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Old 10-Aug-2015, 3:38 PM   #3
Retired A/V Tech
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,569
My guess is electrical interference from power line noise, which is worse on VHF than UHF.

I have a VHF-Hi channel that is marginal. I can receive it sometimes when the SNR is 15-16 dB. When the noise level is higher, I can't receive it because the SNR is less than 15 dB. I am able to measure the signal and the noise in an adjacent unused channel, and my Sony TV gives me the SNR in the Diagnostics Screen. My old SLM (signal level meter) has an AM detector for the audio so I can hear the signal and the noise. It's the meter on the left; the newer meter on the right can also measure noise and signal.

I can sometimes track down power line noise using a portable AM radio. Try both ends of the AM broadcast band where there is no station. If you rotate the radio, you see a directional effect.

If you have a radio with an AM detector for VHF (like an aircraft band) that can also hunt noise. Using an FM radio doesn't work, because FM rejects noise.

MFJ makes a power line noise detector.

The power line noise could be coming from inside your house from LED or CFL lamps, or switchmode power adapters; try cutting off circuit breakers.

Is the coax grounded with a grounding block? That helps reject interference.
Installing the fm filter brought UP the signal on WBSF 46 by 15 points! Not sure why it would do that for so high of a UHF station, but it did.
Good; that's from fundamental overload.
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If you can not measure it, you can not improve it.
Lord Kelvin, 1883

Last edited by rabbit73; 4-Sep-2015 at 12:32 AM.
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