Instrumentation & Measurement Magazine 26-5 - 13

In the case of antenna pattern measurement, sampling requirements
typically result in very long measurement times.
Fig. 1a shows a diagram of standard antenna measurement
with one test probe. The antenna under test (AUT) is placed
on a two-axis rotation platform that can move in elevation (θ)
and azimuth (φ). In many cases, the AUT can be characterized
by either using it as a receiver or a transmitter. When the AUT
transmits, the test antenna acts as a receiving probe. The AUT is
rotated to every angle of interest, and a network analyzer is used
to measure the gain and phase of the antenna at that orientation
over several frequencies (f). This can be a time-consuming
procedure. For example, a pattern sampled at 64 angles across
elevation and 32 angles across azimuth has 2,048 sampling
points. If the apparatus takes 1 second to move and 1 second
to settle, and has a 0.1 second sampling period per frequency, it
will take about 177.5 minutes to measure the full pattern across
all 32 frequencies.
Due to the long measurement times associated with antenna
measurement, efforts have been made to accelerate
the process [8]. Fig. 1b shows a multi-probe antenna measurement
system. In this case, test probes are arranged in a
ring in elevation around the AUT. Instead of physically rotating
the AUT in elevation, an array of radio frequency (RF)
switches can be used to sequentially select each probe around
the ring. Since RF switches can be controlled almost instantaneously
compared to physically rotating the AUT, this
significantly accelerates measurement. Assuming the same
parameters used in the single probe example, a multi-probe
system would only take 110.3 minutes. This is a significant
improvement but is still a substantial length of time. Even
though some of the system's movements have been eliminated,
the same number of samples are required, resulting
in long measurement times. Using traditional sampling, the
only way to further accelerate measurement is to reduce the
number of samples, but this will decrease resolution and degrade
performance.
Simple Implementation
Unlike traditional sampling, CS can decrease the number of
samples without decreasing resolution. CS uses compressed
samples, which are random sums of measurements. In the
case of compressive antenna pattern measurement, this can
be accomplished using the same multi-probe hardware used
in traditional multi-probe measurement. Instead of activating
the RF switches sequentially to scan the ring of probes,
random combinations of RF switches are activated simultaneously.
The signals are combined in an RF splitter and
measured using a network analyzer. A number of compressed
samples are taken at each position as the ring rotates in azimuth.
Once the process is completed, a recovery algorithm is
used to convert the compressed samples into the measured
antenna pattern [9], [10].
Fig. 2 shows the normalized mean squared error (NMSE) of
a compressive antenna measurement system for a 64 32 32×× ,
3D antenna pattern in elevation, azimuth, and frequency; and
for a 64 32× , 2D pattern in just one frequency. The error is plotted
vs. the sampling ratio, which is the ratio of the number of
compressed samples to the total number of data points. Using
a sampling ratio of 0.2, i.e., only 20% of the number of samples
required in traditional measurement, a relatively low NMSE
of close to -30 dB can be achieved. Using the same measurement
parameters described above, a compressive antenna
measurement system will only take 23.3 minutes, compared
to the 110.3 minutes of a standard multi-probe system, or the
177.5 minutes of single-probe system. The results for the 2D
pattern will be addressed later.
RF
switches
Test
antenna
Elevation
actuator
θ
φ
Network
analyzer
RF cable
Azimuth actuator
(a)
(b)
Fig. 1. (a) A single probe antenna measurement system; (b) A multi-probe system. The single probe system requires the AUT to be rotated in elevation and
azimuth, while the multi-probe system accelerates measurement by only rotating in azimuth, using RF switches to select probes arranged in a ring in elevation
around the AUT.
August 2023
IEEE Instrumentation & Measurement Magazine
13
φ
Network
analyzer
RF cable
Azimuth actuator
AUT
Splitter
θ
AUT
Test
antennas
RF bus

Instrumentation & Measurement Magazine 26-5

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