Topband: 160 Tower on 80

[Herbert Schoenbohm]

Glenn,  If the tower as is, with the top loading on 160 causing it to be 
long for 80 meters for an easy feed,  you can make a simple decoupling 
sleeve skirt and feed the tower with a gamma rod on 80 meters so the RF 
then sees a shorter tower.  There might be some impact on the 160 meter 
feed configuration but most likely that would only some retuning of the 
160 match.  But apart from that the separate 80 meter gamma rod above 
the decoupling sleeve and finding the sweet "point" without the need for 
a trap or a dedicated L/C circuit at the base of a tower that appears to 
long, is one possible solution.  In summary just shorten the tower 
electrically on 80 by a simple decoupling skirt of 4 wires coming down 
the tower and flaring out at the base as you may require. A fixed or 
variable capacitor between the coax feed and the gamma rod to remove the 
inductive reluctance of the rod should be all that is needed.  In 
essence you have then created a coaxial vertical dipole.  At the point 
where the 80 meter feed line leaves the tower near the base, I think 
most would recommend isolating the feed line with a ferrite ring. I also 
remember Earl Cunningham K6SE, (SK) did an article on a variation of 
this some time ago and he seemed pleased with the performance on 80 
meters. Earl, I understand, did not employ a skirt but just put an 80 
meter gamma rod on the tower used on 160.  I think however Earl's tower 
was a bit shorter and yours as described most likely approaches 1/2 wave 
on 80 meters.  A decoupling 4 wire sleeve changes all that.


Herb Schoenbohm, KV4FZ

On 2/28/2015 9:15 PM, Glenn Biggerstaff wrote:
> Hi all, I have a 90 foot Rohn 25 tower with an insulated base and insulated guy wire sections for top loading ,base fed for 160 meters. It work great ,but I would like to use it on 80 meters as well.
> The 3 ideas I have considered are voltage feed  at the base  with a resonant LC network  at the base, but I am a little worried about the voltages present at legal limit power. Second idea,disconnect the top loading and put a trap between the top loading and the tower to divorce the top loading on eighty then an L network at the base for 80. Third idea, run a wire as a sloper either a quarter wave fed against ground or a 1/2 wave dipole from the tower.
> Any thoughts or alternative ideas would be greatly appreciated.
>
> Glenn WW4B
> _________________
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