ABOUT BALUN
May be baluns are still a mystery
for hams; the only way to understand it is learning what it is and how to use it.
The word balun means balanced-unbalanced: it's used to adapt a balanced device to an
unbalanced one; in a balanced device (as larger type of antennas) we have on both terminal
the same voltage respect to the ground (if not so it's an unbalanced device);
a dipole with direct feed is balanced, a coaxial cable is unbalanced.
So, when we connect a balanced device to an unbalanced one the following occurs:
we have a dipole with coaxial cable direct fed (i.e. RG213); normally, transmitting, there are 2 currents on the cable:
I1, which flows trough the central wire of coaxial cable and from transmitter goes up to the dipole;
I2, which flows (for the skin effect) on the inside part of the copper shield;
The two currents, equal and
opposite, humble itself and we have no radiation from coaxial cable.
The two currents cames on the dipole to be irradiated; part of it cames back; the one on
the shield cames back trough the external side of the shield (no more the inside: so we
have 2 current on the shield, I2 and I3); the value of this current (which we'll call I3)
depends to the impedance value of the external side of coaxial cable respect to the ground
(in a word if it will find high or low resistance);
if impedance will be high, I3 will find high resistance and its value will be low; if
impedance will be low, resistance will be low and I3 value will be high; in this way I3
will radiate RF and the external side of the coaxial cable will radiate as a third wire of
dipole: it's as we have a dipole with 3 wire (see picture 1); as consequence the radiation pattern will
be distorced (see picture 2);
the big problem is that this new wire is often near TV and telephon cables irrading
directly on those: so we have more probabilities to cause TVI.
This new wire, when its impedance is low, change the dipole impedance so we can have high
S.W.R; this is the reason why (without balun) varying the coaxial cable length the S.W.R.
changes.
Let's see now why use a balun; to delete this current I3, we need an high impedance for RF
on the external side of coaxial cable; so we have high impedance casually for some cable
length (odd multiple of 1/4 lamda) or using a balun: so the first reason to use a balun is
TO AVOID THAT ON THE EXTERNAL SIDE OF COAXIAL CABLE FLOWS A CURRENT I3.
The simpler balun is a coil with some coaxial cable turns just belowe the antenna feed
point: this inductance make the cable impedance (we mean the external side of cable
shield) higher so that the RF current will find an high resistance and its value will be
very low (that will not disturb I2 and I1 which flow inside the coaxial cable).