----- Original Message -----
From: "Tom Sessions" <k4rv@mindspring.com>
To: <towertalk@contesting.com>
Sent: Tuesday, January 18, 2005 9:03 AM
Subject: Re: [TowerTalk] horizontally polarized antennas and salt water
> Has anybody actually placed a REAL horizontal antenna close to salt water,
> then placed an identical REAL antenna back several wavelengths and A-B'd
> the difference over various times and propagation conditions? If so, what
> differences, if any, were observed?
>
> 73
> tom K4RV
>
This experiment is tougher than it might seem, if you want real quantitative
results. I've given quite a lot of thought to how one would do this sort of
evaluation using beacon signals, etc. The real problem is that "various
times and propagation conditions" is tough to average over and compensate
for in a even half rigorous way.
You'd need to have BOTH antennas up simulataneously and be sampling the data
from both simultaneously (at least on a time scale comparable to the
variations in propagation... say, every few seconds)
The other approach is to measure far field patterns with some sort of probe
(like RELEDOP or the blimp borne system used in England). This would remove
the uncertainties from relying on a ionospheric path, and would give you
data in the third dimension.
I suspect that some SW broadcasters have some sort of data along these
lines. They use horizontally polarized curtain arrays, and, surely, some are
close to the water, and have been moved, and they have field strength data
from their intended area of reception. A company like TCI that sells SW
antennas might have data as well.
The HF over the horizon ocean radar folks (like CODAR) might also have some
test data.
The real question is whether they've published it, in a useful fashion.
Hams could probably give you a "well, it seemed to play better" or a "I
worked DXCC on 5 watts" kind of answer. Hams are unique in that they
(generally) are willing to wait for that 1% of time when the propagation is
open, with their particular antenna situation, and also, are willing to tune
around looking for stations to which there is a propagation path. Antenna
performance for hams is very subjective, and is probably affected more by
the shape and size of back and side lobes more than raw forward gain. A
change from 10 dB F/R to 20dB F/R is a huge difference in operating, but
corresponds to a tiny change in forward gain.. When you consider that most
paths are actually NOT on the peak of the "main lobe" of the antenna
pattern, but well down on the lobe (particularly if you have an antenna
where the main lobe is at a high elevation), it gets even trickier. If
you're at the -10dB point on the lobe, you're at a place where the gain vs
angle is changing quite rapidly, and small changes in lobe angle or signal
angle of arrival can have huge effects on signal strength. The folks
running stacks with switching are well aware of this effect (because a stack
creates an interferometer with very sharp peaks and nulls).
I think that the modern electromagnetic modeling progams are sufficiently
accurate and well validated to give you an answer that is within the
measurement uncertainty of the physical experiment. The challenge in the
model is accurately characterizing the test conditions, and the uncertainty
in that is much greater than the differences you'll see from other effects.
For instance, the reflection coefficient of that sea water changes with
angle of incidence, sea state and wind - which is what those HFOTH radars
use for measuring currents and waves. A flat earth approximation (no waves)
and changing antenna heights, is about as good as you're going to get,
without a huge amount of work.
Another way of saying this is that if Ham A sets up the experiment and
collects the data, that will work for Ham A's antennas and Ham A's two test
locations, but that the results probably won't be applicable to Ham B's
experiment with Ham B's antennas at Ham B's location. Or, at least, the
simple model will predict Ham B's results about as well, and no worse, than
Ham A's data.
Jim, W6RMK
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|