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[quote:e37329608f=”PhilB”]Can you comment on this recent post from the Community Radio forum. I am particularly wondering about the statement “buried ground radials need not be longer than the antenna is tall, since a shortened, loaded vertical antenna has a more compact near field”. (etc)[/quote:e37329608f]
The performance of radial ground systems with MW vertical radiators was measured by Dr. George Brown and others (all of RCA Labs) in a landmark study in 1937. The conclusions of the study were adopted by the FCC and many other regulatory agencies, and have been proven accurate in thousands of data measurements ever since.
This study measured the base impedance values of vertical radiators of five heights ranging from 22 to 99 degrees, and their corresponding field strengths when using buried radials of varying numbers and lengths.
Both the base impedance of a vertical radiator referenced to a perfect, infinite ground plane, and the field strength produced by that radiator at a given distance per watt of radiated power can be calculated accurately using appropriate equations.
The study showed for radiator heights of 22 to 99 degrees that a ground system of at least 113 buried radials, each at least 0.41 wavelength long produced measured base impedances and field strengths that were within a few percent of their calculated values for a “zero ohm” ground connection (a perfect connection to a perfect Earth). The field strength from radiators shorter than about 5 degrees falls rapidly from the theoretical maximum, as even this elaborate ground system has losses greater than the natural radiation resistance of these short radiators.
They further found that as the number of radials is decreased, so does their effective length. So radials can be shortened when fewer of them are used. But antenna systems with fewer, shorter radials don’t perform as well — ground system losses rise, which means more tx power is lost in heating the ground, rather than being radiated as a radio wave.
Here is a significant quote from this study: “When the radial wires were 45 feet long, the measured (base) resistance was practically independent of the number of wires. Evidently, most of the earth loss occured in regions beyond the periphery of the ground system.”
This study demonstrates that the effectiveness of a radial ground system is independent of the electrical height of the radiator — which differs from the statements in the Hawaii Ham Radio webpage.