Nearfield Systems Inc.
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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?
6. What is the significance of the 3 lambda AUT/probe separation distance?
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?
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.
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.
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!
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.
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.
6. What is the significance of the 3 lambda AUT/probe separation distance?
A NF measurement needs to be taken at a point in space where the NF probe is sampling the AUT radiating field. If the probe comes too close to the AUT it can start to detect the AUT reactive NF (fields that are coupled to the AUT and that do NOT contribute to radiation - just AUT reactance). This is typically within 1 wavelength (lambda) of the antenna. In this region the probe is not able to detect radiating fields since the reactive fields are dominant. We therefore have to ensure that the probe is outside of this region and NSI have found that 3 lambda is a safe distance. There is no maximum distance and the NF theory works even if the probe is at infinity. However, maximizing the distance leads to power loss due to space attentuation. It is accetable to reduce the distance from 3 to 2 lambda if needed, but one does run the risk of the NF probe detecing the reactive near-field, so this should be avoided unless some experimental data is available to mitigate the risk.
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.
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.
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.
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.
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.
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.