I modeled my 97-foot Rohn 25 with three yagis on it, and the result
seemed to indicate that 50 ohms plus some XsubL was to be found at about
50 feet. I tried a single-wire shunt there, spaced about 30 inches from
the tower, but the series capacitor would not give me even a rough
match. I tried just below the first yagi, at 69 feet, and it was not
usefully better, so then I put it back down at about 50 feet and used an
omega match. With that and a radio at the bottom of the tower (BCI
makes my MFJ unusable on 160), I was able to get it tuned in less than 5
minutes, with a 2:1 SWR bandwidth of about 70 KHz.
Aside from requiring a second capacitor, I don't see any serious
downside to the omega match.
73, Pete N4ZR
The World Contest Station Database, updated daily at www.conteststations.com
The Reverse Beacon Network at http://reversebeacon.net, blog at
reversebeacon.blogspot.com
On 4/25/2010 8:31 PM, Jon Zaimes AA1K wrote:
> Wayne,
>
> I've usually found the 30-40 ft. height works fine if the tower is
> electrically 1/4 wave or less. But if it it is longer than 1/4 wave the
> longer shunt is needed.
>
> 73/Jon AA1K (/1 in Maine this week)
> www.aa1k.us
>
> On 4/25/2010 10:10 AM, Wayne Rogers wrote:
>
>> 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
>>
>>
>>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
>
>
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
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