George and others that have responded: Thanks.
George provided the guidance I was looking for. I am still confused though
with the references I have seen to a 'shunt fed tower' where the rod goes
all the way to the top. Obviously a 40' gamma match is easier to implement
than a full height (90' in my case) shunt feed. Is there any benefit to
going all the way to the top?
Thanks again for the help. I have learned quite a bit just monitoring the
reflector, and really appreciate the wisdom of rthe group.
Wayne N1WR
> --------------------------------------------------
> From: "w8uvz" <w8uvz@voyager.net>
> Sent: Saturday, April 24, 2010 10:27 PM
> To: "Wayne Rogers" <n1wr@chesapeake.net>
> Subject: Re: Topband: Shunt fed tower
>
>>
>> Hi Wayne
>>
>> I'm not Herb but I will try to answer since I started this thread.
>>
>>
>>> Herb: What do you mean "sweet point?" This has always been a mystery
>>> to
>>> me.
>>
>> That's the magic point of connection (gamma rod to tower) that will match
>> with series cap to your feedpoint. We normally recommend using an Omega
>> Match with uses two caps; one series, one shunt.
>>
>> Then finding the "sweet spot" isn't so critical.
>>
>> That's the method that the article writer, W4OQ recommended. And he
>> provided a graph to determine the length of the gamma rod needed. I
>> found, with his article and graphing, that 40' rod from the feedpoint was
>> easy to match with the Omega. So I just always used that length and the
>> Omega and it was easy.
>>
>> We build up a "breadboard" Omega match using low voltage air variable
>> caps, put low power to the system until it was resonant. Then measured
>> the capacitance of the breadboard caps and made up the final Omega Match
>> with HV components that equaled the breadboard value. Worked great.
>>
>>
>>
>> I have a 90' tower with a TH-7 etc on top that I am getting ready to
>>> feed. I have heard "feed it at the top (just under the beam)" and I
>>> have
>>> heard 55' up. How does one go about finding the right spot to attach
>>> gamma
>>> rod to the tower? I plan on using 1/2" hardline outer conductor for the
>>> line.
>>
>> The article helps a bunch. But with the Omega mentioned above, 40' would
>> do it.
>>
>> If you can get your hands on the Ham Radio May 1975, its a big help. If
>> you can't get ahold of the article, send me a biz size SASE and I'll mail
>> u a copy.
>>
>> 73 George W8UVZ
>>>
>>> Thanks to all for the good advice on this reflector and 73, Wayne N1WR
>>>
>>> --------------------------------------------------
>>> From: "Herb Schoenbohm" <herbs@surfvi.com>
>>> Sent: Saturday, April 24, 2010 6:20 AM
>>> To: "w8uvz" <w8uvz@voyager.net>
>>> Cc: "Phil LaMarche" <plamarc1@verizon.net>; "Topband"
>>> <topband@contesting.com>; "Kyle N4NSS" <n4nss@yahoo.com>
>>> Subject: Re: Topband: Shunt fed tower
>>>
>>>> w8uvz wrote:
>>>>> Hi Phil
>>>>>
>>>>> There is an excellent article in Ham Radio May 1975 by John True, W4OQ
>>>>> on
>>>>> "shunt-feed systems for grounded vertical radiators". His system uses
>>>>> an
>>>>> omega match for matching the gamma rod (or wires) to the radiator.
>>>>>
>>>>> Using large diameter tubing or multiple wires for the gamma will
>>>>> result
>>>>> in
>>>>> more usable bandwidth on top band.
>>>>>
>>>>>
>>>>>
>>>>
>>>> Using the other solid aluminum shield on discarded 3/4 inch CATV cable
>>>> end runs (called by some 750 or .750) spaced 24 to 36 inches from a
>>>> towers is a significant improvement over a single #8 wire. Yet a
>>>> skeleton cage around the tower and taped at or near the "sweet point"
>>>> of
>>>> 3 or 4 wires is even better.
>>>>
>>>>
>>>> Herb Schoenbohm, KV4FZ
>>>>
>>>> _______________________________________________
>>>> UR RST IS ... ... ..9 QSB QSB - hw? BK
>>>
>>> _______________________________________________
>>> UR RST IS ... ... ..9 QSB QSB - hw? BK
>>>
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
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