[TowerTalk] Vertical arrays on sloping terrain - opinions please

[Jim Lux]

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
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