VarAC - HF discussion forum

I thought it may be of interest to read about an example of a very long Vara DX connection using the "Verbose SNR" mode on VarAC. In this mode, the two involved stations exchange the locally measured SNR values at approx. 30sec intervals. The idea is to get an SNR graph over time. This is useful to see where things are going or to compare wave propagation on different days. In contrast to that, a simple ping is always only a snap-shot. Given the time-variant nature of HF, that is usually not very meaningful and reliable.

Lenz, DL8RDL, and I often maintain Verbose SNR VarAC connections on 17m over many hours via the long path VK3 - DL. The nature of this path is, that many or most "ground" reflection points are on sea-water. Unlike reflection on solid ground, sea-water provides near-perfect reflection without significant losses. In a way, seawater acts like perfect metal for the radio wave. That is so because seawater has a dielectric constant as high as 60.

Yesterday we linked up around 0700 UTC on 17m and the link ended at 1400 UTC. Over the time of 7 hours, the signals varied by over 30 dB due to the changes in the ionosphere. Just before 0800 UTC I noticed a sharp drop in my SNR (at DL8RDL) and was not sure why. Reciprocity rules out any propagation effects because they would affect both our SNRs and not just one of the two. So, I put my money on some temporary local noise or QRM at DL8RDL's QTH.

The "N" in SNR is never constant (noise). The variation depends a lot on the local environment (rural, urban, sub-urban). This is why the SNR graph is not a smooth curve. I can often see in the SNRs I measure locally, when my neighbours run gadgets, which don't have EMI filters. Other on-off type RF noise sources are plentiful in cities and suburbs.

DL8RDL has a good hex-beam and I use an Ultrabeam UB6-20. The VOACAP prediction matches well with our 7-hour 17m connection (see below). You can also see, that 20m would not have worked very long, if at all .... Give 17m a try occasionally. You may be surprised.

73

Dieter, VK3FFB