Whenever the question of baluns for RX antennas comes up,
there seems to be a perception that these baluns must
be miniature versions of transmit baluns, consisting
of common mode chokes (coax loaded with ferrite). For
receiving, you do far better with miniature transformers,
such as the ones MiniCircuits sells, like the T1-6.
This will give a much higher common mode impedance
on the low bands than you can get with a common mode
choke. This impedance will simply be the reactance of
the pF or so of capacitance from primary to secondary.
The bandwidth will cover BCB through 6 meters easily.
Concerns have been raised that these
transformers will have distortion if there are strong
signals nearby. I've never seen this, but if you
have a problem, you can make homebrew ones with
BN-70 binocular cores from Amidon. With a true transformer,
you can disconnect either leg of a dipole, and leave
the other one connected, and your receiver will be totally
dead.
Rick N6RK
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