On Aug 19, 5:35 pm, Owen Duffy wrote:
I am not sure whether you are solving a coax problem, or validating the
LP100.

Well, the LP100 and MFJ259B readings are virtually identical. That
gives me some confidence in both instruments. The goal was to tune a
matching section for my loop. Which BTW,
I did and it is working fine. However, the SWR is only reaching a
minimum of 1.6:1 because I didn't get the full 2:1 step down ratio.
Now, I am not going to lose sleep over that, but it seemed like a good
opportunity to understand what happens outside the textbook.


The impedance plot certainly clears away questions about whether you have
properly accounted for velocity factor. The phase plot has a strangely
flattened minimum.

Yeah, I was surprised by that too - but I was pleased to see that - if
it is true. I know the loop won't be bandwidth limited by my matching
section.

I wonder about your confidence in the 54 ohm load, and your answers to
Richard about choking the cable suggests something non-ideal.

Confidence in that it really is 54 Ohms? Well, I wouldn't bet my life
on it, but all my instruments seem to agree that its 54 Ohms. I
choked the cable because it was driving me crazy. No seriously, I was
building the matching section to use on my loop, so you have to slide
those ferrite beads on before you solder on the PL259s. I didn't put
them on because I thought they were necessary when I measured
the matching section, they just happened to be there at the time.

You could try a test with s/c or o/c stub (or both), so eliminating the
54 ohm load. Of course, in such a test, the line loss becomes more
important. It also tests your instrument at extreme mismatch, so you
might want to validate it on o/c, s/c, and some reactive 100:1 loads
using known coax sections.

I measured it with an open end with the MFJ and it had zero reactance
right in the range the LP100 showed it not have any reactance.

I have had good luck with Belden, so maybe I'll order some of that in
75 Ohms and see if its any different.

-Scott, WU2X