80 / 160 semi-long-wire end feed

[W4DNR]

A question about end-fed wires against a metal tower as a counterpoise.


The Rohn 45 is grounded but insulated from the guy wires.
If I go half way up the  Rohn 45 tower @ 70 feet and feed a wire out to a neighbor's tree at 50 feet up .... apx 140 feet away,
I could cut the wire at 60 feet and have a end-fed 1/4 wave 80 meter antenna
or if I extend out to 120 feet, low impedance on 160 but crazy high on 80.

Is there an intermediate length that would not be so high impedance on either band, or should I just go for a trapped end-fed 1//4 for both bands ?

I'm looking for a cloud burner on both bands.

Thanks

Don W4DNR

[WBear2GCR]

you could have both.

put a trap at the resonant point for 80m.
it's a parallel L & C forming a notch filter, making a high-Z at the freq of interest,
which is the same as cutting the wire, so out of the range of the notch, it's just a small series L... so the RF "sees" the whole length.

               _-_-

[N1BCG]

That arrangement is more complex than it may initially seem. The tower isn't going to present a low impedance ground at your feed point. In fact, this will have similar characteristics to an off center fed dipole where one radiator is 70' and grounded at the end. At 80 meters, that would be near where the voltage loop (peak) is.

Perhaps modeling software can reveal the patterns and feed point results for 80 and 160. In any case, a very interesting project!

Another possibility, if you have a good ground system around the base of the tower, would be to shunt feed it via a series capacitor. There's a 50 Z point somewhere up that tower, incredibly.

[W4DNR]

you could have both.

put a trap at the resonant point for 80m.

Wonder how a shorted 1/4 wave of open line would work at the end of the 80 meter section ?

I am using an 80 meter top-fed  sloper  against the tower .... more vertical than horizontal and it works well out several hundred miles.

My neighbor offered a tall tree and now I can run wires horizontal...  more possibilities.

I'm converting a Broadcast transmitter to 160, so I'm trying NOT to create  sparks and arcs above the neighborhood with a high impedance feed point.


Thanks Guys !!

Don W4DNR