[Elecraft] More KAT500

[Don Wilhelm]

There is a common error in assuming that a 4:1 balun should be used with 
450 ohm (or 300 ohm) transmission line.

The impedance at the shack end of the line has nothing to do with the 
characteristic impedance of the line, and the feedpoint impedance may 
actually be quite low.  If you have a 5 ohm feedpoint impedance, a 4:1 
balun will bring that down further to 1.25 ohms producing an impedance 
that most any tuner will find difficult to match.

Bottom line - do some analysis to see what feedpoint impedance you are 
really dealing with.  If you cannot model the antenna or if you did not 
measure the length of the feedline, you can at least measure what you 
have with an antenna analyzer.  If the impedance on all bands of 
interest is high, then a 4:1 balun may be appropriate, but if the 
impedance is lower than 100 ohms, then a 1:1 balun will produce an 
easier load for the tuner.

73,
Don W3FPR

On 11/15/2012 7:31 AM, Jim Wiley wrote:
> Using those voltages as limits, that indicates that the KAT500 could
> handle power levels of about 250 watts at a 14:1 SWR.  Any more could
> result in damage.  I would suggest replacing the 300-ohm line on the
> antenna with 450-ohm ladder line, and inserting a 4:1 balun between the
> antenna and the KAT500.  I suspect that this will reduce the SWR
> excursions, and may get you "inside the envelope" for what the KAT500
> can handle.
>