Reg Edwards wrote:

How do you select a good choke balun from amongst all the others.
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The important characteristic is common mode impedance. Tie the two input
conductors together and the two output conductors together, and measure
the input-to-output impedance. This can be done in various ways, most
easily with an antenna analyzer, or by other means. The higher the
impedance, the better. 1000 ohms seems to be a good number to shoot for,
with 500 ohms probably being ok for most applications. The angle of the
impedance doesn't matter unless you're running high power, in which case
it might be necessary to keep the impedance more reactive than resistive
to prevent excessive core heating.

A good choke balun for multiple band use will have a high common mode
impedance on all bands on which it'll be used. Narrower band ones can be
connected in series as an alternative.

Roy Lewallen, W7EL