On Fri, 09 Jan 2009 08:04:23 +0000
Paul Ormandy <zl4pw@orcon.net.nz> wrote:
>
>>Would be interested to hear any experiences or
>>dimensions,
>>particularly for the helical toploaded option.
Paul,
We used 18 m Spider-pole inverted L-s on 5K0T and VK9WWI.
Also, for a while, I used at my home QTH an 18 m
Spider-pole with a short horizontal wire and a loading
coil that was located just below the horizonal wire
junction.
During the DXpeditions inverted L's horizontal wire was
about 28 meters long. The antenna worked great (partly
because we had sea-water for ground). At home I had room
only for a 15 meter long horizonatal wire so I used the
loading coil to resonate the antenna. On air comparisons
indicated that the inductively loaded Inv L was weaker
than my other antenna, which is a tuner fed 21 m tall
vertical. The vertical was one S unit better (per N4IS)!
It appears that the top-loading inductor was creating
higher losses than the base located antenna tuner. (Which
is built with large inductors and high quality
capacitors.)
My recommendation is to try to resonate the antenna with
the horizontal wire, perhaps with a number of wires to
increase the capacitive top-loading. If that is not
sufficient, build a high quality L network to match the
antenna feed-point to the coax. (You may need that anyway
to match the feed-point impedance.) Generally it is easier
to instal high Q inductors at the base of the antenna than
at the top. I have not tried a helically wound loading
coil but I would guess that its losses would also be
appreciable. Alternatively, you could install a loading
coil somewhere around 6 - 8 meter level on the
Spider-pole, where the coild diameter can be still
reasonably large to give you a decent Q (and lower
losses).
One piece of advice with the Spider-pole: make sure that
the section joints are solidly fixed to each other (use
tape and hose clamps), otherwise the sections tend to
slide back into each other as the antenna flexes in the
wind.
Another point: you may gain more by installing a good
ground system than the differences between the various
loading methods!
GL es 73,
George
AA7JV
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