From: gary@ke4zv.uucp (Gary Coffman)
Newsgroups: rec.radio.amateur.misc
Subject: Re: Tuning Open-wire fed dipo
Date: 27 Aug 92 14:58:47 GMT
Reply-To: gary@ke4zv.UUCP (Gary Coffman)
Organization: Gannett Technologies Group

In article <714856965.F00001@tdkt.kksys.com> Claton.Cadmus@f100.n282.z1.tdkt.kksys.com (Claton Cadmus) writes:
>Richard writes:
>
>RI>I have a 130 foot long dipole, center fed with about 80 feet (very
>RI>approximately) of this common brown insulated "open wire" feedline.
>RI>I use it on 80 and 40 meters and my problem has to do with the tuner
>RI>and the way the antenna tunes.
>
>RI>of the coil.  On 40 meters it tunes and works beautifully.  I get good
>RI>bandwidth, both caps dip the swr smoothly, and the signal gets out
>RI>like Gangbusters.
>
>RI>On 80 meters, the only thing I change is the taps on the coil from the
>RI>feedline.  At the high end of the band, all works well, but as I QSY
>RI>down into the 3700 range I get screwy results.  The dips in the swr are
>RI>not sharp, the position of the taps has little effect, and when I go
>RI>down to 3600 or below it will not tune at all.
>
>I believe the reason for your problem is the antenna becomes resonant
>at about 3600Khz.  The requirement to match the antenna no longer exists
>and your tuner will not do a 1:1 resistive match.  On forty meters this
>antennna is a full wave diople and presents a very high impedence that
>the link coupled matching unit is designed to accommodate.

The requirement to match the antenna to the source still exists, but the 
impedance that must be transformed has drastically changed. This is one of 
the special cases where changing feed line length can be helpful. Since
the feeder is not matched, it can be used as a transmission line transformer 
to get an impedance the tuner *can* handle. Of course that could screw
up operation at 40 meters. Or, you can simply use a different configuration 
tuning network designed to handle the lower impedance presented by the 
antenna/feedline combination. You can't just throw the matchbox away.


>Suggested solutions:
>
>1. Add 10-20 ft of wire to each end of your dipole.  This may be
>difficult but will solve your problem.  If the dipole is 80 ft up these
>wires could simple hang.  As an alternative you could install two small
>loading coils at the center feed point, one in each side.  Something
>like ten turns of insulated #14 wire wound on 2-3 inch dia. PVC forms.
>This has the effect of electrically lenghtening your antenna.

This will work too, but loading coils near the feedpoint are in a
high current node on 80 meters and would be lossy. Better to add
length at the ends. This will also screw up 40 meter performance.

>2. Don't use the tuner in the lower portion of the 80 meter band.  Feed
>it through a 1:1 or 4:1 balun transformer.  You may need to try both.
>Don't worry about the 450 ohm feedline.  The important thing is to
>"transform" the unbalanced output of the transmitter to the balanced
>feed of your antenna system.

This might work, if your transmitter has an adjustable output network.
But if it's the typical no tune rig, it'll probably still complain
and go into VSWR foldback limiting. 

What you really need to do is redesign your tuner to use split stator
capacitors that will allow it to match over a wider impedance range
without excessive Q. What's wrong now is that the L/C ratio has gotten
out of bounds of good matching network design.

Gary KE4ZV