305 BROADCAST AKG/7M Посібник користувача - Сторінка 7

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FM Antenna User Manual

Some Criteria For Evaluating Antennas

5. Did you measure the antenna erect , free and clear of

metal objects?

Side mounting too close to the tower can detune an antenna. The required

spacing distance between the antenna and any other metal object decreases

as the operational frequency increases. Some good numbers per our factory

test procedure for omnidirectional antennas are from 100 ft. at 30 Mhz to 5 ft.

at 900 Mhz.

6. What is t he DC continuity measurement using an

ohmmeter?

Some antennas have direct ground lightning protection. These normally

measure as a DC short between the connector's inner and outer conductor but

will be the proper 50 Ohm impedance at RF. See lightning notes in the catalog

specs to determine if this antenna model should measure as an open or a

short.

7. Did you have the opportunity to substitute an identical

antenna?

If the second antenna measures OK under the same mounting conditions, the

technician's first antenna is probably defective. If the second one yields the

same bad result, the problem is unlikely to be the antenna. Perhaps the

transmitter is not operating on the expected frequency. Substitution of a

dummy load is an option if a second antenna is not available but the test is

less meaningful because it is actually testing only the cable.

8. When was the antenna installed?

It could either be new and defective or had performed nominally for some time

before failing. It is a good practice for technicians to test products on receipt

before transporting them to the job site. Manufacturer's warranties cover only

manufacturing defects, not damage from an improper installation. An example

would be mounting a standard antenna upside-down. This would put the drain

hole at the top where it could collect water and cause the product to fail over

time. Factory options given to an inverted antenna include reconfiguring both

the drain hole location and any electrical beam tilt.

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