VHF Low Reception

rabbit73

New Member
Jul 17, 2020
1
1
S.E. VA
10 dB of noise margin is usually too little for low band reception. I know that TVfool does not consider the 4 dB of NM that the FCC subtracts due to noise. I believe that the rabbitears NM calculation does so, but I have never verified it.
I'm a new member here, but have been helping with reception problems on other forums for more than 10 years. My particular interest is reliable reception on VHF where the noise level is much higher than on UHF, which reduces the SNR and requires stronger signals.

I'm in a retirement home presently on lockdown, so I'm focusing on indoor VHF reception.



When Trip was developing his new Signal Search Map, he asked me to be a Beta Tester. I noticed that the results for his TVStudy-based Signal Search Map didn't track with the results of a TVFool report. This was his answer:

Trip PM Quotes about correction factors for VHF noise

08-15-2018, 05:47 AM
Extract from PM
Re: Strength Search Test Results

Quote:
Originally Posted by Trip in VA

The minimum field strength for TV is a calculated figure that assumes things like receive antenna gain and line loss and the like, but does not take into account factors like VHF noise. Those values are 28 dBuV/m for low-VHF, 36 dBuV/m for high-VHF, and 41+(20*log(f/615)) dBuV/m for UHF (where f is the center frequency of the channel). In my Longley-Rice maps, I use 41 dBuV/m as the cut-off level for all three bands, which effectively provides a 5 dB correction for high-VHF and a 13 dB correction for low-VHF. I've done the same thing with the Strength column here. Then the Margin column is largely a placeholder while I try to figure out if I can get the path information out of TVStudy (LOS, 1Edge, etc.) but is designed to be akin to Noise Margin on TVFool; it subtracts the minimum field strength values from the aforementioned (uncorrected) minimum values, which is why the order of the margin column doesn't always line up with the strength column.
08-16-2018, 02:26 PM

Quote:

Originally Posted by Trip in VA
The FCC proposed raising VHF power levels across the board, and there was so much push-back from broadcasters that they let it die. The broadcasters wanted it handled case-by-case and not with a broad brush, for whatever reason. Of course, stations are complaining about it now, but in 2010, it was exactly what they wanted. But the planning factors do not account for VHF noise or for the fact that most antennas sold today are not capable of receiving VHF (especially low-VHF) well. That's why my maps have those correction factors, and I would argue they're not enough.
END OF EXTRACTS FROM PMs

I used this indoor folded dipole antenna for tests on CH3:



The channel 3 signal had to be no less than -57 dBm for minimum required SNR for reliable reception:





Even on VHF-High a signal has to be stronger because the noise level is higher on VHF-High than UHF:



It's worse on VHF-Low.



 
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Tower Guy

SatelliteGuys Pro
Nov 1, 2005
680
56
Hi Rabbit;

In your room I’d build a full wave delta loop behind the dresser with the apex on top and the wire scotch taped to the back of the dresser. It could be fed at either the top of the delta, or the middle of the bottom. I did a similar thing for channel six, but I taped a rectangular full wave loop to the back of a dresser. My dresser doesn’t have a mirror.
I fed it with a 300 ohm balun with good results.
 
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