|[AMRadio] 50 ohms simple question|
bbruhns at erols.com
Thu Apr 5 17:59:02 EDT 2007
I'm not sure why, but we usually don't use a wideband transformer for transmit, except in solid state transmitters. We do use
adjustable-tap and adjustable coupling tuners to match impedances for transmitters, though. In my experience, I got the best
results by tuning these transformer-like things to the operating frequency. I think the tuning is necessary because of the
relatively poor coupling between large coils - resonant tuning improves the coupling.
In tube equipment, the usual pi-network was able to adjust to a wide range of impedances, which simplified life by making
additional impedance transformation unnecessary. However, large deviations from the design impedance (50 ohms) results in high
voltages or high currents, so this is convenient but not ideal. Also these designs are unbalanced.
But there are 4:1 and I think 6:1 baluns out there, etc These are wideband transformers, and they transform impedance and
change from balanced to unbalanced as well.
A balun is not an absolute necessity, either. A lot of stations just center-feed a half-wave dipole with coax. This works, but
there is some RF on the coax shield and this can be a problem. With a self-resonant half-wave dipole, I have found that if I
connect a coax feedline about 6% off-center, with the longer leg going to the shield and the shorter leg going to the center
conductor, the feedpoint is at an RF zero. I call this zero-point feed.
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