amdx wrote:
Is that because you think MFJ is junk, or because
there is no easy relationship?



It's a noise bridge. It works by putting a signal into an unknown tuned
circuit and looking for null points. If the antenna is not connected or
totally nonresonant, all of the signal coming out of the oscillator
goes to the meter. If it is totally resonant, it all goes out the antenna.

Before the MFJ, people used wide range noise generators, and receivers
tuned to the frequency you wanted to measure. You tuned the circut
to get a null in the receiver.

The more sophistocated ones had a variable resistance, you could adjust to
compensate for extra inductance or capacitance in the circuit.

How the impedance meter actually works I don't know, but I will hazzard a
guess. If you were to place a meter at the known resistance, you could get
an indication of impedance, possibly voltage, possibly current flow.

So what they do is calibrate the meter so that it reads 50 ohms at the
center, then place a 25 ohm load and read where the meter is. They do this
at several known resistances, and then make a meter scale. It is not a
precision instrument, so the same scale can be reproduced for the entire
production run.

It's only useful in context of the device it is in, and can't be used
for something else. Again a guess, but mine is that it is less than 10%
accurate (i.e. wider than 45ohms to 55 ohms reads 50 ohms).


Geoff.


--
Geoffrey S. Mendelson, N3OWJ/4X1GM/KBUH7245/KBUW5379