[FoxHunt] gamma-matched quad antennas

[Dale Hunt, WB6BYU]

  After doing a lot of experimenting with EZNEC it appears
the simplest solution (presuming this is actually a problem
for anyone) is to use a T-match instead of a gamma match.
A good starting point is to use the same dimensions as the
gamma match - basically move the gamma rod down to where it
is centered on the midpoint of one side of the quad loop
and use twice the capacitance from each end to the driven
element that you used at one end of the gamma match.  Then
split the rod in the middle and use series feed with a
choke balun.  Another approach might be to duplicate the
gamma match going the opposite direction and feed them 
using a 4 : 1 balun.

   For the K0OV 4-element quad from the transmitter hunting
book the calculated dimensions are 3 3/4" for the spacing
between the points on the driven element and 11.5pf for
the capacitors.  At least those should give a good starting
point for further adjustment.

   Using direct feed without a gamma or T match the SWR on 
this antenna is less than 2 : 1 at 146.5 MHz, and it gives 
almost a perfect match at 148 MHz.  The W4RNL 3- and 4-element 
quads mentioned in earlier discussions are also suitable for 
direct feed with 50 or 75 ohm coax.  Generally speaking, 
direct feed will usually give a more balanced pattern as any
sort of match hanging on one side of the loop will shift the
resonant frequency, like adding a capacity hat.  This leads
to having more effective current on one side of the loop 
than on the other, and the resonant frequency will change
depending on where the loop is fed.  On the EZNEC plots I can
see a slight shift in some of the minor lobes, but there is
no noticable azimuth skew using either the gamma or T match
using 1 degree measurement intervals.


   My next step is to try modifying one of my 2-element quads
to use the T match and see how it works.  If I discover any
surprises I'll post it here, but otherwise I've probably taken
up enough bandwidth and those wanting to discuss it further can
contact me by direct email rather than on the list.

      - Dale  WB6BYU