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Electrically short antennas are used for licensed AM broadcasting primarily for “Class C” stations, which are used for short range, and have between 250 watts and 1 kW of power. If antennas shorter than 1/10 wavelength are used, the RF field strength at the base of the anenna is very high, and the high electric field interacts with the soil, causing the ground resistance to be higher than for lower field strengths. The usual remedy for this situation is to shield the soil in the vicinity of the base of the antenna with sheet metal.
Part 15 AM hobbyists have noticed the high field strength of short antennas when they get a mild electric shock from touching the antennas of their very low-powered transmitters. A Part 15 AM transmitter might generate 100 VAC, or so, at the antenna. The short antennas produce high voltages because of their high capacitive reactances. When tuned to resonance by a loading coil, the resonant circuit produces a high RF voltage. The much higher power applied to the antenna by a licensed broadcast station produces a very high voltage.
The ground current of a short antenna tends to be concentrated near the base of the antenna. Some people have thought that this means that the ground plane of the antenna does not have to be very large. This is not true. Beyond the solid sheet metal near the base of the antenna, long radials should be attached to the sheet metal.
For a Class C station, the FCC requires a field strength of 241 mV/m at a distance of 1 km when 1 kW is applied to the antenna. This requires an antenna efficiency of more than 64.5%. It is this field strength requirement that has prevented licensed AM broadcast stations from using antennas that are very short.
The “Class A” clear channel stations are required to have a field strength of 362 mV/m at a distance of 1 km for 1 kW antenna power input. This makes electically short antennas out of the question for these stations. Even a quarter wave tower can’t produce this much field strength. This is a job for a more directive antenna, such as a 5/8 wavelength monopole.