At 08:55 AM 2007-09-28, Brad Smith wrote:
> >You left out a few specs on your antenna system. Are they separated
> >by 1/4 wavelength? Do you have 1/4 wavelength 75 ohm lines connected
> >with a coax-T and an additional 1/4 wavelength of 50 ohm line
> >connected between one antenna and one of the 75 ohm lines? Did you
> >tune each element with an open circuit at the feedpoint of the
> other element?
>
>The antennas are spaced 1/4 wavelength are are fed with equal length
>lines of 50 ohm
> coax and a 1/4 wave 50 ohm line is inserted in one line. Should
> I have used 75 ohm?
The idea of the 75 ohm lines is to raise what might be a 50 ohm feed
to 100 ohms so you can parallel the two feeders and get somewhere
near 50 ohms again. However, as the model shows, given the effect of
mutual coupling, things aren't be quite that simple.
How long are the two equal length feedlines and what velocity factor
are they? I'll run my model and see if the SWR is similar to what
you see. You may be able to pick a feeder and phasing line length
that minimizes the phase, current and SWR errors.
BTW, if I recall correctly, the Comtek box uses a 90 degree hybrid
with a dump load. That makes the SWR look better at the expense of
losses in the dump load. The effect of mutual coupling on the
driving impedances of the two elements is still an issue and phase
and current errors can still result.
73, Terry N6RY
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