> Hi Gerald,
> You can use a Delta Electronics OIB2 HF Operating Impedance Bridge to
> measure the input Z under power. Would need a constant
> carrier level for
> the meter to null easily.
> 73
> John
> K5PRO
OIB's are quite handy. I have an OIB3 and always wanted an OIB2.
You can also do high-power swept Z measurements using a conventional vector
network analyzer (i.e. one without an integral test set) combined with an
external amplifier and directional coupler. We use this arrangement for
testing and tuning broadcast antenna systems (antennas, lines, combiners,
etc.) where there can be several watts of RF coming back toward the test
equipment from other adjacent broadcast stations in proximity or on the same
combiner.
Ron Rackley wrote a paper covering this for AM broadcast antenna system
measurements, where a high-power source is often necessary to overcome all
of the RF coming back down the line from other AM stations, and likewise to
isolate the test equipment from such damaging levels of RF - Google it for
details. It also includes design details for a directional coupler for the
AM BC band with notes for an alternative version to cover 3-30 MHz with a
claimed directivity of better than 40 dB.
--- Jeff WN3A
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