Topband: unipole

Herb Schoenbohm herbs at surfvi.com
Tue Sep 15 10:17:35 PDT 2009


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 will 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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