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For the technically curious, and with credit to Reg Edwards, below the line are his calculated parameters for a 3-meter Part 15 AM vertical and its loading coil, for the conditions shown.

The inductance of the loading coil offsets the capacitance of this electrically short antenna, resulting in the values in bold face black (below). This antenna system impedance would be an acceptable SWR match to most transmitters designed for a 50 ohm load.

Note that even though the antenna is fairly well matched to 50 ohms non-reactive by the loading coil, the DC resistance of that coil has >400 times the radiation resistance of the 3-meter whip. Therefore the coil absorbs the majority of the transmitter power, and much of what’s left is lost in the resistance of the ground connection. This results in poor radiation efficiency for the configuration (see the line in red text below).

This should give Part 15 AM-ers some insight into the reasons why the coverage of these systems is limited — although still capable of giving useful service for the purpose it is authorized.

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H. Height overall, metres … 3.20

P. Coil height as percent of Ht … 5.0

U. Upper antenna dia, mm. … 6.0

L. Lower antenna dia, mm. … 6.0

C. Coil length, mm …… 100

F. Frequency, megahertz ……. 1.00

J. Coil diameter, mm ………. 150.0

G. Ground loss resistance, ohms … 6.0

Overall antenna height 0.011 wavelengths.

Height of coil midpoint 0.20 metres above antenna base.

Coil inductance 769.03 micro-Henrys

Number of turns on coil 76

Coil wire diameter 0.87 milli-metres = 20 AWG

Self-resonant frequency of coil 2.02 MHz, with self-capacitance.

Coil Q 311 incl effect of self-capacitance.

Radiation resistance 0.05 ohms transformed to feedpoint.

Coil wire loss 20.57 ohms .. .. ..

Antenna conductor loss 0.02 ohms .. .. ..

[b:1c39f5027e]Feedpoint resistance 26.64 ohms

Feedpoint reactance 0.1 ohms [/b:1c39f5027e]

[b:1c39f5027e][color=red:1c39f5027e]Radiating efficiency 0.18 percent.[/color:1c39f5027e][/b:1c39f5027e]

Loss relative to 1/4-wave vertical 26.9 dB when both above same ground.

Efficiency of 1/4-wave vertical 85.9 percent when both above same ground.

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