GR Bridges were standard equipment for anyone working on AM broadcast
antennas. They are often still used. Probably the most commonly used
instrument today is the Delta OIB-1 or OIB-3 Operating Impedance Bridges.
Portable, accurate and easy to use, they also require a source of
excitation. They have a built in detector, though it is common to use a
more sensitive external detector.
They can be driven by a signal generator or directly by up to a 5KW
transmitter (up 50KW with external adaptor).
Their main claim to fame is the ability to measure complex impedances in
a directional antenna system where the "cold" Z is very different from
the "hot" Z due to mutual coupling in the array.
I had a problem matching my 160 tower due to mutual coupling to a second
tower only 36 ft. away. My MFJ analyzer said everything was fine, as
would a GR Bridge. The Delta OIB gave very different measurements.
After I detuned the second tower, the MFJ and the OIB agreed.
Since many hams use more than one tower, they may encounter similiar
problems.
73...Don, K9NR
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