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@gmail.com>
> To: "Tower and HF antenna construction topics." <towertalk@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@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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