Topband: Vertical Question

[Rick Karlquist]

Jim Murray wrote:
>
> Any 160 meter vertical can also be driven on 80 meters as a voltage
> fed antenna.  You just need a different matching network at the base.
> You don't need a trap.  Traps only become necessary if you want
> to operate over more than a 2:1 frequency range.
>
> Rick N6RK
>
> Rick,
> Can you or anyone else elaborate a little on this.  The matching network
> to use my 160M Inv. L. on 80.  I presently have a 2:1 UNUN at the base and
> all is well.  Not quite sure, with the exception of running another wire
> for 80M, how I can match it for 80.
> Best 73,
> Jim

In general, a vertical or inverted L, with or without top loading,
that is resonant on 160 meters with a low (<50 ohm) input impedance
will also be resonant on 80 meters, but with a high (>1000 ohms)
input impedance.  To match this high impedance to 50 ohms, you
need a shunt reactance from the antenna to ground of about 200-300
ohms and a series reactance from the antenna to the center conductor
of the 50 ohm input connector that is also about 200-300 ohms.
These can consist of either a shunt C and series L, or shunt L and series
C.  You have to determine the exact values experimentally.
To cover the whole 75/80 meter band, you will need to be able to
vary the shunt element.  The series element should work OK at
one value for the whole band.

I have a top loaded 90 ft vertical that is resonant on 160 meters
w/o a loading coil.  It is also parallel resonant in the middle
of the 80 meter band and I can drive it with a big air variable
and a big air core coil as a matching network.  It works well.

Rick N6RK