The "earth loop" antennas have their adherents, and seem
appealing because of the potentially large size possible.
The simple radiation resistance formulae suggest that loops
should always lose out compared to verticals as the frequency
decreases (Rrad proportional to f^2 for a vertical, f^4 for a
loop), although any real situation is likely to be more
complex than this. Do any measurements of efficiency for this
type of antenna exist?
My feeling that ground loops may work out a lot better than may be first
thought at VLF is based on several factors :
The US submarine service uses them for ELF / ULF - at 60 to a few 100 Hz -
and I always look first to what others have done before in the professional
field. They usually get things mostly correct and saves reinventing wheels
each time.
The skin depth of soil at 9kHz varies from 28m in rich (high conductivity)
pastoral soil to 90m is sand / rock, meaning that the vertical dimension of
the loop becomes quite large.
There is no problem whatsoever matching low frequencies to the inductive or
resistive loads that a ground loop will present - it is the need for large
inductors and high voltages that is the real killer (in all senses !) for
wire antennas at these freqs. All that is needed is a lot of wire trailed
out over the ground for a long distance.
The 1/F^2 for E field vs 1/F^4 for loops works against us as antennas get
small, but does mean that as the amount of copper goes up, the efficiency
rises, to a simple approximation, as the fourth power of length so small
improvements to antennas should give more dramatic results.
My copper water pipe loop for 73kHz in its early days (2 turns of 10mm
diamter pipe, 3m overall diameter), was only 10dB down on the 7m high tee
so and much simpler to match to. The high circulating currents in the
matching capacitors can be simply catered for by using banks of parallel
capactitors in parallel. The water pipe antenna had 40A current in it and
using 30 x 3.3nF 1700V caps in parallel they just got slightly warm at 200W
G2AJV had considerable success on 73k with his large loop.
Andy G4JNT
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