What else is in the vicinity (within 10 meters) that is metallic?
As an aside, there is a lot of anecdotal stuff that points to models
creating numbers that come out low when doing things with wire. I've
had that experience myself.
Insulation will make the wires seem longer due to velocity factor. End
effect will make them seem longer yet. Especially at 21 MHz, the two
together could drop the frequency significantly.
Don't use PVC, it's quite lossy AND has a worse velocity factor. PE or
Teflon insulation only if you must use insulation.
I would try PE/Teflon/bare first, and see what happens.
Also do not loop the end of one element through the end of another
element using an egg insulator, or use wire to support/go between two
ends using egg insulators.
The issue with end effect is that the end has a measurable capacity to
the rest of the world. Insulators usually require looping the wire
through something and then back onto itself, making the capacity at
the end to the rest of the world a lot higher than the theoretical end
of a wire suspended by magic in a model. Since this part of the
antenna is a very low current, high voltage place, a little capacity
has a lot of effect. The percent effect on element resonance frequency
increases with the square of the frequency.
If you can determine the center of performance (combination of forward
gain and f/b and resonance), adjust the driven element shorter by the
% that the frequency is low. (This is NOT a cut and measure for best
SWR deal.) Note the actual length removed (not the percent).
For each parasitic compute the same percent against the parasitic's
length, and AVERAGE that with the actual amount of the driven element
length adjustment.
The absolute length adjustment for end effect at a given frequency is
the same regardless of the overall length of the element. While the
adjustment for insulation is proportional to the length of the
element. That's why the average as a guesstimate for parasitic
elements.
73, Guy.
_______________________________________________
Antennaware mailing list
Antennaware@contesting.com
http://lists.contesting.com/mailman/listinfo/antennaware
|