Another point I will add -
If you intend to switch directions by moving the feedpoint to
either side of the 90deg phasing line, be SURE to float the shields
of the UNFED side of the phasing line. By this, I mean use a DPDT
relay to select feeding one or the other side (coax inner and
shield) of the phasing line. Do not tie all the shields together.
I had found several years ago that using a aluminum box containing
the relay with SO-239s for each antenna, feedline and the phasing
line (5-connectors) I could never seem to get the FB over about
10-12dB. Using a plastic box and isolating the phasing and antenna
lines (DPDT switching which side of the phasing line was fed)
improved the FB to over 20dB.
Apparently, the shield is as important to generating the phase
delay as the center conductor, and tying the shields together
disturbs the delay line function.
Another small (3dB or so) contributor to a better F/B I found on my
40M 4SQR was to add 9 ferrite beads over the RG-11 coax at the base
of each vertical, as a RF isolator. Evidently, the unbalanced
currents in the feedlines were effecting the pattern or picking up
common mode signals.
73 bill n4lg
At 01:13 PM 9/27/2007, Terry Conboy wrote:
>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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