Operation
|
15
Performance
The free space far field performance of the 3159 is very different to its performance in the near field semi-anechoic
chamber environment under which it typically operates. In the EMC chamber there is more coupling to the chamber floor
and the DUT which loads the antenna and could result in undesirable resonance effects that increase the VSWR of the
antenna despite the isolating benefits of the balun.
The VSWR of the antenna provides a good indication of expected performance but the actual measured
Ē
field strength
under test conditions in the chamber will be the most accurate. The antenna balun is manufactured and measured
separately as a preliminary performance test, with a 200 Ohm resistor placed across the output of the balun in place of
the elements. The VSWR plot shown in Figure 2, is an example of such a measurement of a version C balun, takenduring
factory calibration. Following successful completion of the balun assembly, the VSWR measurement was repeated on an
OATS with the antenna elements fitted. The antenna was mounted horizontally with the balun at 1.5m above the OATS
without ferrite tiles below the antenna. The measurement was performed with and without the full complement of
extensions for comparison. (See Figure 3.)
Figure 2: VSWR Measurement of the balun with 200 ohm load resistors
Figure 3: VSWR of antenna with and without extensions, Horiz.,no ferrites, on OATS
Without
extensions
With
extensions
