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Re: Topband: unipole

To: Glen <>
Subject: Re: Topband: unipole
From: Herb Schoenbohm <>
Date: Tue, 15 Sep 2009 13:17:35 -0400
List-post: <">>
Glen wrote:
> Hello,
> I would like to speak with anyone running a unipole on 160M.
> At first, it seemed resonant on 4.03Mhz and 7.3Mhz. 160M is a mystery right 
> now...added
> about 25 feet to
> each wire run on a piece of PVC drain tile up at 70' point.  Seems to find
> resonance now about
> 3.65Mhz and 6.6Mhz.......  No resonance that I can
> find on 160M......I suspect that my tower feed lines may be a source of 
> trouble.  I have all
> the 7/8" hardline
> and rotor cable coming off at the 40' point and traveling down a guy wire 
> towards the shack.
> Glen....Your tower may be to long at RF for 1.8 resonance.
With a 120 foot tower and beam(s) on top you might have enough top loading that 
the tower structure appears to long for 1/4 wave 1.8 Mhz resonance.  You need 
to also isolate the 40 foot ABG side connected wires whatever they are, and 
look to match the structure with the cage feed regardless of the actual 
resonance frequency which is probably below 1.7 Mhz. Also bring your cables to 
ground or create chokes to isolate them at RF.  A 40 foot drop off point will 
really make things very problematic. Also you should not worry that much about 
actual resonance of the vertical tower but rather look for a proper match if 
the physical height is certain and the offending rotor cable and coax are 
isolated. If you are using a cage feed I would look for a much lower point 
(between 35 to 55 feet) to connect the cage to the tower (the connection at the 
top can remain)to find the "sweet point" so the bottom of the cage will be 
close to the feed point impedance of 50 ohms if coax fed.  There wi
 ll be some inductive reactance in the cage which can be removed by a series 
capacitive of equal value.  If you want to save some trips up and down the 
tower you might want to consider an Omega match...or a CLC matching network 
that has a great range of impedance transformation. Remember there is no magic 
in resonance but rather getting a good efficient transfer of power to a metal 
vertical tower.  If matched properly at the feed point, I doubt if there is 
much measurable signal difference to worry about between a 100 foot 
(non-resonant) cage fed tower or a 134 foot one that is  "resonant".

Hope this helps,

Herb Schoenbohm, KV4FZ

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