At 09:20 AM 10/31/2005, Jim Idelson wrote:
>I am looking at installing an 80m 2-el or 4-sq vertical array on sloping
>terrain. the terrain slopes down from approximately NW to SE. I believe the
>ground will be 15 to 20 feet higher on the northwest side of the array.
>
>My plan is to use radials on the ground, not raised radials. Here are the
>questions.
>
>1. What is the impact of sloping terrain? Does it matter much?
It matters, but it's hard to predict how much it matters. HFTA only models
horizontally polarized antennas (where the calculations are much easier,
because the reflection coefficient is really high almost independent of the
earth dielectric properties..). I've been looking for a "simple" method to
model this kind of thing, but, so far, I haven't found anything that is
practical for quick and dirty implementation. OTOH, if you happen to have a
copy of one of the fancy 3D electromagnetics codes around...
This is an area of extreme interest in the wireless industry, where the
vagaries of propagation with various polarizations and large objects (in
terms of wavelength) are common. The IEEE Transactions are full of
exceedingly dense papers that attack one piece or another with excruciating
detail.
I'd be happy with a clunky, good enough model, even if it took 5 days of
run time on a PC.. it's not like I'm going to model moving my HF array at
60 mi/hr down the freeway.
>2. What's the best practical way to deal with this situation?
Put the antennas up, tune for maximum smoke, operate.
If you wanted to obssess a bit, you could make a series of measurements of
the field strength at some distance away as you test antennas in pairs,
putting a variable phase shift in one of them. From that, you could
calculate the mutual interaction matrices, including the far field effects
of the terrain. Once you've got the interaction model dialed in, you could
synthesize a more optimum phasing network.
This is a problem of some significant interest in many fields these days
(including my professional work, where I'm trying to do essentially this,
at, variously, 1.26GHz, 7 GHz, 8.4 GHz, and 32 GHz). If you happen to know
a PhD candidate looking for a thesis topic, you've got a nice one.
I'd love to have a little box that had a tunable HF receiver, a data
logger, a GPS receiver (for time hacks and frequency cal), and some sort of
data link that one could deploy around an experimental antenna so that you
could make this sort of measurement on a regular basis. Here in Southern
California, you could distribute a dozen or so around on suitable quiet
locations, and make antenna pattern measurements much easier for everyone
around. Something like an ICOM PCR-1000, a Z3801, a TNC, and a computer
would work... Probably $2-5K all told, after packaging, etc.
Hm.. all we need is a few hundred thousand dollars.
>73,
>
>Jim Idelson K1IR
>email k1ir at designet.com
>web http://www.k1ir.com
>_______________________________________________
>
>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.
>
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_______________________________________________
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.
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