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Quad - Single vs separate feedlines

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Subject: Quad - Single vs separate feedlines
From: (Paul Ferguson)
Date: Wed, 1 Jan 1997 21:46:15 +0000
Three years ago, I bought a Lightning Bolt 2 element, 5 band quad. I
decided to go with a quad became of the small turning radius and
reputation for good performance at low height. I mounted it on an
HDBX-48 tower. I think the quad is good value and I was impressed with
the mechanical construction. It performed well on all bands except 10
meters. I use an R-7 vertical for comparison and usually there is a
big difference. On 10 meters I did not notice much difference. The SWR
was a bit over 2:1 on some band edges. I used the preset reflector
stub lengths and never tried any tuning.

This quad uses a single feedline into a 2:1 matching xfmr where the
driven elements are tied together. I made an EZNEC model of the quad
and found the model also showed good performance on 20 thru 15 with 12
meter gain down a bit, but 10 meter performance was very poor. The
gain was low and the major lobe elevation angle was high. I
experimented with the model by deleting elements and found 10 meters
was affected by the 12 and 20 meter driven elements. I tried models
with 2 feedlines -- 20/17/15 & 12/10 and 20/15/17/12 & 10. I also
tried 3 feedlines -- 20/17/15 & 12 & 10. I could not resolve
interaction problems. I decided to rebuild the quad someday using a
separate feedline for each band. There may be an easier solution than
separate feedlines, but I did not find it with the model variations I

Hurricane Fran came thru Raleigh, NC with 80 MPH winds and dropped a
tree on the reflectors. The boom and driven elements came thru
undamaged. Nature eliminated my procrastination on the redesign. Now
was the time.

>From each driven element I used a 1/4 wave length of 75 ohm coax to a
switch box on the boom. From the switch box to the shack, I used 52
ohm coax. The combination gives a match at the antenna for about 113
ohms. I also used about a foot of 1/2 hole ferrite beads on each
feedline connection to the driven elements for a current balun. I
designed the switchbox to keep the feedlines open when not switched
in. The model showed shorted 1/4 wave stubs disturbed the performance,
but open stubs did not.

The model of the redesigned quad shows good performance on all bands.
It shows the 10 meter f/b ratio is poor because of interaction with
the 12 meter reflector. I decided my goal was good gain and was not
concerned with 10 meter f/b.

The new quad on the tower shows good performance on the few 10 meter
openings I have seen. I now see a big difference in comparison with my
R-7. The quad seems to perform fine on all bands. I have not yet tried
to tune the stubs for f/b.


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