[HCARC] Best Info On Balun's I have seen

[Kerry Sandstrom]

Gary,

What Kurt N. Sterba  says is correct, however, I would like to put a little 
different slant on it.  A dipole in free space is a balanced antenna and 
should be fed by a balanced transmission line or the transmission line will 
radiate and distort the radiation pattern.  That is absolutely true.   Not 
many of us have our antennas mounted in free space.  For 80 and 40 meters in 
particular, the antennas are seldom more than a quarter wavelength from the 
earth.  They are definitely not in free space.  Typically they are not even 
symetrical: one end will be higher than the other, one end may be near a 
building or in trees, paret of the dipole may run over a driveway which has 
steel mat buried in the concrete, etc.  The typical dipole as installed by 
hams is not balanced.  Many of the dipoles I've seen are not even in a 
straight line.

There is a second way that feedlines pick up RF currents and reradiate them 
and distort the radiation pattern.  That way is if the feedline is in the 
radiation pattern of the antenna.  For a dipole in free space the feedline 
should be brought out perpendicular to the dipole and at its center.  If the 
dipole and feedline are truly balanced, there will be no pickup of the 
radiation by the feedline.  Take a look at most ham dipoles.  Most hams run 
the feedline however they have to and it's seldom perpendicular to the 
antenna for the quarter or half wavelength it should be.  No matter what 
most hams do, there feedline will pick up some of the RF and reradiate it.

So where does that leave us?  Our dipoles are seldom symmetrical and 
balanced.  Our transmission lines are in the radiation fields of the antenna 
and will pick up and reradiate thus distorting the already distorted 
radiation pattern of the dipole.  Because of its proximity to ground the 
radiation pattern is already distorted.  We have little control of most of 
these factors.  I wouldn't and don't worry about feeding a dipole with an 
unbalanced coaxial cable.

Note that this still doesn't address the problems of nonlinear ferrites and 
high power RF.

Now lets look at history for a couple minutes.  In the early days our bands 
were all harmonically related; 160, 80, 40, 20 and 10 meters. If you put up 
a half wave dipole for 160, all the other bands were an even harmonic of 160 
so the dipoles were a multiples of a full wave dipole on all the other 
bands. The impedance of a full wave or even multiple of a full wave antenna 
is several hundred Ohms.  It makes sense to feed this ntenna with a 450 Ohm 
open wire line.  The only band where you have a real mismatch is on 160 
where every ham antenna is a compromise.  Hams used antenna tuners which 
were balanced to match to their transmitters.  None of this really applies 
to a modern ham station.

The last paragraph you quoted from Sterba is something you have to be very 
careful with.  A transmission line has a characteristic impedance.  for most 
coaxial cable it is 50 or 75 Ohm.  For parallel conductor line it is 
typically between 75 and several hundred Ohm.  If your system is matched, 
that is the impedance of the load, the antenna, is the same as the 
characteristic impedance of the transmission line, no matter how long the 
line is, the input impedance of the line terminated in the matched antenna 
will be the characteristic impedance of the line.  If the characteristic 
impedance of the transmission line is not the same as the load impedance, 
then the input impedance of the line will depend on the length of the line. 
Whenever the line is an integral multiple of a half wavelength long, the 
input impedance of the line will be the same as the input impedance of the 
antenna.  Whenever the line is an odd integral multiple of a quarter wave 
long, the line will act as a "quarter wave transformer" and the input 
impedance of the line will be the characteristic impedance of the line 
squared divided by the load impedance.  For a 300 Ohm line and a 50 Ohm 
antenna impedance, the input impedance of the line is 1800 Ohm, a challenge 
to match!  For othe line lengths the impedance will be between these two 
extreme values but it will have a large reactive part which is also fun to 
try to match.  The infamous Smith Chart is the easiest way to find out the 
input impedance given the line length and antenna and characteristic 
impedances.  The impedance of the antenna is not always easy to get.

Bottom line:  In my opinion for a practical low HF dipole a balun adds 
nothing.  For VHF and higher it is important but not for HF.  The most 
practical transmission line is one that is as close to the antenna impedance 
as possible which to me suggests 50 or 75 Ohm coax.  Any remaining 
mismatches can be handled by an antenna tuner in the shack.  Once you get 
something working you can make changes and see if they improve things or 
not.  But only make one change at a time!

Kerry