To match or not to match - Impedance transformation
One of the most widely used antennas with HAM's, is the simple but effective dipole.
Under normal circumstances you will have some form of balun between the dipole
and your coax, to transform from balanced antenna to unbalanced feedline.
Most new ham's use the so called "ugly balun" which consists of 8-10 turns of
your feedline coax, coiled on a form of some sort. This works as the outer
shield of the coax effectively becomes an inductor which will prevent the
commo mode current from creeping down your feedline.
BUT, and theres a big but in there (no pun intended) this "balun" is not
balanced, and will only equalize the current in one leg of the antenna.
But even more important, it will ONLY do so effectively on one frequency.
The normal way to make a "good" balun is to wind a core with a bifilar winding
about 8-10 turns, and then connect the coax to one side and the antenna
to the other. this is a valid way to do it, and will force equal current in both
antenna legs in a wide frequency spectrum, limited by the core material.
But theres an even better solution available.
the idea is to have a third winding which is used to de-magnetize the core
and give the balun better balance when used with a multiband dipole
like the ZS6BKW i was using at the QTH at the time of writing.
When the dipole is used at a non-resonant freq. with a tuner at the bottom
it put's alot of stress on the balun, with core loss and heat as a consequence.
This balun, wound on a T240-43 core, has an extremely wide load balance
due to the third balance winding.
it made a world of difference at my QTH on the lower bands !
Click either image, or click here to read the PDF about the topic.
if you are a dipole user, it is well worth the read...trust me on that.