It was only an idea, but at least I can share the thinking behind it...
The aim is to use a HF (20-10m) beam with insulated elements for top loading
the whole tower on one or more of the LF bands, so we need to connect the
insulated elements to the boom. We could use a short jumper wire, but only if
we could guarantee finding the exact electrical center of the element;
otherwise there would be some risk of detuning the elements away from their
normal operating frequency.
(This could be a particular problem with SteppIR elements because the true
electrical center is not easy to identify or connect to.)
My engineering compromise would be to use an inductor of a few microhenries,
which would behave more like an RF choke on the higher bands so the exact point
of connection would not matter quite so much. There would be no detectable
effect on the resonant frequency on the lower bands because the inductance is
relatively small and positioned quite close to the top.
50uH would have a higher reactance at HF, but that amount of inductance might
also begin to affect the LF resonance. A higher inductance also involves a
higher voltage gradient across the coil, and anything resembling a plate choke
carries a risk of parasitic series resonances. You'd need to model the whole
structure to be certain of the numbers, but my gut feeling is that "a few
microhenries" would be a better compromise... and almost certainly better than
a plain piece of wire.
73 from Ian GM3SEK
>-----Original Message-----
>From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
>Jim Thomson
>Sent: 06 August 2014 02:27
>To: towertalk@contesting.com
>Subject: [TowerTalk] Fwd: Insulated elements, Yes - No?
>
>Date: Tue, 5 Aug 2014 22:54:31 +0100
>From: "Ian White" <gm3sek@ifwtech.co.uk>
>To: <towertalk@contesting.com>
>Subject: Re: [TowerTalk] Fwd: Insulated elements, Yes - No?
>
>W4TV wrote:
>>
>>The advantage of using insulated elements mounted above/below the
>boom
>>is that one does not need to correct for the boom. There is no need to
>>adjust the elements for shortening due to passing through the boom as
>>is the case with the Hy-Gain "clamshells" or other mounting designs
>>that use aluminum plate and conductive (U-Bolt/saddle) mounting. Even
>>1/8" thick PVC or fiberglass tubing is enough to eliminate nearly all
>>"boom effect" at HF.
>>
>
>Insulating the elements will certainly reduce the correction for the
>boom and mounting plate, but does not eliminate corrections completely.
>There is still a proximity effect, which will obviously be greater for a
>mounting that uses thin insulating sleeves than it would be if the
>element is spaced farther away from the metalwork on substantial
>insulating blocks.
>
>Corrections for element mountings are subject to some uncertainties, so
>it is always preferable to use a mounting method that involves smaller
>correction in the first place.
>
>Mounting methods that involve metal-to-metal contact will also require
>that same good contact to be maintained for the entire lifetime of the
>antenna, because corrosion between the element and its clamps may cause
>the element to become detuned. An insulated element mounting avoids
>that
>uncertainty, completely and forever.
>
>If it is necessary to connect the center of the element to the boom,
>either to reduce "static noise" or for top-loading purposes on 80/160m,
>the best method is to use a small air-wound inductor which behaves more
>like an RF choke on the higher bands.
>
>
>73 from Ian GM3SEK
>
>
>## OK, so how big a choke do we need for a given band ? Will a 50 uh
>choke suffice.... like
>say 12-18 ga magnet wire wound on a 1 inch solid form...sorta like a plate
>choke. Are insulated eles
>better or worse for lightning and rain static, or static build up etc. Why
>bother with the choke.... just grnd the
>center of all the eles to the boom with a short wire. If it’s a static,
>lightning
>issue, I would think you would want all the
>eles grnded to the boom, not just the extreme ends.
>
>## watch out, if the eles are groiunded to the boom via wire, the boom
>could then resonate on bands you don’t want.
>The KT-36A boom will resonate on 40m for example. If instead a choke
>was used to grnd the ele, then maybe not, depending on
>choke value.
>
>Jim VE7RF
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