>From: km1h@juno.com (km1h @ juno.com)
To: <topband@contesting.com>
>Date: Thu, 19 Mar 1998 11:52:40 EST
Snip...
>
>If I go to a 4SQ this summer I will probably use the Comtek
>approach.
>
W7DD (ex KV7S not N7DD) here in town is using the comtek box to
drive an 80 meter 4-square of 4 wire dipoles hung from a central
tower support. Seems to work quite well.
>For the purpose of this discussion and to be of more general use
>to others why not confine this to a single full size element with
>full size radials. Phasing, etc just makes it more complicated.
>
I'll run it both ways. Somtimes an aray will have a slightly
different TOA than a single element.
>Very Poor Soil, I would estimate 1 mS/M. Solid granite rock is
>from right on the surface to 2' below. The soil itself is gritty
>sand with a thin covering of forest dirt. Very dense ground cover
>vegetation....millions of wild blueberries and huckleberries.
>This was an apple and pear orchard 50+ years ago and now
>overgrown with oak, maple and pine trees in the 40-70' range.
>
Let me get this straight. You have a site with lots of
blueberries and nearby fishing and you have time to spend
worrying about an antenna?
Seriously, How much of the vegetation will be sticking up above
your radial plane? Vegetation in the near field is pretty lossy
stuff too. You might be able to benefit from raising the radial
plane above the vegetation if possible. It would still need to
be more dense than four or even six radials though.
>
>I can understand with HF and VHF horizontal Yagis but not for a
>160M vertical. If rocky hilltops were superior then that is where
>the BCB towers would be.
>
Most BCB station operators don't get advertising revenue from
being able to claim DX listeners (there are exceptions of
course). So most don't care about a site that is good for
launching signals beyond their daytime coverage area.
>I have a very simple question: With my soil AND terrain, what is
>the lowest TOA I can expect......with any # of radials? I
>suspect it is around 15 degrees or higher...per ON4UN and the
>ARRL ANtenna Manual.
>
I think I have enough info about your setup now to make a stab at
a reasonable estimate of this for you. I'll probably get time to
do the work by next week sometime.
Snip...
>>I don't pretend to be able to explain it but I have now been
>>bitten several times by inferring relative effeciencies from
>>feedpoint impedance measurements. And I was later sorely
>>disappointed by actual field intensity measurements which were
>>done to confirm the work.
>
>
>Over what soil conditions?
>
Various. Ranging from 3 mS to 15 Ms or so. Mostly in the 3 to 5
mS range. Conditions around here are VERY spotty and vary over a
pretty large range. It is one of the things you have to be very
careful of when doing antenna measurements.
When I was working at Fort Huachuca (US Army Test and Evaluation
Command Antenna Test Facility near Bisbee AZ), we had several
different sites to use in the local area. These sites ranged
from solid igneous rock (very low conductivity) to the salt lake
bed in Wilcox Playa (very high conductivity) to the artificial
ground plane on the East Range at the fort (near perfect). This
ground plane was a 500 foot radius area covered with 2 inch
copper welded intersection mesh. the central mesh area was
surrounded by a radial field that went from the mesh out to 1/2
mile radius (I was told - I'm certain about the mesh radius) with
a copper #10 radial wire every 1/2 degree.
The radials didn't really add much effect for most of the antenna
work I was involved in (mostly because of the wavelengths
involved). But the mesh was quite effective indeed.
Snip...
>Would you think that my extremely poor soil is the reason the
>elevated radials appear to work well? With true ground somewhere
>around 50' + below the surface the coupling effect is
>minimized. This might tend to explain why BCB sites over
>excellent ground do not benefit from elevated radials.
Good point. If the ground is already extremely good. the
elevated radials really won't do much. But then you wouldn't
expect the full screen to have the full 5 or 6 dB effect either.
I believe the case Tom R. was involved in which he mentioned in
previous posts saw pretty much the full 5 dB improvement when
they went from elevated radials to full screen. So in that
particular case, apparently the ground under the array was pretty
much average.
It isn't all that useful to think about how far down "true
ground" is. True ground is the surface you stand on most of the
time. Unless there is a lot of tall vegetation. Then the true
ground surface for RF loss purposes starts somewhere overhead.
A better way to think of it is: "How far down into the earth does
the skin depth of this poor soil let my fields penetrate?" The
larger the volume of earth that is interacting with the fields
around the antenna, the more loss there will be.
Snip...
>I will decouple one of the elements and just use a single
>element. I will then use a local station 4 miles away to give me
>readings when I go from 6 to 12 and 12 to 24 radials. With a lab
>quality step attenuator that should yield acceptable accuracy.
So far, so good.
>The first 6 additional radials are already in place but not
>connected. Will also monitor the 2:1 VSWR bandwidth.
For the measurement you are tring to do, it would be much better
if the existing radials weren't there - connected or not. I
don't know how to predict the exact effect they might have. But
if they weren't there, there would be no lingering question.
We're only talking about a few dB difference here after all.
Snip...
>It (NEC4 - EG) has reportedly undergone serious real world
>testing to fine tune the algorithms. Perhaps N6BV could be
>persuaded to do some NEC-4 modeling at the ARRL and write an
>article.
>
This would be quite useful.
>>1. Possible use of antenna design that does not include earth
>> return current dependency for the feedpoint (some form of
>> vertically polarized loop)
>
>
>What does EZNEC have to say about a full size loop, vertically
>polarized, base 8' above the ground? F/B is of no concern to me
>but maximum ERP is...and at the lowest possible angles.
>Can 2 loops be installed at right angles, switched or phased?
>But this should be another subject.
>
Yes they can.
Snip...
>>Unless you are talking about doing this rabbit mesh over a
>>quite large area, I don't think it would gain you much on an
>>antenna this large. I assume you were talking about the area
>>near the base of the antenna here.
>
>I thought that the maximum current density and therefore the
>highest losses are at the base.
This is true. But this is also where the radials come together.
So your six existing radials form a sufficiently dense screen out
to a distance of about 16 feet. At that point, they will be about
16 feet apart. I would tie a perimeter wire between the radials
at this radius and start the mesh "radials" centered between the
wire radials connected to the perimeter wire and run them out
from there. That will give you the maximum radius of screen with
sufficient density to be an efective near field screen.
But the screen density will begin to fall again very soon. You
would then have effectively 12 radials out to a radius of 66
feet. But the screen would only be full density out to 40 feet.
So I'm wondering if it wouldn't just be easier (and cheaper) to
run 24 elevated radials from the 16 ft perimiter wire out to 66
feet. You would then have a full density screen out to 66 feet.
Then, If you wanted to "go all the way", you could run a
perimiter wire at 66 ft radius and go from there to 140 feet with
60 radials. This would give you a full sized screen with a
density nowhere less than 0.03 wavelengths between conductors.
By the way, according to the paper on _measured_ vertical antenna
systems by Brown. Lewis, and Epstein ("Ground Systems as a Factor
in Antenna Efficiency", by G.H. Brown, R.F. Lewis, and
J. Epstein, Proceedings of the Institute of Radio Engineers,
Vol. 25, No. 6, June 1937, pp. 753-787. - thanks to W1FV for the
reference), it appears that at 66 foot radius, with 24 radials,
and a full sized 1/4 wave radiator, you may already be within 2
dB or so of all you can get. So your comment about near field
losses being concentrated near the feedpoint is appparently
validated by measurement.
>I asked about installing 4 or 5 50' x 4' meshes in a spoke
>pattern from the base. Overlaps would be soldered at many
>places. So the question still remains....should the mesh be
>installed at the same height as the radials OR on the ground and
>then use a very low loss connection to the elevated radials???
I would keep the mesh in the same plane as the radials.
>
>>>Would running a few.... 4 to 10 maybe.... very long radials
>>>help? Long is 1000-5000' . Elevated or on the ground?
>
Snip (initial response) ...
>
>I was thinking more about efficiency at my already high TOA. I
>can run 10 or so down the hill, thru the woods and into several
>swamps.
>
This few very long radials would be a waste of time and material
effeciency wise.
Sorry it took me so long to get to respond to this one. But the
questions deserved more time than I had during the work week to
devote to them.
73, Eric N7CL
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