> Finally, if you want to see if a balun is working, one simple crude
> test is to monitor the VSWR while running your hand around and along
> the feedline (outside the antennas immediate feed). Large changes in
> VSWR indicate poor balun decoupling. The simplest way to feed small
> Yagis is to bring the feed line from the driven element back behind
> the reflector and use rear mounting.
I've seen this done before.
If you test for common mode isolation by moving you hand along a
cable, most baluns would look very good. Even very poor baluns.
The only way to tell if a balun is working or not is to either
measure feedline radiation, or to measure common mode feedline
73, Tom W8JI