Amateur Weak Signal communications

Weak signal communications refer to communications between two stations where the stations are not necessarily able to hear the communication signals with the human ear.

Decoding weak signal amateur radio with the aid of computers.

The signals are usually decoded with software in a computer with the help of what is known as digital signal processing via a sound card. How did the idea of weak signal communications develop in Amateur radio? Well it is well known that the strength of a signal at the receiver mainly depends on the size of the antenna of the receiving station. The power of the transmission at the transmitting station and the distance between the two stations and normally the "quality" of the space between them. If any of these are in any way less than favorable the signal will probably not be "heard" by the receiving station. But the theory states that the signal is not gone it is still somewhere shaded by the noise caused by other electrical transmissions that cause the hiss we normally hear.

Efforts to bring weak signals out of oblivion:

So a group of radio amateurs decided that there had to be a way to get the weak signal shaded by the noise processed by digital means so that the human ear can hear it or the computer can decode it. All sorts of tricks were designed like:

• To induce a pseudo signal that will remove the noise because it is out of phase - although crude it worked well and many a marginal signal was drawn out of the noise in this way.
• Another attempt was to shift the freq of the oscillator of the intermediate frequency and get the intermediate frequency out of the pass band of the noise. This sounds very technical but it actually meant that the noise was moved away from the signal.
• When computer processing of sound and audio signals was introduced it opened a whole new world of possibilities - and that is where we are today.

Even bouncing signals off the moon over 350 000 km from earth became possible.

Armed with the right equipment and computer the amateur radio operator can literally use an antenna - point it at the moon and "listen" to or at least decode another operator’s signal coming back off the moon where the moon is used purely as a reflector. The first contact like this was done between a station on the east coast of America and one on the west coast on a frequency one would usually require both stations to be able to "see" each other in a configuration that is known as "line of sight".

Not only boucing signals off the moon but also meteor showers.

Recently amateurs have started to even bounce signals of meteor showers. Very few people realize that there are a lot of meteors that enter the atmosphere daily and it was discovered that with careful planning and good equipment and digital signal processing with the aid of a decent computer. The author personally holds the South African record with meteor scatter contacts of over 1800 km with fellow Amateurs on two of the frequencies that usually require line of sight propagation. He has also done several contacts via the moon with stations all over the world including contacts with stations from every continent on one particular band.

I truely believe that our imagination is our limitation in weak signal communications by amateur radio!

To read more about these and other weak signal communication techniques:
EME for Africa
Sam in Russia
Dave in USA
Dan in Switzerland

Written by JD
South Africa
22 December 2006