[AMPS] MFJ-259B Question

[Peter Chadwick]

Goose says:

>But for getting in the ballpark, the MFJ works fine.

All systems based on a directional coupler lose accuracy as the load departs
from the design impedance. This is because  at 'high swr', quite large changes
in load impedance can produce quite small changes in the reflected power. There
was a very good article on this some 18 or so years ago in the Hewlett Packard
Journal, in an article describing a component measuring meter.

It was shown that with a 50 ohm directional coupler system, loads of around
10kohm can give answers between , if I remember, 3k and 30k or something like
that, all depending on the directivity of the coupler.

The same applies to network analysers, of course.

The old GR or equivalent bridges or even the just pre-WW2 Boonton impedance
meter don't have this particular problem - just different ones - like size and
weight and power consumption and no direct read out, and they're not made any
more, and the few that are around tend to be expensive.

73

Peter G3RZP



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