An antenna transmits and receives signals over the air. Therefore, antennas are used to transform guided electromagnetic waves or signals into electromagnetic waves that propagate in free space, and vice versa. To create radiation, there must be a time-varying current or an acceleration (or deceleration) of charge. Generally, passive antennas can both transmit and receive energy and are thus reciprocal.
Antennas are used in all wireless systems from IoT devices to microwave and millimeter-wave imaging systems, such as radio telescopes. The exact behavior of an antenna is related to the geometry of the conductors and dielectrics in its structure, and there are a wide variety of antennas to accommodate various application requirements.
Different antenna types include dipole, monopole, horn, vivaldi, yagi, patch, microstrip, paraboloid, log-periodic, wire, grid, loop, array and many more. Each of these antenna types is designed for a specific purpose.
Before connecting an antenna into a wireless system, the engineer looks for the specifications and characteristics of the antenna, such as radiation pattern, efficiency, directivity, polarization, and beamwidth, that are suitable for the given application. Antenna measurement techniques refers to the testing of antennas to ensure that the antenna meets specifications or simply to characterize it.
The antenna measurements are then conducted to confirm the antenna under test meets the parameters as specified in the datasheet. Antenna measurements can be regarded as the experimental validation of the parameter values given in an antenna’s datasheet.
Typical parameters of antennas are S-Parameters, Antenna gain, Antenna Efficiency, bandwidth, radiation pattern, beamwidth, polarization, and impedance.
The gain of an antenna is the ratio of the power radiated (or received) per unit solid angle by the
antenna in a given direction to the power radiated (or received) per unit solid angle by an isotropic antenna fed with the same power. The gain is maximum in the direction of maximum radiation (the electromagnetic axis of the antenna, also called the boresight)
Antenna Gain in the most simple terms describes how well the antenna converts input power into
electromagnetic waves or radio waves headed in a specific direction. Mathematically, Gain is defined as: Gain = Efficiency X Directivity.
The directivity of an antenna is defined as the power density of the antenna in its direction of maximum radiation in three-dimensional space divided by its average power density. The directivity of a half-wave dipole antenna is 1.64 or 2.15 dB. An antenna radiating equally in all directions is an omnidirectional antenna. The directivity of the hypothetical isotropic radiator is 1 or 0 dB. Typical antennas used for cellular and IoT applications are omnidirectional in nature. This allows a better chance of receiving attenuated signals even though there is no direct line of sight.
Antenna Efficiency is the ratio of the power delivered to the antenna relative to the power radiated from the antenna. An ideal antenna theoretically radiates all the power delivered making it a 100% efficient antenna. However, an ideal antenna does not exist. In the real world, a highly efficient antenna is one that is well matched and has minimum power loss due to absorption, heat or other system losses.
Efficiency = Pradiated / Pinput
The network parameters of an antenna are used to determine the appropriate matching network and interconnect needed to connect an antenna to a transmitter/receiver.
The input impedance of an antenna is a ratio of the complex, input (source) voltage and current at the antenna port. In the case of many antenna designs, the raw antenna impedance is not matched to common transmission line impedances, such as 50 ohms or 75 ohms coaxial transmission lines or waveguide interconnect. Hence, a matching network is used to transform an antenna’s innate impedance to the impedance of the transmission line, waveguide, or directly to the transmitter/receiver front-end. How well an antenna impedance is matched to a desired port impedance is given as a VSWR and reflection coefficient measurement.
The VSWR, also known as standing wave ratio (SWR), is the ratio of the maximum voltage and minimum voltage of a standing wave developed at the port of an antenna. For the most efficient transmission and reception, an ideal antenna has a VSWR of 1:1.
The power PM is matched to the transmission line by a generator and is available to the antenna. The antenna will accept some power (Pin ) and reflect some power depending on the matching VSWR. Reduced by losses in the antenna caused by RL, the remaining power is radiated from the antenna (Prad ) to generate a certain radiant intensity (I) in different directions from the antenna aperture.
Effective isotropic radiated power (EIRP)
The EIRP denotes the absolute output power in a given direction. If no direction is defined, the direction of maximum radiation intensity is implied. The EIRP is the power an ideal isotropic radiator requires as input power to achieve the same power density in the given direction. EIRP is the power accepted by the antenna multiplied by the antenna gain, or radiated power multiplied by the directivity.
The bandwidth of an antenna is not an inherent antenna characteristic but is defined as the range of useable frequencies in which the antenna performs as specified with respect to a freely defined set of parameters. Commonly, the frequencies for which matching is acceptable (e.g. VSWR of less than 1.5 or 1.4, as defined by the antenna manufacturer) define the antenna bandwidth. Antennas typically exhibit broadband behavior that may extend beyond the antenna’s bandwidth.
The half power beam width defines the angle between the first points for which the EIRP of an antenna pattern is 3 dB below peak value, with the peak EIRP direction typically being roughly in the center of the beam width. In addition to the desired beam direction, typically additional beams are formed by the antenna. These beams are called side lobes and are numbered by their order from the main beam.
The side lobe level is often given relative to the peak beam level and identifies how good the antenna manages side lobe suppression, which is especially important for beamforming using array antennas
Antenna polarization is a description of the way the electric field (E-field) or magnetic field (H-field) of a wavefront oscillates. If the field lines oscillate within a single axis, they are known as linearly polarized. A linear polarized antenna may be horizontal (0°), vertical (90°) or cross polarized if at an angle other than 0° or 90°. An elliptical polarization is when the field oscillations of the wavefront orbit around the origin and are either traveling clockwise (right hand elliptically polarized) or counterclockwise (left hand elliptically polarized). As a special case of elliptical polarization, the E-field lines may perfectly rotate around the origin and form a circle, which is known as circular polarization.
It is important to have receiving and transmitting antennas with matched polarization, or some of the power of the signal will be lost due to the polarization mismatch. This is known as the polarization loss factor (PLF), which can be described with the equation:
Where 𝝋 is the angle offset of the polarization of the antenna compared to the polarization of the wavefront.
Antenna element mutual coupling and isolation
In the case of antenna arrays, multimode antennas or multipolarized antennas, there will be some electric and magnetic coupling between the antennas or ports. The isolation between the elements is a measure of the signal strength in a given element and the strength of that signal coupled into another element. The isolation is usually given as a worst-case figure, but often must be characterized in order to implement multi-input multi-output (MIMO) or beamforming algorithms.
S-parameters are a complex matrix that shows Reflection/Transmission characteristics (Amplitude/Phase) in the frequency domain. S11 and S22 are reflections on port 1 and port 2, respectively. These two ports can be used to measure antenna return loss, impedance, and voltage standing wave ratio (VSWR).
These parameters can be determined for a single antenna or two antennas sharing the same ground plane. S-parameters are a complex matrix that shows Reflection/Transmission characteristics (Amplitude/Phase) in the frequency domain.
The magnitude of the return loss shows that when a certain power is delivered to the load (Antenna), how much power is reflected back in (dB). It also identifies the antenna resonant frequencies. graphical representation of a typical antenna impedance known as impedance analysis on a “Smith Chart”. This allows an easy way for an engineer to determine the mismatch between the antenna’s impedance and the load impedance (typically 50 ohms). A good matching network reduces the mismatch and allows maximum energy transfer in the form of power to the antenna and eventually into free-space as electromagnetic waves.
S21 and S12 for a two-port network identify the effect of one port on the other. S21 is the metric describing the effect on port 2 due to port 1 and vice versa. These parameters are used to determine insertion loss and isolation between two antennas sharing the same ground plane separated by some spatial distance (“d”).
During the design process, antenna development is largely based on simulations and software tools that can design antennas based on a defined parameter set. When it comes to final production or even integration, the real-world parameters can easily deviate from the simulation results. Especially in highly integrated systems where antennas are surrounded by other components, the antenna characteristics can change significantly and simulations have to take all other components of the system into account.
Complex simulations can take a long time, while measurement times with modern test systems are down to a few minutes. Clearly, measurements are still the surest way to determine the real-life performance of the antenna.
For antenna testing, the most fundamental device is the VNA [Vector Network Analyzer]. The simplest type of VNA is a 1-port VNA, which is able to measure the impedance of an antenna [which is equivalent to measuring S11 and VSWR].
Except for polarization, the SWR is the most easily measured of the parameters above. Impedance can be measured with specialized equipment, as it relates to the complex SWR.
Measuring the antenna performance is critical for a successful antenna transceiver design. Testing begins with return loss and impedance measurements. Tests for Efficiency, Gain, Directivity, Effective Isotropic Radiated Power (EIRP), Total Radiated Power (TRP) and Total Isotropic
Sensitivity (TIS) are required to fully quantify the design measurements for an antenna. These parameters are typically measured in a fully anechoic chamber environment.
The antenna efficiency is measured in an anechoic chamber by feeding some power to the antenna feed pads and measuring the strength of the radiated electromagnetic field in the surrounding space. A good antenna, in general, radiates 50 – 60 % of the energy fed to it (-3 to -2.2dB).
The antenna pattern is the response of the antenna to a plane wave incident from a given direction or the relative power density of the wave transmitted by the antenna in a given direction. For a reciprocal antenna, these two patterns are identical.
Antenna pattern measurement
A specific distance away from an antenna where the electric and magnetic fields of the electromagnetic radiation pattern from the antenna are perfectly orthogonal is called the far-field of an antenna. The parameters of an antenna are typically measured in the far-field. The radiation pattern of an antenna is determined by the three-dimensional shape of the antenna’s far-field radiation.
Typically, an antenna measurement utilizes plane waves for testing the antenna under test. The test setup uses source antennas or transmitting antennas with known characteristics so that field incidents on the antenna under test are approximately plane waves.
Measuring radiation pattern requires a sophisticated setup including significant clear space (enough to put the sensor into the antenna’s far field, or an anechoic chamber designed for antenna measurements), careful study of experiment geometry, and specialized measurement equipment that rotates the antenna during the measurements.
The required equipment for antenna measurements include:
- A reference antenna – An antenna with known characteristics (gain, pattern, etc). The Reference Antenna should of course radiate well at the desired test frequency. Reference antennas are often dual-polarized horn antennas, so that horizontal and vertical polarization can be measured at the same time.
- An RF Power Transmitter – A way of injecting energy into the AUT [Antenna Under Test]. The Transmitting System should be capable of outputting a stable known power level. The output frequency should also be tunable (selectable), and reasonably stable (stable means that the frequency you get from the transmitter is close to the frequency you want, does not vary much with temperature). The transmitter should contain very little energy at all other frequencies (there will always be some energy outside of the desired frequency, but there shouldn’t be a lot of energy at harmonics, for instance).
- A receiver system – This determines how much power is received by the reference antenna. The Receiving System simply needs to determine how much power is received from the test antenna. This can be done via a simple power meter, which is a device for measuring RF (radio frequency) power and can be connected directly to the antenna terminals via a transmission line (such as a coaxial cable with N-type or SMA connectors). Typically the receiver is a 50 Ohm system, but can be a different impedance if specified.
- A positioning system – This system is used to rotate the test antenna relative to the source antenna, to measure the radiation pattern as a function of angle. There are many ways to perform this rotation; sometimes the reference antenna is rotated, and sometimes both the reference and AUT antennas are rotated.
Note that the transmit/receive system is often replaced by a VNA. An S21 measurement transmits a frequency out of port 1 and is records the received power at port 2. Hence, a VNA is well-suited to this task; however it is not the only method of performing this task.
A multitude of antenna pattern measurement techniques have been developed.
What is Passive Antenna Testing?
A typical passive antenna environment is one in which the antenna port is isolated from the rest of the RF front end. The antenna port of the DUT is directly connected to a signal generator set to a desired frequency and amplitude. The DUT is placed on a turn table typically rotating 360 degrees (Azimuth). A receiving antenna, typically a horn antenna or patch antenna with dual polarization, is placed on a boom that moves from zero to 180 degrees (Elevation).
The DUT antenna transmits radio waves which are picked up by the antenna for the receiving instrumentation. Measurements are recorded continuously at several angles of Elevation (theta = 0 to 150) and Azimuth (Phi = 0 to 360) to provide a 2D or 3D view of the antenna radiation pattern. Efficiency, gain, directivity and EIRP are also calculated through software algorithms performing numerous computations.
Free Space Ranges (Anechoic Chambers)
Free space ranges are antenna measurement locations designed to simulate measurements that would be performed in space. That is, all reflected waves from nearby objects and the ground (which are undesirable) are suppressed as much as possible. The most popular free space ranges are anechoic chambers, elevated ranges, and the compact range.
Near field and far-field measurements
All common antenna properties are defined in the far field (FF) region. Per IEEE definition, the FF is the region where the field of the antenna is essentially independent of the distance from a point in the antenna region. In other words, in FF, free space and plane waves are assumed.
An ideal plane wave consists of infinite parallel wave fronts, which are planes of constant
phase. To achieve infinite parallel planes, infinite energy and space are required, rendering
the mathematical constraint impractical. However, assuming a small portion of the wave
front, the deviation from a plane wave will decrease with increasing distance from the
source. To minimize the deviation, sufficient distance and free space around the direct line of sight (LOS) are required.
Far-field range (FF)
The first technique developed was the far-field range, where the antenna under test (AUT) is placed in the far-field of a range antenna.
IEEE suggests the use of their antenna measurement standard, document number IEEE-Std-149-1979 for far-field ranges and measurement set-up for various techniques including ground-bounce type ranges.
Generally, the far-field distance or Fraunhofer distance, d, is considered to be
where D is the maximum dimension of the antenna and is the wavelength of the radio wave. Separating the AUT and the instrumentation antenna by this distance reduces the phase variation across the AUT enough to obtain a reasonably good antenna pattern.
Outdoor test ranges
Outdoor test ranges are large testing facilities in open space with either one or two towers for the probe antenna and AUT. Open area test sites (OATS) provide no shielding or only limited shielding against interference from other radiators in the environment. They are still used mainly for very large antennas or low frequencies due to the large required FF distance.
Near-field range (NF)
Due to the size required to create a far-field range for large antennas, near-field techniques were developed, which allow the measurement of the field on a surface close to the antenna (typically 3 to 10 times its wavelength). This measurement is then predicted to be the same at infinity.
Planar near-field measurements are conducted by scanning a small probe antenna over a planar surface. These measurements are then transformed to the far-field by use of a Fourier transform, or more specifically by applying a method known as stationary phase to the Laplace transform
A third common method is the compact range, which uses a reflector to create a field near the AUT that looks approximately like a plane-wave.
Anechoic chambers are indoor antenna ranges. The walls, ceilings and floor are lined with special electromagnetic wave absorbering material. Indoor ranges are desirable because the test conditions can be much more tightly controlled than that of outdoor ranges. The material is often jagged in shape as well, making these chambers quite interesting to see. The jagged triangle shapes are designed so that what is reflected from them tends to spread in random directions, and what is added together from all the random reflections tends to add incoherently and is thus suppressed further.
ETS-Lindgren, MVG and Rohde & Schwarz are some of the leading providers of fully anechoic
chambers, each providing their own unique solution to obtain antenna parameters.
Whenever a DUT is measured, the measurement results should not be dependent on the test setup. Therefore, calibration is required for each measurement setup. For calibration of conducted setups, a network analyzer can straightforwardly measure the scattering parameters of the used cable. The frequency response information and reflection coefficient of the cables are used by the test instrument to compensate for these effects. Successive measurements of a DUT will feature a clean and well-defined signal at the DUT input and the received signal parameters will reflect the actual parameters at the DUT output. In other words, all measurements are relative to the newly defined “calibration planes” located at the DUT connectors..
Modern signal generators and signal or spectrum analyzers can use the scattering parameters of a test setup to move the calibration plane of their measurements, similar to the port calibration of a network analyzer. Depending on the test equipment capabilities, the instrument may be able to compensate for frequency response in amplitude and phase, optionally taking reflections into account.
What is Active Antenna Testing?
A typical active antenna test environment is one in which the antenna is not isolated from the rest of the RF front end. In other words, the overall system (antenna plus RF front end) is measured to quantify performance. The figures of merit to qualitatively evaluate the antenna systems are TRP and TIS. These parameters are measured in a fully anechoic antenna chamber.
TRP (Total Radiated Power) is the measure of power radiated by the antenna when it is connected to a RF transmitter averaged over a 3-D sphere.
TIS (Total Isotropic Sensitivity) is the measure of the average sensitivity of a receive antenna system when averaged over a 3-D sphere.
Antenna Testing in European Space Agency
The Microwave and (sub)mm-wave RF Material Characterization Laboratory allows characterizing, at a very early design stage, the RF proprieties of materials when developing new antenna technologies. Parameters such as complex refractive index (and thus dielectric constant and dispersion losses) are extracted from RF measurements of polymers, ceramics and foams, materials widely used in space and ground antennas. In addition, the Laboratory can also characterise the RF losses in reflective surfaces, critical in high frequency radio based instruments for space science and Earth observation. These parameters can be measured very accurately across a very broad range of frequencies, allowing high fidelity designs in turn boosting the competitiveness of the European space sector in international markets.
The two anechoic chambers, the CATR and Hertz, enable accurate verification of radiating structures, payloads and complete satellites, from early design stages, up to final performance testing right before shipping the satellite to the launch site. Radio waves are EM radiation just like visible light, and they behave in the same way.
Tests are performed on individual antennas as well as antennas integrated with other antennas or subsystems. Testing the antennas integrated in the complete satellite structure is often the only way to ensure that the spacecraft is equipped with a compliant and reliable antenna (and communication) subsystem as the high number of antennas and deployable appendixes featured in modern satellites can cause unwanted interference when operated together.
Measuring an antenna’s performance can provide a wide range of benefits
Designers can optimize an antenna’s physical construction and radio performance based on product size, typical usage, manufacturing costs, and RF exposure
Users benefit from increased range and improved signal quality
Better transmission reliability can be achieved regardless of the position or orientation of the device
Reduced power consumption extends battery life
Proper antenna placement relative to digital circuitry can increase receiver sensitivity
An optimized antenna design will create a robust base station channel and produce better network communication performance