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
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