[Elecraft] KAT 500 and open wire

[Don Wilhelm]

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.
>