305 BROADCAST AKG/7M Panduan Pengguna - Halaman 9

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FM Antenna User Manual

Some Criteria For Evaluating Antennas

Notes

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.

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