[QCWA] Baluns part 9

[Jeffrey D Angus]

I have mentioned he importance of using the correct balun
ratio between different impedance feed lines.

Use of a Smith Chart is a valuable tool to show exactly
what happens between the load and the source with
various lumped reactive or either series or shunt pieces
of feed line of various impedance.

As an example of how to use one based on a previously
cited example of a feed line combination.

6' of RG-213 4:1 balun and 50' of 300 ohm balanced
line.

For just one single frequency:

One wave length is 130.9'  at 7.150 MHz.
With a velocity factor of 66% for the coax that would
be 86.4'. and with a velocity factor of 98% the open
line would be about 128.2'

The open wire line looks like 140 degrees of transmission
line and the coax looks like 25 degrees of transmission
line.

The model for a Smith Chart then would be:

50 ohm source (center) -> 25 degrees of 50 ohm line ->
140 degrees of 75 ohm line -> load. Note: 300/4 = 75.

Even if the antenna looked like a perfect 300 ohm load,
the impedance present to the source (tuner I assume)
would be an SWR of 1.5:1 and look like 60 -j20.

To make it appear as a 1:1 50 ohm load to the tuner, the
antenna feed impedance would need to be close to
260 +j100 (A 260 ohm resistor in series with 2.2 uH coil)

The point I am trying to make here is that the most effective
transmission line is where it appears to be one continuous
impedance from end to end.

And that is where the proper selection of a balun comes into
play.

Jeff-1.0
wa6fwi