[TowerTalk] Shunt Fed Tower - 160 Meter Gamma Match Question

[W2RU - Bud Hippisley]

Chuck --

I believe your gamma rod is too close to your tower.  

My current gamma rod is seven feet from the nearest side of my 90-ft top-loaded Rohn 45.  That's the result of modeling with EZ-NEC, initially undertaken because I got looking at the gamma match dimensions on my old Cushcraft 4-element 20-meter beam, and wondered if it might not be appropriate to scale the spacing of the gamma rod from the driven element by a factor of eight (the ratio of 20 meters to 160 meters).  With that gamma spacing, my tap is around 50 feet, and I don't need an omega capacitor.

At my previous QTH, I had 125 feet of Rohn 25 with a few feet of 2-inch mast sticking out the top.  Sometimes it had a 3-element KLM 40 on top, sometimes nothing.  My gamma rod was 80 feet total, and only about 2 feet from the nearest leg of the tower, but I don't think my tap point ever exceeded 70 feet.  But my omega (shunt) capacitor was closer to 1000 pF fully meshed, and it usually ran at 1/3 to 1/2 full mesh.  Initially it didn't work as shown in the omega match drawing from either Paul Lee's book or Bill Orr's book (I can't remember which one), but when I switched the top end of the omega capacitor to the "wrong" side of the series capacitor, I was able to get a match.

In both installations, all guys were insulated within a couple feet of connecting to the tower, and I had/have around 20 radials of varying length.

Bud, W2RU


> I tried Bud, W2RU's suggestion of switching the shunt capacitor, but it made 
> the receive level lower.
> 
> I moved the gamma tap to 70 feet. I connected the center conductor at the 
> top and the shield at the bottom of the coax gamma "rod" I am using, to 
> increase the series capacitance.  That increased the receive level 
> substantially.  I can hear all kinds of signals early in the evening.  The 
> background noise is S-9 on SSB.  This is as good as the receive gets on a 
> vertical here, but I am not getting a match.
> 
> Do I need to "bite the bullet" and move the tap up to 90 feet like Larry, 
> K4AB says?  Or has someone used more capacitance with these approximate 
> heights?  I feel I am close due to the good receive.
> 
> Chuck W5PR
> ----- Original Message ----- 
> From: "Larry K4AB" <larry.k4ab at gmail.com>
> To: "Tower and HF antenna construction topics." <towertalk at contesting.com>
> Sent: Monday, February 15, 2010 10:50 AM
> Subject: Re: [TowerTalk] Shunt Fed Tower - 160 Meter Gamma Match Question
> 
> 
>> Chuck,
>> 
>> I've installed a lot of 160 shunt feeds.  I would start by raising the 
>> gamma
>> attachment point.
>> 
>> Usually 2/3 the height of the tower would be fine. In your case 90' should
>> work well.
>> 
>> GL!
>> 
>> 73,
>> Larry K4AB
>> 
>> On Mon, Feb 15, 2010 at 10:35 AM, Chuck <w5pr at swbell.net> wrote:
>> 
>>> I am trying to shunt feed a tower for 160 meters.  I have a 135' Rohn 55
>>> tower with several good sized antennas on it.  Guys are insulated.  I 
>>> have
>>> run an old piece of coax up 35' about 2' off the tower for a gamma.  I 
>>> have
>>> about a 150 pf variable in series and 200 pf to ground.  Ground is 2 -140
>>> foot radials, elevated 10'  (In the past I think I have used larger
>>> capacitors for this.)
>>> 
>>> I do not get any match on the series variable with 200 pf, 100 pf and 0 
>>> pf
>>> to ground.
>>> 
>>> What is the easiest thing to do now?
>>> 
>>> 1.  Shorten the radials?
>>> 2.  Increase the series capacitance?
>>> 3.  Increase the capacitance to ground?
>>> 
>>> Is my gamma too short?
>>> 
>>> Thanks!
>>> 
>>> Chuck W5PR
>>> _______________________________________________
>>> 
>>> 
>>> 
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