Variometer Loading Coils for LP AM operation
The use of a short antenna under FCC Part 15 rules with the AM88 will require a matched antenna if any kind of range is needed beyond the typical home or apartment coverage. Since short antennas are highly reactive and have a low radiation resistance component, typical values being 3000-5000 ohms of capacitive reactance and 0.01 to 0.2 ohms radiation resistance, a matching network will be needed to permit RF current flow in the antenna. For example, consider a situation in which the AM88 operating at 1600 KHz is fed directly into an 8.5 foot CB whip antenna with no matching arrangement, and assume a voltage level of 2 volts RMS to the antenna. Also, assume a typical antenna impedance of 0.1 - j 4000 ohms, and a loading coil loss resistance of 20 ohms and ground loss resistance of 100 ohms (caused by effect of imperfect ground) The total antenna circuit impedance will be the sum of these, or an impedance of 120.1 - j4000 ohms. Of all this impedance, 0.1 ohm is radiation resistance, the rest (120 ohms) loss. The net impedance is 4002 ohms (vector sum of 120 and j4000) in magnitude, which we will call 4000 ohms for simplicity.
With 2 volts RMS, current flow is 0.5 ma into the antenna. Radiated power is (Current squared) x (Radiation resistance) or 0.025 microwatts. With a loading coil to cancel the reactance, the antenna looks like a 120 ohm resistance and the current flow is now 16.6 milliamperes. The radiated power is now about 28 microwatts, 1120 times as much over the unmatched whip. In theory, this should give a range increase of 33.4 times. Adding a matching network to match this antenna to 50 ohms would give another (1.5 times) boost in antenna current, and this would further increase range, since the effective radiated power would increase by about 2.25 times. Therefore, if the 8.5 ft unmatched whip antenna gave a 50 foot range, using a loading coil and matching network would in theory give a range of 33.4 X 1.5 X 50 ft or 2505 feet. This is about half a mile. This shows the importance of matching the antenna to the AM88.
Using an 8 to 10 foot vertical antenna, a loading coil in the 400-600 microhenry range would be needed for operation at the high end of the AM band (1600-1700 kHz) and as much as 4000 microhenries for the lower frequencies (520-600 kHz) A simple way to tune the antenna is to use a variometer coil. These are coils with two windings, one inside the other, with variable mutual inductance (coupling). This technique avoids the use of erratic slider contacts, switches, and losses in unused or shorted out coil turns to provide a wide range of adjustment, and is very simple to adjust. The photos below show two coils, the smaller one for 1600 kHz and the larger for 520 kHz. These are made from 3 and 4 inch PVC pipe and use no special parts. Construction details should be fairly evident from the photos. Several taps are brought out on the bottom side of the coils to allow a wide variation in inductance. The smaller unit is approximately 300-600 microhenries, the larger is 2500 to 4500 microhenries. #24 wire was used in the windings. These photos should prove useful to those wishing to build or experiment with one of these coils.
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