Hope everyone interested gets a copy of this. Thanks to all who responded.
So far, most of the responses were from those also looking for an answer, and
from a few people who were willing to speculate. A couple of people had
opinions based on limited amounts of empirical investigation or modeling.
The most popular answers were:
1. "Ask W8JI", and
2. Set up the verticals so they are at the same absolute height over a true
horizontal [try to cancel out the effects of the sloping terrain]
I was concerned about (2) introducing significant differences in
characteristics of the individual elements of the array when they are mounted
at very different heights. This would range from the element at the highest
point of the terrain being essentially ground-mounted to the element at the
lowest terrain being raised by 15 feet or so. W1FV deals with this problem by
raising the feedpoints of *all* the elements, using raised radials and a ground
screen so that ground proximity effects are less pronounced. See
http://www.kc1xx.com/antennas/160_array.pdf.
N0UU reports, "Moxon (before he quit radio and went to photography and then to
the great radiator in the sky) in "HF Antennas for all locations", p. 140,
wrote, "Vertical polarization tends to be unattractive when the ground is
sloping. ....the image being tilted back so that it makes little or no
contribution to the wanted field strength. ..... Situation is even worse with
the antenna normal to the slope......." All this with the verticle on the
slope. Some modeling in order for a vertical back a 1/4 wavelength from the
slope and on flat ground."
73,
Jim K1IR
> Jim,
That's a pretty steep hill.
K4SQR told me the feedpoints should be at the same elevation. That caused me
to find the flatest part of the hill I could for my 40m 4-sqr. The max
difference in feedpoint elevation is about 3 feet, so I didn't bother to
compensate. I don't think this affected array performance at all. 15-20 feet
difference on 80m would probably translate to 7-10 feet difference on 40m,
which would have been enough to get me thinking about compensating.
I couldn't find anything on the subject in ON4UN's book. You might try
dropping him an e-mail. W8JI is also an expert on phased arrays (and a lot
of other things) so an e-mail to him might provide some useful answers.
Either one of those guys could tell you the science behind the answer. Also
try searching the TowerTalk archives and posting a query. The subject has
come up before, though I don't recall seeing a good answer.
If ON4UN and W8JI don't have a definitive answer for you, my advice is to
build the array without compensating for the slope of the hill. If the array
doesn't perform as expected (easy to tell from gain relative to a
dipole/monopole, F/B and dumped power), you could raise the feedpoint of the
downhill element(s) by using insulated tower sections. For 80m I don't think
you would have to slope the radials up to the feedpoint -- they could be in
a ring around the grounded tower base. Best to ask the experts about this,
though.
73, Dick WC1M
> -----Original Message-----
> From: Jim Idelson [mailto:k1ir@designet.com]
> Sent: Monday, October 31, 2005 12:21 PM
> To: TowerTalk Post; CQ-Contest Post; YCCC
> Subject: [YCCC] Vertical arrays on sloping terrain - opinions please
>
> 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?
> 2. What's the best practical way to deal with this situation?
>
> 73,
>
> Jim Idelson K1IR
> email k1ir at designet.com
> web http://www.k1ir.com
>
>
<
Jim Idelson K1IR
email k1ir at designet.com
web http://www.k1ir.com
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