On Wed, 18 Mar 1998 18:53:18 -0700 Eric Gustafson Courtesy Account
LOTS of SNIPS follow to preserve the peace.
>>My verticals are a full 1/4 wave long with 130-150' radials.
>I'm assuming there are two verticals phased as described. Got
>any idea how much current is present at the feedpoint in each
>radiator when endfire phasing is in use? I don't need an
>absolute number just the correct ratio.
If ON4UN is to be believed the ratios are close to 1:1 with just a 90
degree phase shift. I am using a 70 electrical degree length of coax to
simulate the 90 degree required but complicated by the mutuals. The F/B
is decent...in the 15 -25dB range for the most part but I do not use them
for RX. I have absolutely no lumped LC networks, just 2 relays and coax.
KISS.
If I go to a 4SQ this summer I will probably use the Comtek approach.
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.
Also, got any idea of
>the earth constants under this array?
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.
>>My site is fairly flat for about 250-300' in any direction and
>>then drops down 400' in anywhere from 1/2 to 1 mile depending
>>upon direction.
>
>Great site! No wonder you do well.
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.
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.
>>Does a noise bridge give a meaningful indication of Ohmic loss
>>reduction? A full size 1/4 wave should be ~37 Ohms with an
>>ideal ground according to the literature. I started out with one
>>radial and added one at a time until I reached the 37 Ohm
>>point...which happened to be 4 radials. I added 2 more for good
>>luck and saw absolutely no difference on the bridge. Is this not
>>supposed to indicate that the Ohmic losses are minimized....?
>>again according to lots of published ham articles.
>>
>>I understand your remark that measuring the FS is the only true
>>test but I have yet to see a plausible explanation why the noise
>>bridge can be misleading. I am talking about a full size
>>antenna...no loading or other excuse to confuse the discussion.
>>
>
>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?
>I have also had similar disappointments when comparing model
>results with what we could achieve in the field with actual
>measurements.
The inaccuracies of NEC-2, ELNEC, AO and MININEC are well documented. I
read now that EZNEC included a fudge factor which very closely agrees
with NEC-4.
I haven't used NEC4 yet but I remain unwilling to
>disregard past experience in the field that happens to disagree
>with modeling results.
Sounds smart to me until you have the opportunity to actually try it.
>I have had better luck inferring loss reduction from system
>bandwidth changes. But even this isn't a perfect indicator
>IMHO.
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.
A FS meter in the far field IS a perfect indicator of the
>quantity you are trying to improve. By the way, some of these FS
>measurements were done at the TOA, not at zero elevation.
I am going to try that soon. 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. The first
6 additional radials are already in place but not connected. Will also
monitor the 2:1 VSWR bandwidth.
>Now I'll have to get and read the article. As I say, I haven't
>used NEC-4 yet so I'm not familiar with all its limitations. But
>I would be shocked if it didn't have some significant ones.
It 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.
>
>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.
>4. Have a serious discussion with myself about how much effort I
> am willing to go through to get the last 4 or 5 dB.
I have been doing that for a few years. Thats why nothing gets done!
>
>The mesh had the effect of improving your average screen density
>to a significant degree.
>
>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. 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???
>>Would running a few.... 4 to 10 maybe.... very long radials
>>help? Long is 1000-5000' . Elevated or on the ground?
>Unfortunately, the longer the radials get, the more of them it
>takes to maintain enough screen density to be effective.
>Here, you are talking about the region from 2 to 10 wavelengths.
>This is sufficiently long that if you actually did screen it
>adequately, you could make a positive effect on the brewster
>angle and bring down your takeoff angle (TOA).
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.
>As a practical matter, you really don't need to go all the way to
>10 wavelengths. Three wavelengths would get your TOA down to the
>5 degree region (I believe, I'll check that one out and get back
>to you.) with the 3dB point much lower than that.
>
>But to get that improvement, and get all but the last 0.5 dB or
>so of it, it would take about 630 radials. You could save a bit
>of wire by reducing the number of radials as you approach the
>base of the antenna to maintain the minimum density for the
>entire screen. But it would still be a MAJOR project.
Time to go fishing!
73 Carl KM1H
>73, Eric N7CL
>
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