[TowerTalk] Strong FM Station Impact on Antenna Analyzer Use in VHF

[Jeff DePolo]

> I am having trouble obtaining resonance (low SWR) at 144 MHz with 2
> commercially made 144 MHz yagis.  I am wondering if both 
> antenna analyzers
> are being adversely impacted by a 50 KW FM station on 107 MHz 
> located about
> 0.5 miles away.  The antennas are being tested at 35 ft out 
> in the open
> supported by a tram.

Assuming the yagi is mounted in the clear and can be rotated, first point it
at the FM station and make a measurement, and then rotate it 90 degrees
(broadside) and take another measurement.  If the apparent match/VSWR
changes as the orientation is varied, that would be a good indication that
the test instrument is being overloaded or otherwise adversely affected by
the high-level off-channel RF.

At 1/2 mile, the free-space path loss is about 70 dB at 100 MHz, so the 50
kW FM would yield about +7 dBm using a unity-gain receiving antenna at that
location.  I don't know those two specific devices well enough to know
whether or not a signal of that amplitude (plus or minus the yagi's
gain/loss at 100 MHz) that far off-channel would be an issue.

					--- Jeff WN3A


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