http://www.reversebeacon.net showing my signal as received by monitoring stations, after I called “CQ” on twenty meters.
One of the other uses of SDR is the formation of reverse beacon sites, which scan shortwave (or other) frequencies for CW (and potentially other) signals, and log them to internet-available web servers. In Figure 1, my call can be seen after I called “CQ” on twenty meters. The station in the far left column is the monitoring station, and gives me some idea about where my signal is going. The frequency is listed, of course, as well as a “ball park” signal strength reading. On this particular day, running only 30 watts to an attic dipole antenna, I was pleased enough with the 20+ db reports from AC0C and KM3T. One can imagine how useful this is, for purposes of antenna tuning, etc.
The site gives me some indication about whether or not I’m wasting my time on a particular frequency/band, trying to elicit communication. Empirically, I’ve found a threshold level, above which I’m likely to have a QSO (around 9 or 10 db). Note that it’s not impossible to have a QSO with lower signals, it’s just less likely. The reason for this is pretty intuitive. The monitoring stations are often monsters, run high on the top of mountains, or outfitted with large, tall towers and antenna setups. When working a QSO, you’re more likely hooking up with Joe, down in the valley, with his attic dipole (ahem). So – it’s obviously necessary to knock the signal down for little Joe. Below some threshold, the monitoring station will hear you, but Joe will not. I’ve already called CQ at odd times, (when the band was theoretically down) – and received a half dozen reports from monitoring stations. In these cases, the band was often devoid of signals that I could copy. In other cases, even the monitoring stations do not log anything, and the band is “truly” down.
I suppose this feature imparts the idea that you’re being closely monitored, and that isn’t always a positive feeling. On the ham bands though, it’s expected.