Don,
I guess I should have pointed out that I was assuming one of those
wide-range, balanced tuners of days of yore. I have one of those
plug-in coil tuners I built long ago, and it will match almost anything,
so feedline length is seldom an issue. Alas, the commercial tuners
available today have much smaller ranges, so one does have to ensure
that the impedance is in range, both of the tuner and the balun.
73,
Scott K9MA
On 1/6/2019 16:44, Don Wilhelm wrote:
Scott,
As I indicated, that is NOT true of just any length of window line.
Some will work while others will not.
With the G5RV, the only thing we know for sure is that on 20 meters,
the impedance at the end of 34 feet of 300 ohm feedline is
approximately 75 ohms.
The impedance at that point will not be the same for other bands, and
that is why a tuner is required.
Once you get RF onto the radiator, it will be radiated. One real
question is just how much loss do you encounter in the feedline. The
other real question is, how much trouble do you have matching the
shack end impedance to the 50 ohm output requirement of the transceiver.
In days of old when we used plug-in coils in the transmitter and tuned
the output tank to resonance, we could tap off those PA tank coils to
match most any impedance. That capability moved into the antenna
tuner with the advent of 50 ohm only transceivers - the first antenna
tuners used balanced plug-in coils which could handle both balanced
and unbalanced feedline, as well as low to high impedance depending on
where you placed the tap(s) on the coil.
The problem with those tuners is that they were not conducive to rapid
band changes - you had to plug in coils for a different band.
The Johnson Matchbox was a great asset to band changing and the proper
taps on the coils, but its matching range was much more limited than
those old-fashioned plug-in coils which provided a very wide range of
matching impedance.
73,
Don W3FPR
On 1/6/2019 5:15 PM, K9MA wrote:
The really attractive thing about open line, and even window line, is
that its loss is so low you can use it at a very high SWR, and take
care of the matching at the station end. This is especially useful
for multi-band antennas. The famous G5RV, for example, will work just
exactly the same fed with window line and a tuner as it will with the
usual matching section.
--
Scott K9MA
k...@sdellington.us
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