Removing the first trap, the 10m one, it's also removed its loading effect
and the tubing lenght before of the 15m trap has to be made longer,
practically a 1/4WL on 15m.
On 20m the loading is originally obtained with both 10m and 15m trap but,
having removed the 10m trap, and with a different lenght before of the 15m
trap (that's now the only loading device on 20m) things will change also for
20m and the whip beyond the trap will have to be different than it was
originally.
In the matter of theorical impedances, if the modified element is used as a
vertical against a lossless GP, on 15m it will be that of a simple 1/4 WL
vertical and on 20m it will be less....if the loading device were lossless.
Pracatically speaking, the possible losses in the GP and in the trap (seen
as 20 loading device) will contribute to raise the impedance both on 15 and
20m and the antenna will show an acceptable 50 Ohm match without the need of
any extra device.
As pointed out by Tom in other threads, beware of the voltages that double
in a vertical element (than in a dipole) and that could cause arcing
troubles in some 15m traps, rated in dipoles, at high power levels.
73
Mauri I4JMY
>
> alsopb wrote:
> >
> > I have the driven element of a TH6 here. I want to press it >into
service for a 20/15 rotary dipole only.
>
> I tried that with half a driven element from a TH-3 and never
> could find a resonant point. I actually wanted to use it as a
> tri-band vertical, but I think the impedance mismatch with not
> having a reflector and director messes things up. 73
> Tom W7WHY
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