### Post by Rich on Jul 5, 2020 10:29:13 GMT

I first posted the following text on a "ham" website, but the essentials of it also apply to the short vertical antennas used for Part 15 AM systems, and may interest some readers here, as well. - Rich

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For perspective — the radiation resistance of a short, vertical monopole is a function of its physical height in wavelengths, and the distribution of r-f current along its conducting surface. This is true no matter what configuration of buried or elevated radials the vertical monopole is driven against.

Radiation resistance is the only

Below are the intrinsic parameters for a thin-wire, unloaded, base-fed, 14' vertical monopole driven against a perfect ground plane (calculated by NEC4.2).

20 meters (Hgt = 73°): Radiation resistance = 21.8 Ω, Feedpoint Reactance = -j161 Ω

40 meters (Hgt = 37°): Radiation resistance = 4.65 Ω, Feedpoint Reactance = -j764 Ω

A "loading coil" at the base feedpoint could offset the -jX present there to produce a resonant antenna system. But that will not change either its radiating height, or its radiation resistance in those operating bands.

If the loading coil is moved to the center of that 14' radiator, then the current distribution between the base and the coil input is nearly linear, and that raises its radiation resistance. It also raises its capacitive reactance at the base feedpoint, so the inductance needed for resonance increases, along with its ESR. The increased coil loss reduces system radiation efficiency, and the added inductance reduces the SWR bandwidth at the feedpoint. So center-loading has its downsides, too.

Radiation resistance is the only

__useful__resistance present in the the Real term of the feedpoint impedance of that antenna system, as all other resistances at the feedpoint do not produce the radiation of e-m waves into space. Instead, those other resistances (loading coil loss, radial system loss, etc) dissipate part of the available r-f energy as heat.Below are the intrinsic parameters for a thin-wire, unloaded, base-fed, 14' vertical monopole driven against a perfect ground plane (calculated by NEC4.2).

20 meters (Hgt = 73°): Radiation resistance = 21.8 Ω, Feedpoint Reactance = -j161 Ω

40 meters (Hgt = 37°): Radiation resistance = 4.65 Ω, Feedpoint Reactance = -j764 Ω

A "loading coil" at the base feedpoint could offset the -jX present there to produce a resonant antenna system. But that will not change either its radiating height, or its radiation resistance in those operating bands.

If the loading coil is moved to the center of that 14' radiator, then the current distribution between the base and the coil input is nearly linear, and that raises its radiation resistance. It also raises its capacitive reactance at the base feedpoint, so the inductance needed for resonance increases, along with its ESR. The increased coil loss reduces system radiation efficiency, and the added inductance reduces the SWR bandwidth at the feedpoint. So center-loading has its downsides, too.