[QCWA] Baluns part 11

[Jeffrey D Angus]

Unless the antenna feed point happens to be purely resistive
at the frequency you want to operate and a value that is an
integer multiple of the feed line you're using, you're going to
require an antenna tuner at the transmitter end.

A couple of good examples.

1. A 1:1 balun feeding a resonant dipole with 50 ohm coax.
2. A 4:1 balun feeding a folded dipole with 75 ohm coax.

The difference between coax and "open wire balanced line"
is the losses involved with the transmission of power from
one end to the other.

Unless you're using a circulator with a resistive load to "trap"
all the reflected power from the antenna, it will be re-reflected
back to the antenna. This works well for VHF and higher
frequencies but is not feasible for HF.

Baluns serve two purposes in reality. First to convert from an
unbalanced source, such as coax, to a balanced load like a
dipole or doublet antenna. Secondly, through an impedance
transformation, 4:1, 9:1 etc, the take the impedance of the
antenna and feed line and bring it down to a value that a
tuner can deal with.

Again, the importance of choosing the correct balun is to make
the two types of feed line appear to be seamless to the tuner.

As an example, suppose you have a distance of 50 feet between
an unbalanced tuner output (i.e., designed for feeding coax between
16 and 150 ohms impedance +/- some reactance). And you have an
Antenna that is going to appear to be between 150-1350 ohms of
impedance.

If you use a 9:1 balun to transition between 50 ohm coax and 450
ohm balanced line, you really CAN put the balun anywhere between
the tuner and the antenna feed point. The only differences you will
see are the losses caused by the coax vs the almost zero loss of the
open wire balanced line.

However, if you choose to use a 4:1 balun between the 50 ohm
coax and the 450 ohm open wire balanced line, the section of coax
is going to act as an additional impedance transformational section
as will the 450 ohm open wire line. At this point, your tuner may or
may not be able to handle the resulting transformed load presented
to it.

Jeff-1.0
wa6fwi