At 09:14 AM 2007-09-27, Brad Smith wrote:
>Since COMTEK is back-ordered at this time, I'm trying to phase two
>75 meter verticals using the old coax trick. When fed directly with
>50m coax, each antenna resonates at 3.8mHz fine, with good
>SWR. However, when I add the quarter-wave matching section, I can't
>get the SWR down to a low level below 2:1 above 3.6mHz, despite how
>much I shorten the verticals. The phasing line is made of 50 Ohm
>coax cut to the followng formula:
> 246/3.8=64.7 x .66 (VF)=42.7 feet.
>
>What am I doing wrong?
Brad,
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?
If this is all the case, the mutual impedances between the elements
will cause the feedpoint impedances to change dramatically from 50
ohms when both elements are driven.
I modeled the elements in EZNEC and set them up with 14 ohms of
ground loss to bring the feedpoint impedance of each to 50 ohms and
resonant at 3.8 MHz. By driving them with equal currents and 90
degree phasing, and the feed impedances change to 33.82 - J 19.02
ohms (leading phase element) and 64.97 + J 18.92 ohms (lagging
element). The gain is 2.1 dBi at 25 degrees elevation with F/B = 24 dB.
Adding the 50 and 75 ohm phasing lines, the common point impedance is
73.3 + J 29.3 ohms (SWR=1.84) at 3.8 MHz. Of course, this doesn't
produce equal elements currents with 90 degree phase difference. The
phase angle is about 119 degrees and the current ratio is 1.26:1,
giving 2.6 dBi gain and F/B=13 dB.
This is a common problem with simple phasing systems. Read this
article from W7EL for one workable approach:
http://www.eznec.com/Amateur/Articles/Simpfeed.pdf . If needed, the
Simpfeed calculation program is available on his web site on this
page: http://www.eznec.com/Amateur/Articles/index.html
Note that most simple phasing systems don't provide a 50 ohm feed at
the common point, but do provide proper phase and currents to the
elements which maintain the F/B of the array.
73, Terry N6RY
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