>From: km1h@juno.com (km1h @ juno.com)
To: <topband@contesting.com>
>Date: Tue, 17 Mar 1998 14:01:21 EST
>
>Eric, Hardy, Bill, Jeff, Yuri....etc. tnx for all the input.
>
>Eric, I will seriously edit your post so as to keep W4ZV
>contented. I also sent off some more questions yesterday to the
>reflector as I am getting educated here.
>
Probably a good idea. Bill has been very tolerant of me so far
on this subject. Thanks Bill.
Snip...
>My verticals are a full 1/4 wave long with 130-150' radials.
>
Snip... (array description for brevity)
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. Also, got any idea of
the earth constants under this array?
>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.
Snip...
>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.
I have also had similar disappointments when comparing model
results with what we could achieve in the field with actual
measurements. I haven't used NEC4 yet but I remain unwilling to
disregard past experience in the field that happens to disagree
with modeling results.
I have had better luck inferring loss reduction from system
bandwidth changes. But even this isn't a perfect indicator
IMHO. 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.
Snip...
>
>Which brings us full circle and back to the Chrisman
>article. I'll ask again, what does NEC-4 have to say about this?
>You have me 99% convinced BUT...
>
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.
>>In my own case, I'm having to face the same problem at a new QTH.
>>Although not particularly space constrained, the thought of the
>>work and expense of the full screen under a full size radiator is
>>daunting. I will definitely be making compromises and
>>tradeoffs. But I will be trying to understand the real
>>consequences of doing so.
>
>
>What would you suggest for a practical compromise?
>
I hesitate to make any specific recommendations since antenna
compromises are extremely site specific. What might be easy to
do and effective at one site. Can be absolutely impossible to do
or not very effective at another. But here are the general kinds
of things I will be considering doing at my new QTH.
1. Possible use of antenna design that does not include earth
return current dependency for the feedpoint (some form of
vertically polarized loop)
2. Trading physical size of the radiator for bandwidth to
minimize the size of the near field zone that must be
screened. This option is limited by the extent to which
ohmic losses in the radiator system can be controlled.
3. Possible use of support structures as parasitic elements. I
live in an area where true omni coverage is not that
important.
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.
5. Reinterperet the rules to consider the power limit as ERP
relative to the (actual) legal power limit delivered to a
lossless full sized array of some reasonable size. And then
buy a big enough PA to achieve equivalence (just kidding).
Snip...
>Every time you reply I learn but that generates more questions.!
>I have 2 for now:
>
>Would installing a high density mesh at the feed point help? By
>that I mean a 2" x 4" grid of welded, galvanized and vinyl
>insulated rabbit cage wire. It is available in 50' x 4'
>rolls. Should it be placed on the ground and connected to the
>elevated radials with wide copper strap or elevated along with
>the radials? At a prior QTH I placed 5 rolls of that cage on top
>of 25000 feet of on/in ground radials. The tower was a shunt fed
>100' with a top load of long boom 10-15-20M yagis (all grounded
>elements) . The mesh reduced my 2:1 VSWR BW from 50 to about
>20KHz. The "apparent improvement" in pileup busting was very
>noticable. The QTH was pretty much surrounded by swamp out to a
>mile or so in most directions. The water table was 2-3' below the
>surface. I still do not understand the reason for the
>improvement.
>
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.
>
>
>Would running a few.... 4 to 10 maybe.... very long radials
>help? Long is 1000-5000' . Elevated or on the ground?
>
>Tnx agn Carl KM1H
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).
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.
73, Eric N7CL
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