Ask half a dozen hams to define a Ground Plane Antenna and you will probably get half a dozen different answers, yet it is thought of as one of the basic antenna types that newcomers will be introduced to in their education.
There seems credible acceptance by some writers that the Ground Plane Antenna was invented by George Brown (more completely Brown, Lewis and Epstein (BLE)) from RCA, and is described in US Patent 2,234,333 for a Demountable Antenna
filed in 1939. The patent does not call the thing a Ground Plane Antenna, but it does describe what could be naively described as a quarter wave vertical radiator with four equally spaced quarter wave horizontal radials (plus some other embellishments).
BLE gives the dimensions for the antenna at 41.5MHz and offers that the feed point impedance is 21.5Ω, transformed up to (the then popular) 70Ω transmission line by his custom quarter wave transformer (part of the invention).
The naively basic Ground Plane Antenna
Lets look at an NEC model of the vertical quarter wave and four quarter wave radials alone (ie in free space).
An NEC-4.2 model gives the feed point impedance as 23.4+j6.11Ω. The reactance is not surprising since the elements are actually slightly longer than a free space quarter wave, and resonance would occur at a little less length. Importantly, the R value is in the ball park of their estimate, so that reconciles reasonable well. BLE do not give a gain figure, but the gain of a lossless model in NEC is 1.44dB.
Above is the pattern, no surprises there (unless you were expecting it to look like a quarter wave monopole on perfectly conducting earth). Continue reading What is a Ground Plane Antenna?