaller. Skin effect loss usually occurs at high frequencies when the signal reaches and moves along the surface of the inner conductor, which causes additional RF losses at higher frequencies.

A skin depth calculator can be found here.

Resistance per unit length is the ratio of specific resistance or resistivity to the area of cross-section of given conductor in Ohm per meter. With skin-effect loss, the resistance per unit length, Rl, and the inductance per unit length, Ll, increase with the square root of the frequency.

Skin effect losses are resistive, caused by the narrowing of the conduction path. In calculating loss, loss per unit length includes the skin effect loss and dielectric loss.

What is Dielectric Loss?

Dielectric conduction loss is caused when the insulating material inside the transmission line absorbs energy from the electromagnetic field developed between the inner and outer conductors. Dielectric loss explains the amount of dissipation of electromagnetic energy or heat of a dielectric material. It is often described in terms of either the loss angle δ or the corresponding loss tangent tan δ. Both refer to the phasor in the complex plane whose real and imaginary parts are the resistive (lossy) component of an electromagnetic field and its reactive (lossless) counterpart. The ratio of two quantities is defined in term of Tan.

What is Loss Tangent?

Loss tangent is the ratio at any specified frequency between the real and imaginary parts of the impedance of the capacitor. A large loss tangent refers to a high degree of dielectric absorption. Loss tangent is the ratio between the imaginary and real parts of the complex permittivity where the permittivity of dielectric is given by:

ε =ε_re − jε_im

When this formula is drawn on an x-y plane, the tangent of the angle between the real and the imaginary quantity is discoverable which can be described as:

tanδ = ε_im/ε_re

Which means the ratio of the imaginary part to the real part of the permittivity is found to be another quantity, i.e. the loss tangent, which is used to express the losses in a dielectric material. In other words, it is the ratio of the imaginary part to the real part or the tangent of the angle between the complex number and the real axis.  This angle is the loss angle and the tangent is called the loss tangent.  Thus, the value of the loss tangent describes how lossy a material is, such that it either represents a very lossy material or a very good conductor.

Measurement of loss using sinusoidal excitation at a particular frequency yields the product of skin effect and dielectric loss functions or the sum of the skin effect and dielectric losses in units of dB. Once the skin-effect loss is derived, the portion of loss attributable to dielectric losses may be estimable.

 

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Coaxial Cable Loss due to Dielectric Conduction

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The coaxial cable provides a transmission line with low loss characteristics that can shield from outside signals and provide a durable yet flexible line that can be used in a variety of applications with a wide range of frequencies.  At microwave frequencies, much of the loss in coaxial cable transmission lines is characterized as metal loss however, losses due to dielectric conduction can be significant for some uses.

What is dialectic conduction loss?

Electrical losses in a coaxial cable create heat in the center and outer conductors. Because most of the heat is generated at the center conductor of the cable, this heat is referred to as dielectric conduction. Dielectric conduction loss is caused when the insulating material inside the transmission line absorbs energy from the electromagnetic field developed between the inner and outer conductors. The dielectric in coaxial cable refers to materials with a high polarizability, is used to indicate the energy storing capacity of the material by means of polarization, and is expressed by a number called the relative permittivity.  A coaxial cable uses the permittivity of the material between the center conductor and shield to determine its characteristic impedance when it is used as a transmission line. Dielectric insulators polarize to oppose an applied electric field which decreases the electric field in the dielectric material and reduces internal loss in the cable.

Construction and performance of coaxial cable

The coaxial cable conducts electrical signals using a solid, stranded, or copper plated steel wire, known as the center conductor or core, surrounded by an insulating layer, the dielectric, which is enclosed by a shield and protected by an outer insulating jacket. The function of the dielectric is to maintain the spacing between the shield and the center conductor but a certain amount of signal energy is dissipated in the dielectric material itself. The ideal dielectric material does not exhibit electrical conductivity when an electric field is applied however, all dielectrics have some degree of measurable conductivity.

Temperature stability is important to maintain in order to gauge and regulate the performance of the coaxial cable. If the temperature of the coaxial cable is elevated too high or too quickly, the cable may become warped and damaged beyond repair. As any heat generated due to dielectric losses is dissipated within the dielectric, it is paramount that the construction of the dielectric material be considered when selecting the correct cable for the specific application.

The most common dielectric material is polytetrafluoroethylene (PTFE) which has lower dielectric losses than PVC. The benefit of using PTFE are that this material can tolerate temperatures ranging from -50°C to +200°C and is often combined with other materials to increase dielectric constant or improve temperature stability. The drawback of this material is that it is sensitive to moisture under voltage stress.

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Coaxial Cable Power Handling Part 3

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In the previous sections of the “Coaxial Cable Power Handling” blog series, a brief overview of coax power handling, and how cable size, cable type, signal frequency, and electron velocity through a cable, influenced the power handling capability of a coaxial cable. In this section, attenuation/loss, environmental factors, VSWR, and cable capacitance, will be touched on in respects to coaxial cable power handling.

Attenuation or Loss

One of the important constraints that limit the power handling capability of a line is attenuation. Attenuation is the loss generated by the inner and outer conductor of the coaxial cable and its dielectric medium. As most of the heat is generated in the center conductor, any heat generated as a result of dielectric losses will be dissipated within the dielectric. The construction of the dielectric is key in determining the power handling capability of the coaxial cable.

Thus, lower loss RF coaxial cables will have a higher power rating than higher loss RF coaxial cables. Additionally, there is a degree of loss as a signal travels along a coaxial cable which is proportional to the length of the cable and is specified in terms of a loss over a given length or decibels over a given length, e.g. 0.5dB / 10 meters.

Environmental Factors and VSWR

Non-optimal operating conditions may require a coaxial cable to be derated. Environmental temperature can induce derating. For example, if the coaxial cable is operating in a high temperature environment, it cannot dissipate as much heat, operating temperature will increase. Hence, a de-rating factor is applied if the coaxial cable is used at high temperatures. Altitude plays a role but only at significant heights.

Thus, if the cable is operating at a high altitude and under reduced pressure, cooling will be less effective and temperature rise within the cable will be greater. Additionally, if the coax cable is operating under conditions where the VSWR is high, de-rating may be necessary due to the variations of high and low current along the coaxial cable such that the power dissipation may cause higher levels of power to be dissipated locally.

Capacitance

The capacitance of the coaxial cable is the ability to hold a charge. Capacitance exists between the inner and outer conductors of the coaxial cable which is proportional to the length of cable as well as the dielectric constant and the inner and outer conductor diameters. The larger the capacitance, the longer it takes a signal to reach full amplitude inside the cable. Thus, a high capacitance is an undesired characteristic when determining the performance of a coaxial cable.

For a detailed look at the data used to develop recommended maximum power ratings for RF coaxial cable power handling, see Power Handling Capabilities of RF Coaxial Cables, 1971, http://www.dtic.mil/dtic/tr/fulltext/u2/912072.pdf. in which “representative coaxial cable types were subjected to high power CW and pulse tests” which were “conducted over a range of frequencies and ambient temperature and pressure attitudes.”

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Coaxial Cable Power Handling Part 2

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In the previous portion of the “Coaxial Cable Power Handling” blog series, we discussed an overview of coax power handling and how cable power handling impacts cable choice. In this portion of the blog series, we will take a deeper look into how cable size and type, as well as signal frequency and electron velocity through a coaxial cable, relate to a coaxial cable’s power handling capability.

Cable Size and Type

Different types of coax cable are used for specific applications and the specifications help to determine the recommended or required coaxial type. For example, in microwave applications where very low loss is needed, a semi-rigid coaxial RF cable with a solid copper outer sheath may be preferred as it offers superior screening compared to RF cables with a braided outer conductor, especially at microwave frequencies.

Although the power handling capability of RF coaxial cable is not a concern for many installations, when using medium or high power transmitters, the power rating or handling capability of RF coaxial cable must be considered. In applications where high power is used, ordinary polyethylene might melt and distort the cable therefore cables with an air dielectric with the center conductor held in place by a dielectric coil that runs along the length of the cable is preferred.

For medium to high power applications, coaxial cables with a Teflon dielectric are preferred as these can withstand high temperatures up to around 160°C. When considering which cable to use, it should be remembered that as the frequency increases the skin effect becomes more pronounced, and coupled with increased losses in the dielectric, this limits the power handling capacity.

Frequency and Velocity

When considering wave propagation over a cable, the creation of higher modes can degrade efficiency, produce higher reflections, and mitigate the power handling capability of the coaxial line. As power is inversely proportional to frequency, the higher the operating frequency, the lower the power capacity of the coaxial line. Length of the cable can mitigate the power handling capability and coaxial cables are often cut to a specific length to act as an impedance transformer or a resonant circuit. If resonant lengths of RF coaxial cable are used, it is necessary to know the velocity factor of the cable.

The velocity factor specifications of a coaxial cable is the speed at which the signal travels within the cable compared to the speed of the signal in a vacuum. The velocity factor specification is a figure which is less than 1. The dielectric between two conductors indicates the velocity factor; coaxial cables composed of a solid polyethylene dielectric will have a velocity factor around 0.66, and those made of foam polyethylene will have velocity factor figures ranging from about 0.80 to 0.88.

For a detailed look at the data used to develop recommended maximum power ratings for RF coaxial cable power handling, see Power Handling Capabilities of RF Coaxial Cables, 1971, http://www.dtic.mil/dtic/tr/fulltext/u2/912072.pdf. in which “representative coaxial cable types were subjected to high power CW and pulse tests” which were “conducted over a range of frequencies and ambient temperature and pressure attitudes.”

The post Coaxial Cable Power Handling Part 2 appeared first on Pasternack Blog.

Coaxial Cable Power Handling – Part 1

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The coaxial cable provides a transmission line with low loss characteristics that can shield from outside signals and provide a durable yet flexible line that can be used in a variety of applications with a wide range of frequencies. Power handling capability, also known as the power capacity, is one of the important criteria in evaluating the performance of the RF coaxial cable. When considering medium or high power transmitter applications, power handling is especially critical, as the incorrect type of coax cable can result in a failure of the cable and possible damage to the transmitter.

Electrical losses in a coaxial cable creates heat in the center and outer conductors and, because most of the heat is generated at the center conductor of the cable, in the dielectric core. Power handling capability is related to the ability of the cable to withstand and dissipate this heat. In power handling, the maximum allowable operating temperature of the materials used in the cable, especially the dielectric, is the ultimate limiting factor. For applications with continuous power, heat loss within the cable is the limiting factor and, when using pulsed power through the coaxial line, operating voltage must not be exceeded. Thermal conduction of the coaxial line generally relates the physical properties of the cable with temperature. Specifically, thermal conductivity (k) is characterized through the quantity of heat (Q) transmitted in time through a thickness (L) in the direction normal to the surface area (A) due to changing temperature. Thus we have the rate of heat transfer in the cable and the thermal effects of the line, also known as power rating of the line.

As a rule, the power handling capability of a specific cable is inversely proportional to its attenuation and directly related to its size. Regarding coaxial cable power ratings, correction factors for ambient temperature, altitude and VSWR must be taken into account. For example,  high ambient temperature and/or high altitude reduces the power rating of by impeding heat transfer from the cable and VSWR reduces power rating by causing localized hot spots in the cable. Which coaxial cable to use depends on the size and type of the line, the frequency of operation, the attenuation or loss, thermal conduction and capacitance. Each of these factors plays a key role in determining the power handling capacity of the line.

 

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Introduction to Coaxial Cable Losses

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Coaxial cable transfers radio frequency power from one point to another and, in the ideal world, the same amount of power would transfer along the cable to the remote end of the coax cable. However, real world conditions include some power loss along the length of the cable. Loss, or attenuation, is one of the most important features to look for when deciding what type of coaxial cable to use in a design.  Loss is defined by decibels per unit length and at a given frequency. Thus, the longer the coaxial cable, the greater the loss. Loss is also frequency dependent, generally increasing with frequency, but the loss is not necessarily linearly dependent upon the frequency. Power loss occurs in a variety of ways:

Resistive loss

Resistive losses within the coaxial cable occur when the resistance of the conductors and the current flowing in the conductors results in heat being dissipated. Skin effect limits the area through which the current flows, which leads to increased resistive losses as the frequency rises. To reduce the level of resistive loss, the conductive area is increased resulting in larger low-loss cables. Also, multi-stranded conductors are often used.  Resistive losses generally increase as the square root of frequency.

Dielectric loss

Dielectric loss is signal energy dissipated as heat within the insulating dielectric of a cable, but is independent of the size of the coaxial cable. Dielectric losses increase linearly with frequency, and the resistive losses normally dominate at lower frequencies and. As resistive losses increase as the square root of frequency and dielectric losses increase linearly, the dielectric losses dominate at higher frequencies.

Radiated loss

Radiated loss in a coaxial cable is usually much less than resistive or dielectric losses, however poorly a constructed outer braid on some coaxial cables may yield a relatively high radiated loss. Radiated power, problematic in terms of interference, occurs when signal energy passing through the transmission line is radiated outside of the cable. Leakage from a cable carrying a feed from a high power transmitter may produce interference in sensitive receivers located close to the coax cable or a cable being used for receiving can pick up interference if it passes through an electrically noisy environment. To reduce radiated loss or interference, double or triple screened coaxial cables are designed to reduce the levels of leakage to very low levels.

Of these forms of loss, radiated loss is generally the less concerning as only a very small amount of power is generally radiated from the cable. Thus, most of the focus on reducing loss is placed onto the conductive and dielectric losses, except in certain applications.

Loss over Time

Loss or attenuation of coaxial cables tends to increases over time as a result of flexing and moisture in the cable. Although SOME coax cables are flexible, the level of loss or attenuation will increase if the RF cable is bent sharply or if there is a disruption to the braid or screen. Contamination of the braid by the plasticisers in the outer sheath or moisture penetration can affect both the braid where it causes corrosion and the dielectric where the moisture will tend to absorb power. Often, coax cables that use either bare copper braid or tinned copper braid experience more degradation than those with the more expensive silver plated braids. Although foam polyethylene provides a lower level of loss or attenuation when new, it absorbs moisture more readily than the solid dielectric types. Cables with solid dielectric polyethylene are more suited to environments where the level of loss needs to remain constant or where moisture may be encountered. Even though RF coaxial cables are enclosed in a plastic sheath, many of the plastics used allow some moisture to enter thus, for applications where moisture may be encountered, specialized cables should be used to avoid performance degradation.

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