[TowerTalk] Gamma Match on 6 meters

[K7GCO@aol.com]

In a message dated 6/16/00 11:52:01 AM Pacific Daylight Time, 
abowen@nettally.com writes:<< 
 At 08:31 AM 6/16/2000 -0500, Lee Buller wrote:
 >
 >Going to the Cushcraft web site, you can see how one is built, but it
 >doesn't show dimensions very well.  I understand the construction and the
 >theory behind it, but I do not understand how they come up with the
 >dimensions.  Is there a formula?  Or, is it just try it until it works?
 =================
 There are many sources for gamma match info. One is Hamcalc by VE3ERP.
 Personally, I have found that the hairpin match is easier to adjust, but
 does require a choke type balun. 

*********Not if you use the ORIGINAL WWII DESIGN.  The coax was inserted into 
the middle of the hair pin center where grounded to the boom and run up to 
the tubing to the feedpoint.  The shield connects there and the center lead 
jumps across to the other feedpoint.  The jump is usually short and needs no 
series Xc.  No additional balun needed.  A 55 year+ design has been sorrily 
forgotten.  

In QST about 30 years ago a hair pin match on a Hy Gain tribander used 1/8" 
aluminum wire.  The author run a coax parallel to the 1/8" wire to the feed 
point where the shield was connected and the center lead across to the other 
feedpoint.  The coax shield was connected to the hair pin boom connection and 
center of the hair pin.  Another short length of coax was run on the other 
side just to balance it electrically and physically.  This was a great 
modification not requiring a series Xc that still gave very low SWR on 3 
bands with no changes of hairpin length.  If the hair pin wire was tubing it 
could be run inside it.  That is a very good design only requiring that the 
DE be shortened slightly to get a low SWR as the hair pin lengthened it.  The 
short length of the high current radiation section that is canceled in the 
hair pin has very little affect on the pattern as seen in Eznec.  It does 
require having both ends of the DE insulated from the boom.  I have the Eznec 
Pro version now and I really like it.

K7GCO  


 I did wish to try a gamma, however, based on writings by W1JR, who has much
 more experience at these designs than I.
 
 For my little 2 element using 5/8" and 1/2" tubing, Hamcalc said to use a
 1/4"gamma arm of 9.7" and 30 pf series C.
 
 I put a piece of #12 house wire inside a 1/4" tube and adjusted to those
 dimensions, but failed to get a match. No doubt, my .15 spaced reflector
 resulted in a somewhat higher feeding impedance. I did not record it
 initially.
 
 I then fiddled with arm lengths, wire insertion depth and element length
 until I came up with a good match. The MFJ 259 is indispensible for work
 like this. Dimensions worked out to be:
 
 53.5" half element length
 
 Gamma arm, 1/4 OD = 13"
 
 9" length of #12 house wire inside the 1/4" tubing. This turned out to be
 about 45 pf measured with my Autek RF1.
 
 Two antennas were built to these dimensions using 1" PVC pipe/tees for
 boom, element and mast supports. Both performed quite well at about 22 ft
 (different locations) during the VHF contest last weekend.
 
 Note that with the insulation used, this arrangement could be a tad lossy
 and probably not good for more than 90/100 watts out.
 
 The reflector was 59" (half length) and spaced about 36"
 
 INK N4 >>

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