 
A bit of theoryAny conductor of electricity, be it a lamp post, an iron bar inthe wall, or a piece of wire is capable of converting electromagneticenergy, i.e. radio waves, back into an electric current. If sucha conductor comes in contact with the ground, energy flows toearth. If we isolate this conductor above the ground, the energycan be fed into a receiver, and signals with a certain frequencyselected for their entertainment value. Sounds simple. But notethat we're talking about energy and not aboutpotential or voltage. Current can only flow when an energy source(the antenna) is connected to a load (the receiver). Every energysource has an internal resistance (or more accurately impedance),and maximum energy transfer will occur when this is the same asthat of the receiver. It is for that reason that most receiversoffer a 50 ohm impedance input, so as to match 50 ohm coaxialantenna cable. In theory the antenna and any switches associatedwith it should also be a constant 50 ohms too. In practice there is no antenna that always has a 50 ohm impedancethroughout the long, medium and shortwave range. Its impedanceis dependent on a number of factors, including its length, height,and shape. Amateur radio antennas: not ideal for SWLsSo many of the criteria which make an antenna good for the amateurradio operator, like radiation efficiency and standing wave ratioare somewhat different when it comes to reception only. In particular,active antennae are generally out of the question for amateurs.Amateurs also have the advantage that many of the frequenciesused for their type of communications are harmonically related(e.g., 3.5, 7, 14 MHz). This simplifies the construction of multi-bandamateur antennae. Another important distinction is that amateur antennae are usuallymore robust than SWL antennae since they carry much more energy.The flip-side of this is that the amateur can tell a lot morequickly if the antenna is actually tuned properly since he getslocal feedback from instruments and remote feedback via contactswith other amateurs. Even if the SWL had the requisite transmitter,etc., he or she is not licensed to transmit anything on the samefrequencies used to receive programs, nor would people listeningto his signal be very happy to help him tune his antenna! So in some sense the SWL has a more difficult problem to solvethan most amateurs. The great bulk of SWL solutions are adaptedfrom commercial or amateur HF communications technology, althoughwe are now beginning to see some things which are appearing firstin shortwave listener sets. (Station name tuning with time/frequencylists, etc.) Are highly directional antennasa good idea?Not really. The fundamental problem is that antenna gain is achievedby increased directivity. Thus the ideal SWL antenna, one withgain in all directions, is impossible. Remember that high gainantennas require steering in the right direction. Most shortwavelisteners don't want the bother of a mechanically rotated antenna.There are electronically steered solutions but these are expensive.There is nothing more frustrating than a high gain antenna inthe wrong direction! Comments? letters@rnw.nl |