Topband: 160 Tower on 80

[Tom W8JI]

> 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.
>

Hi Glen,

Every antenna is also a transmission line. Every conductor making up an 
antenna has a surge impedance. That surge impedance, along with several 
other factors, determines the base voltage.

If the conductor is uniform size, lossless, not coupled very well to space, 
and infinitely thin, the voltage at some points along the length can 
extremely high. As it is made thicker or loss is added, voltage greatly 
decreases no matter what the length.

This is why wide broadcast towers, even 1/2 wave tall towers, can have 
reasonably low impedances at the base.

A 90 foot tower against lossless perfect ground, with hat wires to make it 
resonant on 160 meters, has about 2kV peak base voltage at 1500 watts on 80 
meters.  The base impedance is only around 1000-1500 ohms.

This is no worse than voltages typically encountered in traps of trap Yagi 
antennas, so it isn't astronomical. It won't be anything at all like feeding 
a vertical #38 AWG wire for voltage or impedance. :)

73 Tom