On Sat, 21 Mar 1998 13:21:30 -0700 Eric Gustafson Courtesy Account
<n7cl@sparx.mmsi.com> writes:
>
>>From: km1h@juno.com (km1h @ juno.com)
To: <topband@contesting.com>
>>Date: Thu, 19 Mar 1998 11:52:40 EST
>
>
>>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.
I just want to simplify it enough so that others can benefit from the
analysis.
>>
>
>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?
Why do you think it takes me years to get motivated?
Since a W1 with 4 8877's went SK a year or so ago I am also beginning to
care again.
>Seriously, How much of the vegetation will be sticking up above
>your radial plane?
The bushes are only a few feet tall....keeps what little soil there is
moist much of the time.
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 will discount the trees for now...just assume an open field. Since the
radials go right over tree branches and are often against the trunk, any
assumption there would be a stretch.
>
>>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.
Mucho Gratias!
>>>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.
Part of my tax dollar and Saddam Hussein is still alive....
>
>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.
Very interesting observation Eric.
>
>>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.
Those references are still suspect IMO. About 4 years between tests,
different consultants and too many unknowns to accept as gospel.
>
>>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.
A few dB is what this has been all about! Do I get those dB by pure
sweat or should I sucumb to the QRO malise that is becoming too darn
evident?
>
>>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.
I will rattle Deans cage the next time I am over his QTH....meanwhile I
am awaiting the QEX update.
>>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.
Lets take this up later....interesting !
>>>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.
That is a great way to pick up 16' Eric....simple enough to even add more
wires in that area. Is the 50' plus 16' a coincidence or does the now
1/8 wave of 66' have any significance?
>
>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.
And at a considerable reduction in total wire....much of which is
overkill close in....brilliant!
Any other effort and response would be most welcome but you have given me
plenty to get started with.
Tnx agn.
73 Carl KM1H
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