[AMRadio] Re: Topband: 160 dipole on 80 & 40


Donald Chester 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.

Don k4kyv





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