Topband: 160 dipole on 80 & 40

Richard (Rick) Karlquist (N6RK) richard at karlquist.com
Tue Sep 13 11:33:26 EDT 2005


> The worse multiband length combination is an electrical 1/4
> wl feeder or multiple used with a 1/2 wl antenna on the
> lowest band. If you have a 160 dipole, you want to avoid a
> "ladder line" feedline near 90-110 feet long.
> 
> The best combination is about a 1/8th wl (or odd multiple)
> feeder, so the best feeder length with "ladder line" would
> be about 50-60 feet or 150-180 feet with your 160 dipole.
> 
> 73 Tom

This is because on 80m, you have a full wave dipole with many
1000's of ohms drive impedance driven through a half wave line,
which puts this same impedance at the tuner.  I don't know of
any commercial tuners that can deal with this.

An alternative to changing the line length is to shunt the 
line with a doorknob capacitor of several hundred pf.  This is
usually easier than changing the line, assuming you can get
the capacitor.  This feature ought to be built into tuners,
but I've never seen it.

Rick N6RK


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