Topband: Spiderbeam pole

[GEORGE WALLNER]

On Fri, 09 Jan 2009 08:04:23 +0000
  Paul Ormandy <zl4pw at 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