[Elecraft] To Balun or not to Balun?

[Don Wilhelm]

Jack,
It is a common misconception that the balun ratio should match the ratio
between the characteristic impedances of the two lines.  That is ONLY true
if BOTH of the transmission lines have loads equal to their characteristic
impedance.

When the load impedance on a line at the far end is not equal to its
characteristic impedance, the inpedance looking into the near end of the
line can vary from a very low value to a very high value - depends on the
length of the line, the actual load, etc.  This is true not only for
parallel line, but is also true for coax line.  This is partly what SWR is
all about.  In general, a designer will chose a length of low loss parallel
feeder to run from the antenna for a specified length.  The reason is to
make the impedance looking into that line at that point equal to 50 ohms, so
you can connect a coax line at that point (either with a balun or direct)
and the coax line will have a low SWR - but the parallel line certainly will
have a much higer SWR on it.  A transmission line cannot act as an impedance
transfprmer unless it is running with an SWR higher than 1:1!!!

Additional note: SWR is not the same as common mode current  - in the case
above, the balun is not needed for impedance matching, but is only needed to
isolate the coax shield from currents flowing on the outside of the coax.

If one thinks about it a bit, this fact should be obvious. Consider the
properties of a 1/4 wave transmission line (quarter wave transformer) - if
it is shorted at the far end, the impedance at the near end will be high,
and when the far end is open, the near end will show a low impedance.  Only
when the load at the far end is equal to the line characteristic impedance
will the near end show an impedance equal to the impedance of the line.

As a result, you may have better luck with a 1:1 balun, or a 4:1 balun, or a
9:1, or some other ratio entirely - it depends on the length of each line
segment, the frequency of operation, and your willingness to tolerate some
level of SWR on the line.

73,
Don W3FPR

----- Original Message -----

I'm in the process of gathering materials for a 1.5 to 2.0 wavelength
80-meter, horizontal loop for my K2-100. The articles that I have read leave
me confused on one issue--well probably more than one--but is it necessary
or desirable to place a balun  1:1 or 4:1 or even 9:1 at the juncture of the
RG8 from the K2-100 to the point where the 450 ohm ladder line begins?

The ladder line will terminate at a corner of the proposed loop?  Some older
QST articles just say feed the loop with 50 ohm coax and let the antenna
tuner take care of any mismatch. Other sources recommend a balun but seem to
think even a 1:1 ratio is OK. Wouldn't a 9:1 balun be ideal to match the 50
coax to the 450 ladder line? I'd like to optimize the system for best
signal.