[Elecraft] Baluns for KAT1

[Don Wilhelm]

----- Original Message -----
> My feedline from my 40m Inverted V is a balanced line, twin lead
> feedline.  I'm not up to constructing my own, so what do you recommend
> in the way of a commercially built one?  I understand from Elecraft it
> needs to be a 1:4 balun which will change the balanced line to an
> unbalaned one.  Thanks.

Donn,
1) Any proper balun will change the unbalanced output to balanced.
2) Whether a 4:1 balun or 1:1 balun will be better depends on:
   a) the length of the feedline
   b) The frequency of use
   c) the characteristic impedance of the feedline.
   d) the feed impedance of the antenna.

The feedpoint impedance into a feedline will NOT be the same as the
characteristic impedance of that feedline unless it is matched at the
antenna end.

What I am saying is that the transmitter (ATU) end of your feedline can vary
from a very low impedance to a very high impedance - and the main factor is
the length of the line in wavelengths.  Even on 40 where your antenna is
resonant, if your feedline is any multiple of a half wavelength, the
feedpoint impedance will be on the order of 65 ohms (about the same as the
antenna feed impedance), but if it is a multiple of a quarter wave long (and
the feedline characteristic impedance is about 300 to 450 ohms, the
feedpoint impedance will likely be in the range of 2500 to 4000 ohms (quite
high).  In between a quarter and a half wavelength, the feedpoint impedance
will vary between these two values.

Only when the characteristic impedance of the feedline is matched to the
antenna feed impedance will the impedance looking into the feedline be equal
to the characteristic impedance.

You can ignore all I have said IF you have a 65 to 75 ohm parallel feedline
AND you are only using the antenna and feedline on 40 meters - if that is
true, use a 1:1 balun because the feedpoint impedance will be on the order
of 60 to 75 ohms.  In all other cases, one cannot predict unless the length
and type of the feedline are known and the operating frequency is known too.
If you cannot predict, try both a 1:1 balun and a 4:1 balun to see which
works better - and you may have to switch them when changing bands for best
results.

In general, choke type baluns (or current type) work with fewer problems and
losses than the voltage type.  A 1:1 choke balun can be as simple as about
15 feet of RG-58 coax wound as a single layer coil on a 4" diameter piece of
PVC.

73,
Don Wilhelm  - Wake Forest, NC   W3FPR home page: http://www.qsl.net/w3fpr/
  QRP-L # 485   K2  SN 0020   mailto: w3fpr@arrl.net