gamma.art

一个很小和天线计算程序

SUBJECT:  Gamma modification for better symmetry & gain

       This subject has been discussed before on the HAM echo and will be 
further discussed here with more detail.

       Most gamma matching devices are mounted to the side of the driven 
element.  This technique is good for matching, but does not provide good 
pattern.  The symmetry of the antenna pattern is distorted to the side of the 
antenna that the gamma match is located on.  Several antenna brands were 
tested with all brands tested showing the same result.  The pattern produced 
by the side mounted gamma is kidney shaped.  This shaping accounts for the 
fact that you can hear a signal on one side of the beam and not the other.  
The side to side ratio is not consistant.  In the case of horizontal beams, 
the gamma hangs down, wasting gain and affording a loss of signal reception 
that could otherwise be used.

       What would it take to move this lost antenna gain and sensitivity into 
the area of the pattern that could be utilized?  Not a whole lot in fact.  The 
entire modification can be done in under a half an hour and for less than a 
dollar.  What results is that the wasted gain to the side of a vertical beam 
is moved forward and the wasted gain under a horizontal beam is moved into the 
horizontal plane forward.  The average gain increase is near 1.5 dB over the 
previous gain rating of the antenna.  What this means is that if your antenna 
produces 10 dBi it will now produce 11.5 dbi.  Your Cushcraft eleven element 
yagi rated at 13.2 dBi will now produce 14.7 dBi.  13 dB is equal to a gain of 
20, 16 dB is a gain of 40, so you can see the improvement. 

       To accomplish this, you will need to remove the gamma match from the 
antenna.  Remove the di-electric innards and replace with a piece of RG-8
foam di-electric about 3-in longer.  Pivot the tuning arm 90 degrees so that 
it faces the first director.  Insert the gamma back into the tuning arm.  Now, 
solder to the center pin of the coax feed point.  If your coax feed point is a 
threaded section, crimp or solder on the appropriate fitting, then tighten the 
nut.  You will need to retune the antenna at this point.

       Fairly easy?  The results are incredible.  Increased gain, better 
symmetry, better side to side and front to back ratios.  This modification has 
been used here for over a year.  When I turn the beam to the side of a signal, 
the results blow away any commercial beam.  My nine element yagi can keep up 
with the commercial eleven element antennas.  Try it out.  I'm sure you will 
enjoy the change.

-WS