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Thank you for the detailed answer. Below are my comments.

An electrical generator is most efficient when the load impedance(1)
is the same as the generator impedance. That is when it can deliver
the most power.
......
However, I was not talking about impedance matching - which is the job of a
transmatch(tuner) - but rather about coupling a balanced and an unbalanced
segment. Some people - like myself till recently - confuse the impedance
matching with the balun coupling. I agree that a balun can sometime do both,
but its main purpose is to couple balanced and unbalanced segments.

The leadin coax + radio has a set impedance, as does the antenna(3).
The 'balun' or 'unun' is the means used to match these two items via
a ratio of windings.

Nope. There are baluns (1:1) which are not designed for impedance matching.

So, I made some research and I got at least a partial understanding :

In a dipole, the electrical potential swings symmetrically above(+) and
below(-) the "ground level"( 0 ) - that's why is called a "balanced" system.
Both poles of a dipole are "hot", in the sense that they generate/receive
useful RF signal - of course out of phase.

By contrary, a coax has the shield set to 0 potential (grounded) and only
the inner wire is "hot" (carries signal). That is why is called an
"unbalanced" line.

If a coax is used to feed the antenna then the coax has only one "hot" wire
(the inner one). The outer shield is "grounded", hence is "set to 0". If a
balun is not used to couple the antenna with the coax, then one pole becomes
RF inert, since is grounded by the coax. In this case, the dipole antenna
becomes a poor inverted L antenna, since only one pole acts as an antenna.
In case that the coax is not grounded, then it is equally bad : the feed
line becomes part of the antenna and it degrades its performance.

The purpose of a balun is to change the phase of the currents delivered by
the dipole such that it creates an additive effect and feed the inner wire
of the coax with added signal from both poles. So basically, the balun
"collects" the signal from both poles, adds them up and pump them through
the inner wire of the coax. It does so through a transformer coupler. For
example, if the dipole is polarized (+x volts, -x volts) , then the balun
feeds the coax with (0 volts, +2x volts). As a side effect, the balun
isolates the antenna and the line such that the line does not influence the
operation of the antenna.

Please keep me honest - tell me whether I got it right.

Yours cordially,

Nic.
http://www.csd.uwo.ca/~nic





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