>From: W8JITom@aol.com
To: <topband@contesting.com>
>Date: Sat, 12 Oct 1996 21:52:19 -0400
>
>All three fields exist very near the antenna.
>
Yes, indeed, they do! Right down to the surface of it, actually.
>There's no such thing as a Magnetic radiator and noise is electromagnetic,
>JUST like a signal.
Yes, in the far field, a radiator is a radiator. Out there you can't tell
if the radiation came from a small loop or a big tall vertical. And yes,
there are radiation fields in the zone we refer to as the near field. But
in the near field area, there are also fields that do _not_ radiate. These
are the energy storage fields.
The antenna usually referred to as a "magnetic radiator" is not really a
magnetic radiator but an antenna which has the characteristic that the
energy storage field which predominates in the near field vicinity of the
antenna is the magnetic field. The magnitude of the energy storage fields
is typically much larger than the component of the "fields" in the same
zone due to radiation. In a lumped element resonant circuit, we would be
talking about the difference between "circulating currents" and the the
real power producing current which is in phase with voltage and brings
energy into and out of the network. This is possibly a poor analogy but the
germ of the idea is in there.
The extent to which the energy storage fields interact with the
surroundings to convert power from imaginary to real (heat) determines the
additional loss due to these fields. Both kinds of energy storage field
can produce losses when interacting with the antenna's surroundings but
they do so using different mechanisms of interaction. Therefore they have
slightly different characteristics in practice depending on the mature of
the lossy material in the vicinity of the antenna.
>Even a small "Magnetic" loop antenna is greatly affected by ground loss
>near the antenna.
Very true, see above. In fact, I think a significant part of the loss
reduction attributed to "magnetic" antennas is due to the much smaller size
of their near field zone rather than the difference in loss due to
interaction with the predominant field type in the near field zone. It is
much easier to keep the smaller energy storage zone off of lossy material.
The tradeoff, if effeciency is preserved, is bandwidth. On top band, a
"magnetic" antenna which was as radiation effecient as a full size 1/4 wave
vertical would not have enough bandwidth to pass a SSB signal through.
>Trying to sort out these fields and blame earth loss on one or the other
>gets us into all sorts of confusion.
>
I wasn't trying to blame losses on one or the other of them but on the
combination of all of them. I still think that understanding that there
are several components of this loss is necessary to formulate a plan to
deal rationally with minimizing the ones that we can minimize without doing
more than is really necessary.
>>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.
>
Somewhere between pretty difficult and impossible. Even what we were doing
was what I would call difficult and had probably about +/- 1dB of
cumulative error. We were measuring the field strength in the area around
the antenna out to a few miles using a field probe (small aperture
reference antenna) hanging below a helicopter. We would take enough data
at a particular range to verify the shape of the main lobe and then we
could estimate the losses from the difference between field strengths
predicetd by theory (no losses) and what we were actually measuring. We
would _not_ have been able to make these measurements with sky wave
propagated signals.
But when signals are as marginal as they usually are for working DX on top
band, 6 dB is usually easily noticeable if not easily quantifiable. If it
wasn't, then why would people so consistently spend money on only 11 dB
worth of amplifier? If a ham has 20 dB of "connection" loss and eliminates
(most of) that with a few elevated radials, even though he still has
significant near field losses, he will perceive a great improvement in his
station performance. It is surprisingly easy to get 20 dB of "connection"
loss. Simply base feeding a short (~30ish feet) vertical against a single
ground rod driven into dry sand will get you easily into the 20 dB of
"connection" loss ballpark on top band.
73, Eric
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