Jason Dugas wrote:

I'm interested in measuring the velocity factor of some coax I have (more of
an exercise than necessity). In order to do this, the MFJ-259 Operations
manual states that the "stub" to be measured should be attached with a
50-ohm noninductive resistor in series to that center conductor of the
Antenna connector on the analyzer.


You may think it says this:
MFJ-resistor-coax_center_conductor--
|
---------coax_shield------------

But maybe it means this:

MFJ----------------coax_center_conductor-resistor-
|
----------------coax_shield--------------------

I don't know what the MFJ documentation says - but if
they want a 50 ohm resistor in series from the center
conductor and the shield, put it at the far end of the
length of coax you are testing. Connect it as shown in
the bottom diagram by soldering the resistor directly
across the center conductor and shield at the far end.

The way I do it is to cut a physical 1/2 wavelength
coax for the frequency, install a pl259 on one end and
connect it to the MFJ. I install a 51 ohm resistor at
the far end of the coax. When that resistor is 1/2
wave away *electrically*, it's impedance will appear
across the PL259 plugged into the MFJ. You need to
shorten the coax by snipping off some of the length
at the far end, then reinstalling the resistor.
When you have the thing at an electrical 1/2 wave
you'll get Z=50 at the MFJ. Your VF is the length
of that piece of coax divides by the original length.

Maybe the MFJ documents a better way - I dunno - but
I've used my way successfully.



What would be a good way of making this connection? I've thought about it
quite a bit; the best idea I've come up with so far is to have a small metal
enclosure w/ two SO-239's mounted. The resistor would go from center
conductor to center conductor of each SO-239. But this requires too many
extra connections & lengths. Is there a better way to do this? Would love
to see anyone else's experimental setup, particularly if there are pictures
or details on the web.

Thanks & 73,

Jason
KB5URQ