On 2/28/2015 5:15 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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I have a 90 foot top loaded vertical made from irrigation
pipe instead of R25. Should be similar electrically.
I have done the voltage feed method on 80 with it, and
it works perfectly. You just have to have the HV components
and be comfortable with designing a simple matching
network. If you were using tuned elevated radials on
160, this would make sense because it would relieve
you of having to have a set of 80 meter radials.
If you have normal ground radials like me, you might
want to consider a relay to switch out the top loading
wires. I currently have an insulated joint at the
60 foot point on my vertical with a relay across it.
When the relay is open, I have a 1/4 wave 80 meter
vertical. In your case, you would have a 3/8 wave
80 meter vertical, that you could tune to resonance
with a series capacitor.
For details on the relay, see my H&K article:
May 2009 - QST (Pg. 66)
Increasing Relay Voltage Handling Capacity
You should put a string of 10 meg resistors
across the relay to drain static charge.
Rick N6RK
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