Weak-Signal means different things to different amateur radio operators. One thing weak-signal does not mean is that all ops are to run QRP or even QRPP. That isn’t to say that one shouldn’t run QRP during weak-signal communications it just means the mode can support higher power levels without interfering with adjacent signals. Many times one will run into discussions about weak-signal communications, sometimes rather heated ones, and the issue of always running QRP on any of the digital modes. Below is a quote that clears up the issue of weak signals (keep in mind that the discussion is about CW/SSB but all digital modes operate on SSB):
“Weak signal — referring to CW/SSB on the VHF+ bands, the word “weak” doesn’t necessarily mean that the received signal is weak. (KC9BQA adds: or that the transmitted power is necessarily QRP, either) It refers instead (roughly) to the ability of a mode to achieve a useful signal-to-noise ratio in a given bandwidth. Stated more simply, CW and SSB “do better” when signal levels are low because their power is concentrated in a narrower bandwidth than FM. This enable contact over longer distances via CW and SSB, all other things like transmitter power and antenna gain being equal. The tradeoff is that FM has better noise-immunity, which is more important for regional (or local) communications.” Ward, N0AX
With that issue out of the way let’s discuss some of the more exotic communications that the digital modes are being used for. Communications of this sort take place in the Earth’s troposphere, such communications have some interesting facts about them.
- Line-of-Sight is the normal communication taking place where transmitting stations and receiving stations are lined up with each other (remember also that the Earth’s curvature of the signals comes into play).
- Tropospheric Scatter is present in all transmissions. This is what causes that fluttering sound (and fading) to a station’s signal when they can barely be decoded.
- Tropospheric Enhancement (refraction) is also common. This comes about on cloudless nights with little to no winds and the ground begins to cool. Stations that are normally fading it and out will appear with more strength. Once the sun re-appears the cool ground and air heats up causing the signal enhancement to disappear.
- Tropospheric Ducting is considered an abnormal, and quite welcome, condition to a band. As with scatter, temperatures play a big part in the creation of these multi-directional openings. This ducting is responsible for some really great DX.
- Rain Scatter, a rare occurrence, takes place in the UHF and microwave bands. These communications are possible due to a heavy band of rain (sometimes with hail mixed in).
- Sleet Scatter, quite similar to rain scatter except it takes place in the winter months and involves ice pellets instead of rain.
- Lightning Scatter, a scary thought here in Florida. A rare and somewhat undocumented phenomenon whereby a lightning strike will create very short opening similar to meteor scatter.
- Aircraft Scatter is simply a case of bouncing signals off of aircraft and flocks birds as well.
- Meteor Scatter is used a great deal on the 6M band and involved bouncing signal off the trails of meteors as they enter the Earth’s atmosphere. For more information about meteor scatter try the American Meteor Society.
- Moonbounce, the big kahuna of exotic amateur radio communications. Also known as EME, Earth-Moon-Earth, this form of communications would take an entire web site to explain.