[Elecraft] Backward balun

[Tom W8JI]

> If you have an mfj analyzer and a 4:1 balun, you should be able to test
> it.
> Just connect up a 50 ohm resistor to the unbalanced side.
> Set the analyzer for the frequency of interest and look at the "R" that
> the analyzer measures.
> Keep the leads going from the analyzer to the balun as short as
> possible.
>
> I wind my own baluns and use this to check them to see how broad banded
> they are.

Since he is stepping down, not up, he would need to measure the balanced 
terminals with the analyzer while terminating the normally unbalanced 50 ohm 
port with 12.5 ohms.

It won't be good, especially at higher frequencies, but some baluns with 
short internal leads will work to some extent mismatched. For example the 
DXE baluns only have about six electrical feet of 100 ohm line, and with a 
4:1 only the upper transmission line is critical for impedance when balance 
is satisfied. In a perfectly balanced load condition the upper core has no 
flux, and the lower core has all the flux behaving as a traditional 4:1 
voltage balun. Only when terminal voltages are unbalanced does the upper 
core have flux.

http://www.w8ji.com/balun_single_core_41_analysis.htm

As for testing, if you do not test with a center tapped load (in the example 
of a 1:1 balun, two 25 ohm resistors in series), and with the split load 
grounded in the center, it is not really a balun test. You much force the 
balanced terminals to be balanced to test how it acts as a balun. With a 
single small resistor you are testing the system only as a transmission 
line, and not really testing any balanced characteristics of the system.

73 Tom