Some Criteria For Evaluating Antennas
Match is only one indicator of antenna quality. VSWR tells us how well the
product’s impedance matches to (absorbs) a transmitters signal, and is easy to
measure in the field. Unfortunately, VSWR does not reveal an antenna’s
efficiency (how well it radiates the signal). This measurement (an antenna’s
radiation pattern) is more difficult to perform in the field. We may presume that
match bandwidth and pattern bandwidth are equal, but this may not always be
true. For example, operating an end-fed antenna below its design frequency
will result in an electrically down-tilted vertical pattern. Usually, substitution
with an identical unit of known quality is the method of choice when a
defective product is suspected.
The typical VSWR for a good antenna is 1.5:1. Although some site engineers
may specify a minimum acceptable value of 1.3:1 there is only a minuscule
improvement. For example at 1.5:1 ratio, 4.0% of the power is reflected back,
creating a 0.18 dB loss.
At a1.3:1 ratio, 1.7% is reflected resulting in 0.07 dB loss. The performance
improvement is only 0.11 dB. It is a good idea to document performance upon
installation. This is usually done by choosing a remote site and measuring the
signal level received from the transmitter. Periodic measurements at that same
location will reveal the amount of any degradation so corrective action may be
taken.
Notes
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FM Antenna User Manual
