[TowerTalk] supergain

[K7GCO@aol.com]

In a message dated 30.01.00 10:52:01 Pacific Standard Time, rick@area.com 
writes:
 
 A quick note here on what the theoreticians say about "super-gain"
 which they call "super-directivity".  This discussion assumes lossless
 elements and matching networks (for now).  The most gain you can get
 additively is a power gain of N (in dB 10 log N) for N elements.  This
 occurs for spacings of around 1/2 wl.  When you go to closer spacing,
 for an optimum driven array, the maximum achievable gain goes up, due
 to subtractive gain.  Unlike the Yagi principle that gain is proportional
 or whatever to boom length, the driven array optimized for maximum gain
 just gets more and more gain as the spacing is decreased.  This is no
 theoretical limit to this gain.
 
 Now when we leave the theoretical world and get back to real world antennas,
 we find that you can actually only get 1 or 2 dB of super-gain before
 the bandwidth, ohmic losses, and tolerance sensitivities eat you alive
 as the theory predicts that it will.  Conventional Yagi designs can also
 get this kind of super-gain, so the decision to use an all driven array 
 would have to be justified on some other basis that gain. 

======With all driven arrays, they tend to have great bandwidth so I say it's 
worth a try and it has been done.
    Another problem is that if you maximize gain, you don't get low sidelobes.

 =======I have a 10M 3 element yagi tuned up in Eznec of .15R and .2D WL on a 
12' boom tuned for max gain. I got 10.12 dBi and 8 dB F/B, 6 ohms Z and 
narrow bandwidth.  The E&H-planes are 54 and 72 degrees.  I have no side 
lobes,   Perhaps I would with a 4 element peaked for gain.
      Since I couldn't find any 6 ohm surplus coax, I determined in the 
computer that 50 ohms could be found about 6.5' from the center.  I used the 
"K7GCO No RF Spill Over Matching Device Configuration."  I cut an oblong hole 
in the center side of the DE and ran the coax down the center of the 3/4" 
tubing about 6.5' where the shield connected.  The center lead jumped a gap 
to the remaining 1.5' that was supported by an insulator.  I connected a 1/2 
WL of coax inside and connected the other end to the MFJ Analyzer at 28.4 
MHz, I fined tuned the feedpoint position and end tips for 1:1.  
    The job is not over.  There is another procedure that no one else seems 
to do.  I place my hand on the center of the DC where secured to the boom 
electrically.  If the SWR changes any, the DE is not electrically balanced 
and will have the Dreaded RF Spill Over--a poor design.  I adjusted the end 
tip so that my hand placement had no affect raising it off 1:1.  I then 
connected the ground of the MFJ to the center of the DE and it's still 1:1.  
That should be done more often.  I have a Polamar RF Meter that clamps on the 
coax to measure RF Spill over and there is none in the shack running a KW.  I 
got the pattern in Eznec on the air and there are no side lobes.  The Dir. is 
2" longer than the DE.  It's a hot beam on receive and transmit.  Reports 
have been great.  It's just 1/2 dB down from my 5 element on a 28' boom--over 
twice as long.  I don't think there is any super-gain here.  No one ever 
tunes it this way.  Although my feed system is very unique, it could be fed 
with a gamma or T.  There is 13 amps in the center of the DE.  It's a 
continuos length in the center so there are no joints.  
     Would you believe that I made a long transmission one time with the 
speech processor on and the DE drooped down like an inverted vee.  This is an 
absolutely true story--I just made it up.  
    I have about 150 KHz bandwidth, it blows them away on the back also and 
helps to keep the frequency clear.  When I call them with this beam--they 
stay called.  I have a way to broadband it now real easy.  It's a great beam 
and have not seen this gain configuration used before and no other beam with 
my ZERO RF spill over feed system.  It would be great for 18 and 12M without 
the extra broadbanding.  
    Question: Would you say this is in the so-called super gain range or is 
that only obtained with closer spacing?  I'll try it.  I can send you the 
patterns.

 It is very easy to illustrate these principles with EZNEC.  Just build a
 four element array with say .1 wl spacing and drive it so that the 
 elements almost but not quite cancel each other out.  The gain will be
 surprisingly high for a .4 wl beam.  The bandwidth will be very narrow
 and the currents will be very high.
 =========Are all element driven or is it a parasitic? 

 Rick Karlquist N6RK
 www.karlquist.com


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