Hi Eric,
This may be the wrong place for a continuance, but I wanted to re-enforce
points you made that are negelected in most antenna articles.
In a message dated 96-10-14 17:01:03 EDT, you write:
>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.
An important point is folklore has it that a loop is quieter by gender, or
that a loop (or other antennas) can be shielded from electric fields. That is
impossible, unless the antenna is made to not radiate or receive at all. We
can only change the characteristics *very* near the antenna under some
special cases.
>energy into and out of the network. This is possibly a poor analogy but the
>germ of the idea is in there.
It's a very worthwhile analogy, because it gives a good picture of many of
the near field storage effects, like Q and bandwidth.
> >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.
> >
>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.
I did mean to imply disagreement with you at all. I was alluding to articles
that draw capacitors from the antenna to earth, and speculate that moving a
130 foot long radial up a few feet or breaking a connection suddenly makes
losses "go away". The coupling doesn't follow the normal distance vs field
strength rules so close to the wire. It's a geometry problem, not a distance
problem, until the wire and earth are far field relationship.
> >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.
My point was the evidence offered supporting elevated radials has always been
indirect comparisons (like measurements compared to FCC estimated ground
conductivity and field strength charts), or gut instinct. Neither of which is
accurate.
>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.
Sounds better than my "on the ground" measurements, but I'm not surprised
they agree so well. Wave angle changes very little with ground system
changes, unless the ground system radiates. Again, it is interesting several
different measurements at different locations by different methods agree.
73 Tom
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