Here is a simple coax to dipole antenna transition that does not work well. (Click to enlarge)
At high frequencies the proximity and skin effects kick in and confine to the current to the surfaces of the conductors. (Click to enlarge)
A simple picture of the currents flowing through a coax. (Click to enlarge)
Feeding a dipole antenna directly from a coax will cause common mode current to flow on outside of the
shield and cause problems. With RF, you always need to be aware of the "hidden schematic". (Click to enlarge)
A current balun (aka "common mode choke") can common mode current from flowing on outside of the shield. (Click to enlarge)
One solution is put a common mode choke right at the feed point between coax feedline and dipole antenna. (Click to enlarge)
Common mode chokes are pretty great and make you carefully think about the rule "Current follows the path of least impedance"). (Click to enlarge)
Sleve choke (bazookah balun) and Pawsey stubs are possible solutions. (Click to enlarge)
Equivalent circuit of sleeve choke (bazookah balun). The quarter wave shorted stub stops current in its tracks. (Click to enlarge)
Produce radiation by forcing a common mode current. (Click to enlarge)
Let's make the outer shield current work for us. Two simple examples are
flower pot antennas and sleeve dipole antennas (Click to enlarge)
Flower pot antenna potpourri (Click to enlarge)
The ubiquitous sleeve dipole is a thing of beauty (Click to enlarge)
Someone made a sleeve dipole using a pepsi can (Click to enlarge)
