A radio direction-finding antenna can be used for a number of purposes, only one of which is finding the direction from which a radio signal arrives. Another use is in suppressing cochannel and adjacent-channel interference. This becomes possible when the desired station is in a direction close to right angles from the line between the receiver and the desired transmitter. Reduction of the signal strength of the interfering signal is possible because the loopstick antenna has nulls off both ends.
Figure 4-14 shows a loopstick antenna mounted in a shielded compartí nei\t for radio direction finding. The shield is used to prevent electrical field coupling from nearby sources, such as power lines and other stations, yet doesn't affec t the reception of the magnetic field of radio stations. The aluminum can be one-half of an electronic hobbyist^ utility box, of appropriate dimensions, or can be built custom from
4-14 Mounting the loopstick antenna in a shielded enclosure.
Harry & Harriet Homeowner Do-it-Yourself hardware stores. The loopstick antenna is mounted by nonmetallic cable ties to nylon spacers that aref in turn, fastened to the aluminum surface with nylon hardware.
The number of turns required for the winding can be found experimentally, but starting with the number called for by the preceding formula. The actual number of turns depends in part on t he frequency of the band being received and the value of the capacitors used to resonate the loopstick antenna.
Most inductors used in radio and other RF circuits are either toroidal or solenoid-wound cylindrical There is, however, a class of inductors that are neither sole-noidal nor toroidal, Many loop antennas are actually inductors fashioned into either triangle, square, hexagon, or octagon shapes. Of these, the most common is the square-wound loop coil (Pig. 4-15). The two basic forms are: flat wound (fig, 4-ISA) and depth wound (Fig, 4-15B). The equation for the inductance of these shaped coils is a bit difficult to calculate, but the equation provided by F. W. Grover of the U.S. National Bureau of Standards in 1946 is workable:
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