[TowerTalk] Open Wire Lengths and Tuners

[K7GCO@aol.com]

   All this coverage on open wire line and tuners has been jogging my memory 
of  60 some years of use.
  The main problem in tuner loss is high reactance and low Z loads.  The 
reactance is from antennas off resonance and open wire feedline other than 
1/4 or 1/2 WL long or any combination +/-.  Ham bands are not exactly 
harmonically related and an 80M dipole resonates at progressively higher 
harmonic frequencies.  They combine +/-.  You can work it out with the 
formula ft = 492 (N-.05) / F where N = # of 1/2 waves.  
       Feed lines can be cut to what I call a magic length that is a 1/2 WL 
or multiple at all bands very close.  For example a 1/2 WL 80M feed line cut 
for 3.562 Mhz will be a WL long and resonant at 7.125, a 3/2 WL at 10.688, 2 
WL at 14.25, 5/2 WL on 17.813, 3 WL at 21.375, 7/2 WL at 24.938 and 4 WL at 
28.5 Mhz.  This average closeness to center of band operation is high.  It's 
reactive length is usually within MB forgiving distance.  The magic length 
for . 66 VF coax is 91' 2" or multiples.  For open wire line of .95 VF, it is 
131' 2.5" and multiples.  Insulated wire and extra spacers more than every 3' 
will lower the VF more and that's why it's necessary to Grid Dip it as I 
explained in a previous E-mail. They are 1/4 WL at 1.781 Mhz.  On 40M and 
above, intermediate magic lengths can be used.  I'll leave it up to you to 
find the convenient lengths--I got you this far.  I have a chart of all this 
in storage.  With the right switches from the flee market, lengths of open 
wire line can be inserted or taken out to change the reactive loads and the 
Rr value.  Changing feedline lengths will either make it easier or harder to 
load.  The SWR stays the same.  This can be a practical and/or a necessary 
procedure with some tuners.  A tuner doesn't have to be balanced to match 
open wire line.  An L-network can be used with a coiled coax behind the tuner 
or use the toroids.  Connect one wire to the tuner output and one to the case 
ground.  There is another technique of either series or paralleling 
inductance or capacitance across the tuner to reduce reactance and some 
change of Z value prior to matching.  Paralleling them requires only one of 
each that can be switched in or connected with clips.  That's easier than 
changing feedline lengths.  The acid test of efficiency (and power out) is 
the thermometer on top of the tuner and the signal reports.
     A good exercise is to draw the current distribution on all bands of the 
antenna and feedline.  This will tell you if you get a Lo or Hi-Z at the end 
of the feedline and the suggested configuration of the tuner for the best 
match.  The MB eliminates most of this.  There are some tricks of making 
multiband antennas all resonate in the middle of the bands.  I'll have them 
in SD.  Ladder line in the blue sky is beautiful.  You will learn to love it 
and it's low one time cost.  This technology can be fit on a 100x100' lot.  
You don't need to be a farmer to use it.  Everyone with "Creative RF 
Intellect" used it in the 30's&40's.
      Someday I will write a book on tuners and antennas for 100' lots--or 
larger.  It will help you enlarge your Antenna Horizons on a retirement 
budget.  I have some designs that are new and very quick to tune.  I have 
been giving mostly all new material here.
 k7gco
    

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