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AntennaMeasurements

FAQ Topics

Antenna Measurements FAQ

1. What is the difference between gain and directivity?

2. How can I determine if a planar near-field is a good solution for my antenna tests?

3. What are the limitations of the MARS post processor?

4. What is the coordinate system reference for far-field radiation patterns obtained on a PNF range? Also, what is the ref plane when doing a back projection?

5. Why is CNF (phi direction only) & SNF angular data density driven by MRE and not probe radial distance?

6. Can near-field testing be done with probe to AUT separation closer than 3 lambda?

7. Compare the Direct Method Vs Comparison Method of gain calibration?

8. Do the NSI SGH NRL curves include estimates of the SGH losses?

9. When testing a CP antenna using an OEWG probe, do I need to measure both probe polarizations?

10. How critical is the phi alignment of the antenna w.r.t the probe?

11. If my antenna is linearly polarized and I do not care about cross-polarization, can I just take a single polarization near-field scan, or do I still need both polarizations?

12. What is the cross-pol accuracy that can be achieved using a standard OEWG Probe?

13. When testing antennas on a SNF range (especially when mounted on a spacecraft) each antenna is often located in a different position on the spacecraft and each antenna is offset w.r.t. the axis of rotation in some way. When a gain horn measurement is performed as part of a gain comparison measurement, is it important to locate the SGH in the same position with the same offset as each AUT of interest or can the SGH be centered over the axis of rotation and can one obtain valid gain measurements in this way?

1. What is the difference between gain and directivity ?

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.
See the gain & directivity slides for an illustration of the concept.

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2. How can I determine if a planar near-field is a good solution for my antenna tests?

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

Planar systems may be used to measure antenna patterns to +/- 70 or +/- 80 degrees.
Planar systems are best suited for measuring high gain, greater than ~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 measurements with spherical or cylindrical near-fields.
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 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.
For additional information on selecting a near-field measurement system, click here.

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3. What are the limitations of the MARS post processor?

The MARS post processor works in conjunction with a small modification to the spherical measurement procedure. For the case of conventional spherical near-field measurements the AUT is typically installed within the range so that the majority of the current sources (i.e. the antenna) are located as close to the origin of the range axis as possible. This is done to minimize the radius of the minimum sphere which reduces the amount of data that has to be collected, and insures that the AUT is displaced as little as possible during the acquisition which tends to minimize the effects of range multipath. The MARS measurement process necessitates the displacement of the AUT from the center of rotation. This is usually accomplished by displacing the AUT in a single axis, e.g. the z-axis by a number of wavelengths (e.g. 20" - 25" @ 2.5 - 3.0 GHz). Also, the best results are achieved when the sampling increment is halved which for a fast system, roughly equates to a doubling of the acquisition time. However, the benefit of all this is that we have found that MARS is capable of improving the chamber performance by more than 10 dB!

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4. What is the coordinate system reference for far-field radiation patterns obtained on a PNF range? Also, what is the ref plane when doing a back projection?

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-Azis 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.

When using the NSI theoretical probe pattern, this is based on the Yaghjian calculation and he also defines the probe coordinates as described above. A phase pattern is not calculated by the NSI software and the phase of the main component is assumed to be constant over the front hemisphere. This is equivalent to assuming that the phase center of the probe is located at the tip of the probe.

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5. Why is CNF (phi direction only) & SNF angular data density driven by MRE and not probe radial distance?

This is a very important concept that many customers misunderstand.

CNF and SNF data sampling density is driven by the NF phase behaviour 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 behaviour 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 behaviour 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.

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6. Can near-field testing be done with probe to AUT separation closer than 3 lambda?

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 NSI2000 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.

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7. Compare the Direct Method Vs Comparison Method of gain calibration?

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 the 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 to the gain calculation menu. 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.

Use the NSI2000 software to calculate a far‑field pattern and select the direct method option for calculating gain. Enter the probe gain in the direct gain menu and the receiver reading that was recorded with the pad and adapter in place of the AUT and probe in the Bypass measurement box. The value of the pad should be entered in the box labeled "Network Offset" as a negative number in dB. The resulting gain will then be calculated using the peak far‑field amplitude determined from the near‑field data.

There is also an APS paper, Gain and EIRP Measurements by Allen Newell that discusses the process in more detail.

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.

The other considerations in choosing the 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 compact range or far‑field range, 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.

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8. Do the NSI SGH NRL curves include estimates of the SGH losses?

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.

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9. When testing a CP antenna using an OEWG probe, do I need to measure both probe polarizations?

Yes, you need to specify in the probe setup a rectangular OEWG probe with Pol0 = 0 deg and Pol1 = 90 deg as you would when testing an LP antenna. When processing to the far-field, you need to specify the far-field principal pol sense as RHCP or LHCP and the software will combine both LP components to produce the right far-field CP sense.

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10. How critical is the phi alignment of the antenna w.r.t. the probe?

The phi angle is measured around the z-axis of the range measured from +X to +Y. A small phi error (dphi) has a minor effect on the principal-pol of the antenna, but a more serious impact on the cross-pol of a linear antenna. Principal-pol amplitude will decrease as the cosine of the angle whereas, cross-pol increases as the sin of the angle. The following table was generated using 20*log10(Cos(dphi)) for principal-pol and 20* log10(Sin(dphi)) for cross-pol. Note that the cross-pol level increases dramatically with dphi (the error). This cross-pol is actually a "false" cross-pol and interferes with the ability to measure the true cross-pol of the antenna. It can be considered as a noise effect in the measurement. It will affect both cross-pol and principal measurements at low levels.

Phi error (deg) Principal-pol (dB) Sidelobe or Cross-pol (dB)
0 0 none
0.1 -0.00003 -55.2
0.2 -0.000053 -49.1
0.5 -0.0003 -41.2
1.0 -0.001 -35.2
2.0 -0.005 -29.1
5.0 -0.03 -21.2
10.0 -0.13 -15.2
20.0 -0.54 -9.3
45.0 -3.0 -3.0

 

In measuring a -20dB cross-pol level or sidelobe, and assuming a 0.5 degree phi alignment error, the noise due to phi alignment error only is 41 dB. This makes the SNR between measurement and noise only 20 dB. So the measurement error will be about +/-0.9dB.

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11. If my antenna is linearly polarized and I do not care about cross-polarization, can I just take a single polarization near-field scan, or do I still need both polarizations?

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 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,

Two polarizations 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.

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12. What is the cross-pol accuracy that can be achieved using a standard OEWG probe?

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.

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13. When testing antennas on a SNF range (especially when mounted on a spacecraft) each antenna is often located in a different position on the spacecraft and each antenna is offset w.r.t. the axis of rotation in some way. When a gain horn measurement is performed as part of a gain comparison measurement, is it important to locate the SGH in the same position with the same offset as each AUT of interest or can the SGH be centered over the axis of rotation and can one obtain valid gain measurements in this way?

The SGH does not have to be located in the same position as each AUT of interest to obtain valid gain data. As long as each AUT of interest (and the SGH) are enclosed by a spherical surface and sampled at the density prescribed by the MRE (maximum radial extent) when measured, the absolute distance from the probe to the AUT and its position w.r.t. the coordinate system origin, are irrelevant.

Hansen discusses this issue in Section 5.4.3 of his book, "Spherical Near-field Antenna Measurements" and shows an example with real data. A conceptual example is to think about calibrating a large reflector with a small SGH and realize that even if one were “to get the separation distance exactly the same”, the power density in close proximity on boresight for the reflector will be lower than for the SGH at the same distance, because the small SGH will have much better focusing ability for the fixed separation distance. This fact, therefore, implies that integration over a large surface would be needed to derive the gain of the reflector, which will be significantly higher than that of the SGH. Therefore, the case is made that keeping the separation distance the same for AUT and SGH is not a requirement for a SNF gain measurement.

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