[Antennaware] Phased Verticals

[Terry Conboy]

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