Down East Microwave

Amateur Microwave Antennas and Equipment beam scatter/redirection. One particularly interesting method of beam blockage is by use of a mirror cemented to the cone of a speaker; the angle of beam reflection varies with the speaker drive (both amplitude and frequency). Another interesting shutter is a chopping wheel—a circular fan blade with periodic apertures, which can "chop" the beam into pulses. In fact, this method is the most used as the chopped beam can convey intelligence by small changes in chopping frequency and the chopped beam can be heavily filtered to remove outside influences (noise) before amplification. A number of articles and pamphlets have been recently published about these forms, see the Foltzer article in the August 1990 OEX and the Atkins articles in March-May 1990 Ham Radio.

Receivers come in two basic flavors: semiconductor (diodes or transistors which are photosensitive) operating either as a source or as a photovariable resistance (with a voltage between 2 and 300 volts), and vacuum tubes (usually photo-multipliers) which have very high gains (as large as 10E + 7) but require fairly high operating voltage (700-2000 V) and relatively complex circuitry. If enough interest is shown, I might be persuaded to do a future column on these interesting "receiving" tubes.

Antennas can be almost anything having light-gathering or amplification properties; of particular interest on reception are lenses of large size, such as flat-plate Fresnel lens. Typical sizes can be 10 x 2 inches, which has a lot more light-gathering ability (10000 x , or about 40 dB) than the 0.1 x 0.1-inch aperture of a semiconductor photodiode. Similarly, beam expanders are often used for collimation of transmitted beams at long distances. Again, this esoteric topic deserves a separate column (or two) all to itself. If enough interest is shown, perhaps a real authority in the field can be persuaded to provide a tutorial for QEX readers.

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