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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