305 BROADCAST AKG/7M Посібник користувача - Сторінка 5

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FM Antenna User Manual

Some Criteria For Evaluating Antennas

Complaints about High Reflected Power, High VSWR, or Poor Match all mean

the same thing. The antenna does not appear to be absorbing as much power

from the transmitter as it should. It must be determined whether the

installation or the product is the cause. Answers to the following technical

questions can help determine if the product qualifies as defective per the

manufacturer's warranty. Our statement will be found on the current Celwave

Price List (we presume all connections are clean, dry, and tight. We also

assume all test equipment to be in good, calibrated condition.):

1. What is the VSWR you have measured?

Check to see if it is a higher number than the Celwave catalog value for this

product. Most of our antennas have a match of 1.5:1 or 1.6:1 across a

specified bandwidth. Performance at a Voltage Standing Wave Ratio greater

than 2.0:1 may be unsatisfactory. Some technicians will refer to match as

"Return Loss" in which case rates of 14 dB or 12 dB apply. Performance at a

R.L. of less than 9 dB may be unsatisfactory.

2. What test equipment did you use?

Check to see that it has been properly calibrated and that any connector

adaptors are of good quality. A poorly matched adaptor will invalidate the

results.

a. Wattmeter/Power Meter?

(These devices are inexpensive and therefore more common but can be

inaccurate, particularly if more than one RF carrier is present.

Technicians who use them will eagerly tell you how many Watts of power

is reflected back to the transmitter but often do not know the actual

mismatch. The forward power measurement is required to calculate the

VSWR or Return Loss number. This can be tricky because some

transmitters have an output stage protection circuit which reduces power

under highly reflection conditions).

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