The purpose of
John Doty (email@example.com)
Date: August 24, 1995
Original source: Usenet's rec.radio.shortwave
of a balun is to block common mode waves (current flowing in the
same direction on both conductors of a transmission line) and
pass differential mode waves (current flowing in opposite directions
on the conductors). While some baluns also transform impedances,
others don't: "1:1" baluns are very common.
There are many reasons to want to block common mode waves, but
for receiving the major one is that noise from your house wiring
can easily reach your antenna by travelling up your feedline in
common mode. It isn't necessary to use a balun to block the common
mode, however: with coaxial cable, grounding the shield short
circuits the common mode.
One source of confusion here is that there are a couple of companies
that make devices they call "Magnetic Longwire Baluns", which
are not baluns at all (they don't block common mode", but are
simply impedance transformers.
There's plenty of theory on the impedance of wire antennas: they
are very predictable. Basically, in the complex plane, the impedance
is a spiral that approaches its central point as frequency increases.
The central point is the "characteristic impedance" of the wire,
typically 400-700 ohms depending on the wire diameter and height
above ground. While matching the cable to this point is never
a perfect match, it is almost always good enough for receiving
frequencies from longwave through shortwave, because natural noise
levels are so high that natural noise picked up by the antenna
dominates receiver noise even if some signal is lost due to the
imperfect match. For receiving VHF and above (where the natural
noise is very low), optimum matching is more important, and it
is also more important when you are transmitting at any frequency.
Is a balun
useless on a long wire antenna?
of all, a true balun can help block common mode noise. Secondly,
we're really discussing matching transformers here. A transformer
that matches a coaxial line to about 500 ohms attached to a wire
antenna at least 15 meters long will transmit enough signal to
allow most receivers to reach the natural noise limit from 100
kHz to 30 MHz. A better match yields stronger signals, but does
not improve the signal to noise ratio. With many receivers, stronger
signals are actually undesirable, due to difficulties with overloading.