|[AMRadio] Re: Topband: 160 dipole on 80 & 40|
k4kyv at hotmail.com
Tue Sep 13 20:31:35 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.
I never had any problem with that configuration, although my basic antenna
was 1/2 wl on 80, not 160. I used about 60 ft of feedline, and the antenna
worked well on 80, 40 and 20. What I liked about the arrangement was that I
was able to use a parallel resonant tuned circuit for each band, because the
impedance was always high, yet almost purely resistive on all bands. The
tuning capacitor was a 200 pf per section split-stator and I loaded the
transmitter into the tuned circuit via a 3 or 4-turn coupling link. To
change bands I used plug-in coils, although a tapped coil arrangement would
have worked if I had gone the trouble of wiring up a hv ceramic bandswitch.
I simply swapped out the coil for whatever band I was on, and set the tuning
cap according to a chart that I made up by trial and error, and it worked
like a charm on all bands.
My present dipole is halfwave resonant on 80m, and is fed with a halfwave
feedline, so it presents a low impedance at 80m, but high-Z on 40, still in
each case almost a purely resistive load. I don't use it on 20, but have
pressed it into service on 10m. Now, I use a separate, pre-adjusted tuner
for each band, so when changing bands I simply switch in the appropriate
tuner and I'm already tuned up. The 80m tuner uses series tuning, and the
one for 40m, parallel tuning. The 80m tuner has a split midway on the coil,
and the split in the coil directly feeds the open wire line. The tuning cap
is the same 200pf/section one I used with the earlier tuner. On 40, the
open wire line is bridged across the coil/capacitor. The cap in this tuner
is a dual 150pf/section.
I can even use the antenna on 160 as a 1/4wl dipole, although it is very
critical to tune up, and has a bandwidth of less than 15 kHz before I have
to readjust the tuner. I switch in about 1/8 wl additional feeder length to
make it work. This also is a high-Z feed, with parallel tuning. On that
tuner, I use a dual 300 pf cap with an additional 50pf fixed vacuum strapped
across the tuned circuit.
I avoid antenna + feedline lengths that terminate midway between a high
voltage point and a high current point on the feedline. Such a length
presents a highly reactive load that is very difficult to tune to resonance
while maintaining an efficient transfer of power, hence the switchable
additional 1/8 wl of feedline. When I first tried the antenna at the odd
1/8 wl, I could arc over a 7 kv tuning capacitor with only about 200 watts,
and although I could get the swr down to unity by tapping down the coil to
attach the feeders, an rf ammeter in one side of the feedliner indicated
substantially less current than I got at that same spot, running the same
input power, with the additional 1/8w switched in, and simple parallel
tuning. Ever since, I have avoided odd 1/8 wl's like the plague.
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