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Antenna Measurements

The radiating fields of an antenna can be separated into three regions: the reactive near-field, the radiating near-field (Fresnell), and the far-field (Fraunhoffer). 

The reactive near-field is located very close to the radiating antenna surface, typically within a distance a of wavelength or two.  Within this region, the fields are changing very quickly due to evanescent waves created by surface currents, and do not propagate.  At a distance of a couple wavelengths, we enter the radiating near-field region.  Here the EM fields are well behaved but are still represented by spherical waves, and the antenna beam has not yet been fully formed.  The near-field region extends to a distance where  the fields start to resemble plane waves.  Beyond this distance is referred to as the far-field (or Fraunhoffer) region and is where the antenna beam has now been developed.

Polarization of a radiated wave describes the orientation of the electric field (E-field) vector varying in time as it travels in the direction of propagation.   If the E-field vector rotates as a function of time along the direction of propagation, the electromagnetic wave is said to be elliptically or circularly polarized.   A circularly polarized wave can rotate in one of two possible senses: right-handed circular polarization (RHCP) in which the electric field vector rotates in a right-hand sense with respect to the direction of propagation, and left-handed circular polarization (LHCP) in which the vector rotates in a left-hand sense. The E-field vector at any instance in time along the direction of propagation can be broken into a horizontal and a vertical component.  A electromagnetic wave that has only a horizontal or vertical component is considered to be linearly polarized. 

Directivity is defined as the power radiated per unit solid angle in a particular direction relative to the total power radiated by the antenna.  Gain is defined as the power radiated per unit solid angle in a particular direction relative to the total power accepted from the source.  Gain, much like directivity, is used to quantify the extent to which an antenna focuses energy.  Unlike Directivity, gain is a measured parameter where as Directivity is calculated. Therefore, gain takes into account additional IR and thermal losses incurred during the transmit of input power to radiated output power.   These losses define antenna efficiency, which is the difference between Directivity and Gain. 

Comparison gain measurement uses a reference antenna of a "known” gain to normalize the test antenna's relative measured power to absolute units (i.e. dBi).   Gain reference standards are commercially available and typically come with gain data based on the manufacturer specifications.  As a result, the manufacturing tolerances typically limit the accuracy of this data to within +/-0.3 to +/-0.5 dB, depending on the frequency range.  From a measurement uncertainty perspective, this means one should never expect to achieve gain measurement accuracies less than that of the standard being used.

To improve gain measurement accuracy, you can choose to have your gain standard antenna calibrated by a certified calibration laboratory, which can provide measured gain data uncertainties in the range of +/-0.1 to +/-0.25 dB.  This can be a costly endeavor where price is driven by the desired uncertainty.

The comparison gain method involves making individual measurements of both the AUT and the gain  standard using the same measurement configuration.   The un-normalized difference between these two measurements and the “known” gain of the standard is then used to determine the AUT gain in absolute dBi units. 

During the measurement process, care must best be taken when replacing the AUT with the gain standard to minimize the introduction of additional measurement errors such as impedance mismatch.  The same RF cable should be used to connect both the AUT and the gain standard.  If additional RF component(s), such as an adapter, are required then the insertion loss of this device must be included in the normalization constant.

No, the NRL calculated gain curves are calculated directivity and do not include ohmic or VSWR losses. Data curves supplied with each SGH represent directivity based on the actual horn dimensions, and not gain. However, since gain assumes a perfect match we do not need be concerned about mismatch, only losses in the horn itself and this is in general in the order of 0.1 dB.

The Three Antenna Gain measurement is the most accurate gain measurement method, but also the most labor intensive.  The beauty of this method is it does not require knowing the gain for any of the three antennas.  However, it does help if one of the antennas is a gain standard for verification purposes.  The method relies on the principle of solving three equations using three unknowns and requires making three separate measurements of all pair combinations.  All three antennas must have the same or overlapping frequency bands, and the phase center of each antenna must be accurately known.

First will be the uncertainty of the gain standard being used.  Other error sources that can impact gain measurement accuracy are impedance mismatches, multiple reflections, room scattering, RF leakage, truncation (near-field only), and RF network variations (i.e. flexing cables, connector repeatability, uncertainty in insertion loss of added RF adapters/cables).

Near-field Measurements

A few points to consider when selecting a planar near-field measurement solution:

  • Planar systems can only provide antenna patterns to +/- 70 or +/- 80 degrees.   If you require a +/- 90 degree or greater Field of View (FOV), this isn't the correct solution for you.  
  • Planar systems are best suited for measuring high  (i.e. > ~15 dBi) antennas.  This is because most of the energy is radiated into the forward hemisphere of the antenna.  Lower gain antennas are better suited for  spherical or cylindrical near-field measurement solutions.
  • The higher the gain, the less angular coverage is needed to capture the significant energy.  A good rule of thumb is to measure sufficient angles such that the sidelobes have dropped 30 dB below the main beam.
  • A reflector's feed system may require that the probe to AUT distance be greater than the typical three to five lambda. In this case, a larger scan area will be required for the same angular coverage.

The probe reference when located at the scanner (x,y,z) = (0,0,0) point is the coordinate reference. The reference point on the probe is the origin of the probe coordinate system that is used to measure or calculate the amplitude and phase pattern of the probe. When using measured probe pattern files, the calibration laboratory should have defined the location of the probe coordinate system. For an OEWG this is normally defined to be in the plane defined by the open end of the probe with the X-Y axes parallel to the side walls of the probe and the Z-Axis centered on the probe parallel to the long dimension of the probe. It could be defined differently, and in such cases the probe's phase patterns would change but the probe corrected AUT pattern should be independent of the definition of the probe's coordinates for a measured probe pattern. Any change in the X, Y or Z location of the probe origin will produce a change in the measured probe pattern that will be equal in magnitude and opposite in sign to the phase correction applied to the AUT phase based on the specified location of the probe.

CNF and SNF data sampling density is driven by the near-field (NF) phase behavior of the AUT radiating field. The MRE (maximum radial extent) defines a surface, centered on the coordinate system origin that encloses all radiating parts of the AUT. It therefore bounds the phase behavior of what we are trying to measure and defines the worst case sampling density. As long as the NF probe is further than the MRE (which it always is since probe radial distance is > MRE) the phase behavior measured will be the same, regardless if the NF probe is at MRE + 3 lambda or at infinity.

It is important to realize that for a SNF system (not a CNF system, since it contains one linear scanning axis) it is only our desire to minimize chamber size and maximize RF power that drives us to move the NF probe closer to the AUT. From a SNF theory perspective we can have the NF probe on the moon and the measurement will require the same angular sampling density as when the probe is close.

It is also important to realize that the SNF theory, sampling and processing can be used on FF ranges to reduce test time if one wants to acquire full 3D data sets since the data can be acquired at a density driven by the MRE, which in most cases is of lower density than what is required in the FF (and that can then be extract through the NF to FF processing). We can then use the SNF theory to reconstruct the field at any point in space, thereby enabling the ability to plot data with an arbitrarily fine resolution.

There are no limits in the theory that require a measurement distance of 3 lambda or greater. The theory actually allows scanning up to touching of the antenna. The recommendation of 3 lambda is a practical one that involves some tradeoffs. In all three of the scanning geometries, planar, cylindrical and spherical, the AMS software auto scan setup calculates a data point spacing based on the assumption that the scan distance is beyond the evanescent region. If you scan closer, the required data point spacing to correctly sample the fine structure of the evanescent field will decrease, requiring some experimentation to determine what the spacing should be and often increase the measurement time. In planar, the closer distance will increase the angular region of validity, or reduce the scan length which are good reasons to go as close as possible. At closer distances, the multiple reflection (mutual coupling) error may increase and this is part of the reason for the recommended 3 lambda distance. With adequate evaluation of data point spacing and multiple reflections, it is possible to make very good measurements with probe to AUT spacing down to 1 lambda or even closer.

The direct gain method is often used in near-field measurements and uses the near-field probe as the gain reference.  It involves performing a "bypass" measurement where the probe input cable is directly connected to the AUT cable, thus bypassing both the probe and AUT.

The connections that should be made for direct gain measurements is to remove the cable from the input to the AUT and the cable from the output connector on the probe and connect these two cables together using an appropriate adapter. It may be necessary to use a fixed attenuator pad along with the adapter to keep the signal level to the receiver from saturating the receiver. It is best to use a pad that will produce approximately the same signal level as when the AUT and probe are connected and the probe is at the peak amplitude position. The value of the pad and the receiver reading should be recorded for entry for use in the gain calculation. The receiver should be left in the same mode, usually B/A that it was in when the near‑field measurements were made. If the receiver was in the B/R mode for near‑field measurements, it should be left in that mode for the direct connect measurement.

Reference the IEEE-APS paper, Gain and EIRP Measurements by Allen Newell for more details on this measurement process.

The direct gain measurement is just as accurate as the comparison method. Both are derived from the basic theory and have the same potential accuracy. The accuracy of the results will depend on the knowledge of the gain standard. In direct gain measurements, the probe is the gain standard and in comparison gain measurements, the reference antenna, usually a standard gain horn (SGH), is the standard.  In most cases, the gain of the SGH is known better than the gain of the probe. The manufacturers curves for the SGH's are accurate to about 0.3 dB, while theoretical gain values for the open ended waveguide (OEWG) probes have an uncertainty of about 0.5 to 1 dB. If the probe has been calibrated using a three antenna gain method, this uncertainty can be reduced to about 0.1 dB.

Other considerations in choosing a gain method are:

A near‑field measurement must be made on the SGH for comparison gain measurements and the far‑field peak from this measurement is used in place of the Bypass measurements. Multiple reflections, room scattering, receiver leakage and truncation will cause some errors in this measurement, and these should be evaluated during the SGH measurement.

The impedance mismatch correction is larger for the direct gain measurements and may require complex reflection coefficient measurements on the AUT, probe, and cables and a calculation of the correction.

The signal level input to the AUT and the SGH must be the same in the two measurements, and this requires using the identical cables for both measurements and having good receiver and source stability. The bypass measurement should be repeated a few times to verify connector repeatability.

The gain values for the probes can be calculated from theory, but the results are not very accurate. The best gain values are obtained from a gain calibration. For most waveguide bands, it is possible to measure the patterns on a CATR, far‑field or SNF range and calculate directivity and estimate the ohmic loss in the adapters and waveguide. The probe gain from this method should be accurate to about 0.3 dB for a good range.

For planar and cylindrical measurements on a linearly polarized antenna, you can often just measure the single matching polarization and get acceptable results. However for spherical near-field measurements, or if you are interested in accurate planar or cylindrical results that are off the inter-cardinal planes, you must measure both polarizations.

For planar and cylindrical measurements where the probe is polarization matched to the AUT and where the main beam is approximately normal to the plane or cylinder axis, high accuracy main component patterns can be obtained using data from the co-polarized probe only. This is true for regions near the main beam and far off axis. For an AUT with the main beam steered far off axis, the cross component probe data may be necessary for high accuracy main component patterns. For spherical measurements where the AUT is mounted with the main beam near the phi axis (polar mount), data must be obtained with both an X and a Y polarized probe. For a linearly polarized AUT mounted with its main beam normal to the phi axis (equatorial mount), the main component patterns can be obtained from data with only the co-polarized probe.

The far-field result of near-field measurements is not fully described unless you take both polarizations. This is particularly true for inter-cardinal cuts. Taking only one polarization measurement will give a reasonable replication of the far-field principal-pol pattern under the following conditions:

1. If a low-cross-pol probe is used,
 2. If you are only interested in principal-plane cuts.

Both polarization senses are required if you want to view the principal-pol pattern off the principal axes or if you want to see the cross-pol. Since the cross-pol pattern is usually measured as a function of the principal-pol level, you must take both pols to make the cross-pol measurement. You may want to do a sensitivity study between single and dual-pol measurements before deciding to take single-pol measurements.

The OEWG has a typical cross-polarization along the two principal planes of -40 dB. If well made, it may be lower than this, but this is typical. If the OEWG is not calibrated and the analytical pattern is used in the processing, it assumes that the probe has no cross-pol. So the estimated error to signal ratio for measuring a -35 dB AUT is -5 dB. Converting this to an uncertainty in dB, gives an estimated uncertainty of 4 dB. If a lower error is required this can be achieved in two ways. If the probe is known to have a much lower cross pol level from previous measurements on this type of probe or if the manufacturer specifies a much lower level such as -60 dB, the estimated uncertainty reduces to about 1 dB. It is difficult to have high confidence in such a low cross-pol unless it is actually measured. The other way to reduce the final uncertainty is to calibrate the probe.

In general, the way to reduce the AUT cross-pol uncertainty is to use a probe with the lowest cross-pol or to calibrate the probe with the best calibration accuracy.

For more information, see Allen Newell's AMTA 2008 paper on cross-polarization uncertainty in near-field probe correction.

Nyquist sampling theory requires that signals must be sampled at twice the signal frequency to reconstruct an image free signal.  This equates to a sampling interval of half-wavelength or less to guarantee an image free far-field region of +/-90 degrees.  However, the sampling interval may be increased to speed up measurement time if collecting data for a reduced far-field angle extent, as is often the case when characterizing satellite antennas.   The alias free far-field angle extent = arcsin (wavelength / 2*d), where d is the near-field data sampling interval.

Far-field pattern data resolution is determined by the FFT size used to perform the near-field to far-field transformation.  Assuming a half-wavelength data sampling interval was used for the near-field measurement acquisition, this will provide an image free far-field angle field of view (FOV) of +/-90 degrees.   The resulting far-field pattern resolution would be 180 / NFFT degrees, where NFFT is the number of FFT points specified for the transformation.   For example, a NFFT = 512 would provide a far-field pattern resolution of 0.35 degrees. 

RF Measurement Systems

Dynamic range is equal to a maximum power signal value over the minimum power signal value at the system's input and is typically expressed as a logarithmic value, dB.  The maximum signal value will be the lesser of  the maximum power available versus the maximum power handling capability of the RF system.  The minimum value is determined by the noise level (i.e. sensitivity) of the RF system.  

This is depends on the smallest signal you wish to measure and how accurately you need to measure it.  For example, if you wish to measured a -40 dB sidelobe  to within +/- 1 dB, you will need at least 40 dB plus 20*Log (10**(1/20) - 1) or 40 + 18.27 = 58.27 dB of dynamic range.

Distributed frequency mixing systems are used overcome RF cable losses at higher RF operating frequencies by down converting to a lower intermediate frequency (IF), resulting in less cable loss.  This is accomplished by locating frequency up/down conversion units close to the receiving and transmitting antennas.   By doing this, the system dynamic range is increased resulting in improved measurement accuracy.  Cable lengths, frequency range and antenna pattern accuracy are all contributors to system dynamic range.  Usually, a full range link budget best determines the answer to these questions.  It also may indicate that other less-expensive solutions can possibly meet your needs.

Click here to see the frequency ranges and compatibility of different RF connectors.

We occasionally hear reports of customers damaging expensive RF equipment by electrostatic discharge (ESD). This note outlines a method for avoiding one common cause of equipment damage. A major cause of ESD-related damage is discharge from the center pin of a coaxial cable upon connecting it to some active device. A coaxial cable is basically a capacitor when the ends are left open. If the center conductor carries any charge, it will go right through your mixer, PIN switch, or network analyzer when you connect it. To avoid damage to expensive equipment, ALWAYS discharge the center conductor of ANY coaxial cable before connecting it to another device. To discharge the center conductor, use one of these methods:

1. Best Method: Connect a load to the cable first, then remove the load and connect to the device -- the charge will bleed harmlessly off through the load. Since most RF cables (and loads) are male, you'll probably want a load with a "barrel" adapter. CAUTION! Don’t try this on a cable that’s carrying any AC or DC power (bias voltages, etc.) or hazardous RF power levels.

2. Short the cable first, using a connector short, the mating connector’s outer conductor, the end of a torque wrench, or some other metal tool. If you use this method, there are several CAUTIONS that apply:
     a. Don’t try this on a cable that’s carrying any AC or DC power (bias voltages, etc.) or hazardous RF power levels.
     b. Make sure the cable isn't connected to some device that will be damaged by momentarily shorting the connected port.
     c. When you touch the tool to the cable, contact the outer conductor first, then gently slide the tool to connect the inner and outer conductors. DON’T force a tool into the connector and bend or scrape the center pin!

Distributed RF sub-systems are often used to minimize the impact of cable loss within a given measurement system but may be more expensive than a single-box solution. Considerations for a fundamental mixing RF sub-system include examination of the system power budget to insure adequate system dynamic range. A broadband RF amplifier for budgetary purposes would be a wise precaution.

There are many factors which can affect the dynamic range including free space loss due to range length, cable attenuation due to cable length, dynamic range of the receiver, receiver IF bandwidth, gain of the range illuminator, and lowest sidelobe level of the AUT. These all have an effect on the RF subsystem power budget. As part of the RF sub-system design (once everything is known) the system power budget will determine whether additional amplification will be needed.

Currently, our acquisition software supports standard Keysight, R&S, Anritsu equipment.

Yes, we currently support Keysight multi-port PNA and Streamline Series USB VNA products for making simultaneous multiple RF channel measurements.  This is often desirable when measuring dual polarized antennas with 2-ports.

Yes, provided the RF equipment has the proper options to support pulsed RF.  We can work with you to determine what options are needed for your particular application.

Test Applications

In general, the best range for your application depends on your accuracy requirements.  We don’t try to sell you one our systems.  We customize a system to meet your needs.  You don’t need to choose the best antenna range for your application.  We’ll do it for you.  Contact us, we are happy to help.

All NPM systems can make far-field measurements.  Some systems make measurements in the near-field and then transform them to the far-field, others use a reflector to synthesize a far-field plane wave in a short distance, while others make direct far-field measurements.  What is right for you depends on your needs to control accuracy, security and your real estate footprint.  Typically, near-field systems require the least amount of space and have the most controlled accurate environment.  Compact ranges provide simpler and quicker far-field results with the advantage of a controlled environment.  Direct far-field systems provide a lower-cost solution at the expense of real estate and pattern accuracy.  We would be happy to discuss your needs with you.

Normally, you try to match the range to the type of antenna.  Base station antennas are long linear arrays.  They are best tested with Spherical or Cylindrical ranges.

Reflector feeds are often medium gain circularly symmetric antennas.  They are often best measured on Spherical ranges like a Spherical Near-field range or Compact range.  Depending on the frequency range and size, a simple far-field range could also work.

Phased array antennas come in planar or curved surfaces.  Matching the measurement system to the antenna helps make the measurements easier.  For planar phased arrays, we recommend a planar near-field measurement solution using either a XY scanner or robotic arm.  For curved surfaces, a robotic arm or a theta/phi system could be used.  These systems can provide near-field measurements which are then processed to the far-field, as well as “Park and Probe” measurements for “element phase-up/calibration”.

Automotive testing is becoming more and more important as more antennas and antenna systems are being added to the car.  We provide both direct far-field and spherical near-field measurements for automobile systems.  NPM has delivered automobile systems large enough to support a full-size SUV.

Services

We offer a 3-day on-site training course that we provide that teaches the basics in antenna measurements, accuracy and practical techniques.  We come to your site and provide hands-on training on your system.  Attendees will walk away with a greater understanding of how to make fast, accurate measurements directly on their systems.

We have experts that can go on-site and diagnose problems and perform maintenance services on your current antenna measurement system.  We have extensive experience in all areas of antenna measurements.  We can assist in providing recommendations and/or services for making your mesurement procedure faster and more accurate.

We pioneered the practical application of the NIST 18-Term assessment.  This assessment evaluates the accuracy of any range.  Our experts can provide you with the tools you need to predict the accuracy of all of your antenna measurements.

NPM has extensive knowledge of  legacy NSI systems and would be happy to discuss with you ways to improve or support your current system.

We are very familiar with Orbit FR hardware and can provide support for upgrading all vendor’s equipment.

At present, we have limited testing service capabilities.  Please contact us to discuss your test requirements.