[TowerTalk] Auto tuners, Baluns and open wire feeders
grants2 at pacbell.net
Tue Jul 17 22:37:28 EDT 2018
Just my 0.02. Count me as a fan of fan dipoles as the way to go.
A good antenna SYSTEM is a different criteria than a simple antenna when
a specific set of goals such as noise, gain, pattern, and reliability
Tuners and open wire feeders are not simple. Obviously tuners are not
as reliable as coax. Open wire feeders if punched web ladder line are
notoriously off spec, fragile, and often lossy. True discrete insulator
open wire line is superior in all respects but is rarely commercially
available. Noise pickup can be problematical since keeping the line
perfectly balanced and antenna also balanced is quite difficult, so
noise and pattern distortion happen. As far as I know there is no
practical balun possible at the feedpoint on open wire transmission line.
If an 80m capable OCF or G5RV or the proverbial doublet are used, then
yes the wire is simple. However, the swr and pattern are essentially
uncontrolled for multiband use. Then an open wire feeder and tuner are
required, not simple and not low noise and not balanced in almost all
installations. A tuner at the feedpoint is elegant, but not simple and
outside maybe not reliable. And QRO outside tuners are NOT cheap. btw
QRO on OCF etc gets to amazing voltages.
So with somewhat more wire complexity in the same space as the
"doublet", an 80/40/20 fan dipole can be placed, fed with coax and well
choked at the feedpoint. Now there are predictable patterns, low noise,
low swr, low losses, balanced, and simplicity and reliability of no
tuner. 15 is within the capability of a tuner rig or amplifier,
depending on how 40m is cut in the band. I found that 12 and 10 swr's
were pretty much swamped by the feedline, 130' of RG213 and they were
also tunable by the rig. However patterns on those bands start looking
like starbursts. and swr losses increased. OTOH judicious feedline
lengths can provide some matching and won't matter on the fundamental
At my new QTH, I put up two "all band" webbed ladder line fed 80m
dipoles up at 65' NS and EW, with a tuner (eventually fried it). 20 and
up pattern was junk, gain, and S/N not so good everywhere. Replacing
them with RG213 fed well choked 80/40/20 fans were a revelation - real
patterns to null QRM and noise, better gain or S/N, and many more DX
contacts. That was the proof for me. Still use them for field day.
Previously, before I knew better I melted the coax in a purchased bead
choke on a G5RV. As K9YC said, there is some band that a G5RV (or other
OCF) will blow up any choke.
IMO, resonant antennas win.
On 7/17/2018 15:27 PM, jimlux wrote:
> On 7/17/18 3:13 PM, Jim Brown wrote:
>> On 7/17/2018 1:50 PM, jimlux wrote:
>>> Yes, you need a choke on the coax at the tuner (which is what forces
>>> the above statement - you don't want currents going down the outside
>>> of the coax).
>> A choke at the tuner is NOT equivalent to a choke at the feedpoint
>> (although a choke both places is not a bad thing).
>> The length of feedline between the antenna and the choke IS PART OF
>> THE ANTENNA, so noise picked up on that length of line is coupled to
>> the antenna, then comes back down the line (as a differential signal)
>> to the receiver.
> I agree - I was assuming the "feedline" is short (single digit feet)
> and reasonably balanced (so that environmental noise would couple
> equally to both sides). And, of course, the tuner itself is sort of
> an asymmetric thing, and if it has a conductive box (or a ground plane
> under the PWB inside a plastic box), then noise could couple in that
> way too.
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