In a message dated 96-10-12 20:15:37 EDT, you write:
>My experience during testing of antennas for military use suggests the
>following:
> First, there is what I like to call the "connection" loss. This is the
> effective resistance of the earth terminal connection of the antenna
> system to the flow of whatever RF current the antenna system is
> attempting to pump into and out of the earth on the wire that connects
> it to earth.
Right on. I like to view this current as the current created because the
antenna needs something for the other terminal to "PUSH" against. The sum of
currents flowing in ALL the wires running up vertically must be equal to the
flow of current into the ground system, or else the feedline will radiate!
> Second, there is the loss due to the interaction of the near-field
> energy storage fields of the antenna with nearby lossy ground,
> vegetation and structures. This loss component behaves slightly
> differently depending on wheter the antenna in question does most of
> its near field storage in the magnetic field or in the electric field.
Here's where most stuff we are exposed to falls apart. It is impossible to
have a time varying magnetic field without an accompanying electric field,
and vice versa. All three fields exist very near the antenna.
There's no such thing as a Magnetic radiator and noise is electromagnetic,
JUST like a signal. Even a small "Magnetic" loop antenna is greatly affected
by ground loss near the antenna. Trying to sort out these fields and blame
earth loss on one or the other gets us into all sorts of confusion.
> As someone earlier pointed out, the "skin depth" of the
> earth is significant at 1.8 MHz.
Up to 50 feet or so, depending on soil.
> If we add just two slightly elevated resonant 1/4 wave radials (one at
> 0 and the other at 180 degrees), we can reduce the "connection" loss
> component to a very small value.
Measure the impedance of a dipole at ten feet. The impedance of one radial
will be roughly HALF the impedance of the dipole. The antenna's impedance can
be (and usually is) less than the impedance of the single radial. Most of the
energy in the radial excites charges (current) in the earth, and is consumed
as heat. Some of it radiates mostly straight up, like a dipole at that
height.
> How significant is the "near-field" component? Usually between 4 and 6
> dB depending on the exact nature of the local earth and surroundings.
Exactly what I measured here, and what someone else measured at an AM BCB
station. Amazing coincidence isn't it, the same amount at three locations
with vertical, the same error in Beverage current tests, and in the test of a
low dipole in Thailand?
>So it is probably the case that both the guy who says "I added 4 elevated
>radials to my vertical antenna and the performance improved greatly" and
>the guy who says "I and others have evaluated 4 elevated radials against a
>full ground screen and the elevated radials loose by 4 - 6 dB" are correct.
It's pretty difficult to measure 6 dB by over the air tests. QSB alone can
mask the change, plus the antennas would have to be over identical soil. In
an A-B test here with a FSM, that's exactly the change I measured. About 5 dB
going from 4 elevated radials to 60 1/4 wl radials.
73 Tom
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