RadioBanter - View Single Post - How much can the impedance of coax vary from its characteristic impedance?

Richard Clark · August 20th 07, 01:33 AM posted to rec.radio.amateur.antenna

On Sun, 19 Aug 2007 20:40:44 -0000, wrote:

You asked if the load is shielded. I was using a MFJ dummy load and
the resistor is inside a metal enclosure. Not sure if this mets your
shielding criteria.

I did have twelve #31 ferrite beads (1.125"X.0.5") on both the input
and output end of the cable, so I can say for sure that aspect was
taken care of. I didn't notice any changing values whenever I would
move the coax around or grab onto either.

Hi Scott,

That seems adequate to the task of differentiating any problem of
common mode intruding into the measurement.

Another easy test would be to simply frequency sweep the terminated
line until you found the 100 Ohms you expect. This may take you well
out of the Ham bands. When you do find 100 Ohms (without reactance)
you can then ratiometrically adjust the length to the required
frequency (which would suggest your problem was probably misjudging
the velocity factor of the cable).

73's
Richard Clark, KB7QHC