Topband: Fw: Shunt fed tower

[Jon Zaimes AA1K]

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 at voyager.net>
>> Sent: Saturday, April 24, 2010 10:27 PM
>> To: "Wayne Rogers"<n1wr at 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 at surfvi.com>
>>>> Sent: Saturday, April 24, 2010 6:20 AM
>>>> To: "w8uvz"<w8uvz at voyager.net>
>>>> Cc: "Phil LaMarche"<plamarc1 at verizon.net>; "Topband"
>>>> <topband at contesting.com>; "Kyle N4NSS"<n4nss at 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
>
>