SNR of short-distance VarAC HF paths

I observed a relatively low SNR at both stations in a short distance (21 km) 10m VarAC link. Both radios showed strong signal levels (around S9 +) but the SNRs were only -7 dB. On my DX paths I usually see positive SNR values. I am wondering whether there is a timing issue in short-distance HF links. Like non-optimal timing at decoding time. Multipath related problems are also possible, I guess. The path is definitely line-of-sight. So there is no ionospheric path involved.

The SNR graph below illustrates the above said. Sure, there are noise peaks and troughs which, show up strongly because the graph y-scale extends over only 14 dB, but the question is: why is the average SNR over a 21 km distance only about -7 dB when the signals are S9 plus at almost no noise at all.

Any comments are most welcome.

Dieter, VK3FFB

2 comments

Comments (2)

irad d

Jul 15

To my best understanding - SNR is only determined by the ability to get the sound out of the noise. Timing would contribute to either being able to decode or not.

I have made numerous local QSOs with no skip (just by the scattered of HF waves going horizontally).

It is always possible that the S level was good but the noise level was just a little bvelow it.

VK3FFB Dieter

Jul 19

Replying to

Thanks for your response Irad. The Vara HF modem derives the SNR from the BER, assuming there is a constant noise level. The S-meter signal levels were s9+ and the noise was very low (less than S1). So, the SNR was in the order of 30...40 dB. Both sides have good antennas over 10m above ground. In-between us is however a large TV and radio broadcasting site on a 600m high mountain. The broadcast antennas are known to passively reflect ham radio signals at VHF/UHF and maybe also 10m. In that case we would get multipath signal distortion. In principle, such distortion affects the BER. Whether this is the case over the given short distance, I can't say. 73

Edited

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