On Aug 19, 9:28 am, wrote:
Hello,

I bought some no name RG-59/U coax (RG-59/U-SP-95 Made in the USA). I
went to cut and tune a 1/4WL matching section for 3.8Mhz. I cut the
coax a little longer because I intended to tune it with my MFJ259B
or.LP100 exactly to the design frequency. I have a 50 Ohm dummyload
that actually is about 54 Ohms @ 3.8Mhz. Plugging the numbers into
ON4UN's program for calculating impedance along a coax line: Given the
coax is 75 Ohms and the load is 54 Ohms, the program reported that if
the line was a 1/4WL long the impedance at the source end would be
about 105+j0 Ohms.

However my actual measurement with both the MFJ259B and LP100 showed
an impedance of 74 + j0 Ohms.

http://remote.wu2x.com:8888/lee/quar...-75-meters.jpg

I plugged in a few numbers into ON4UN program and calculated it would
take coax that had a characteristic impedance of 64 Ohms to see the
transformation that I am seeing.

Is there any error in my logic here? If this coax really is 64 Ohms,
then I'd like to find something that really is closer to 75 Ohms so I
can achieve the 2:1 ratio that I intended.I still have another 100
feet of it and can do more tests with the tools I have on hand.

73,
Scott, WU2X

My rule-of-thumb is that I shouldn't be surprised if the actual
impedance of coax is anywhere within ten percent of the nominal
value. A while back I went looking for precision 50 ohm coax to be
used in a test system that among other things does a calibration of
test instruments, and the impedance of the coax really does matter.
Even cables that cost several hundred dollars for a one or two meter
length (that's admittedly with connectors attached...) didn't
guarantee impedance closer than a couple ohms out of 50.

Add to that that I just went looking for typical loss specs for RG-59-
type cable on the web and the first one I looked at lists it as 70 ohm
line, not 75. Although the expected impedance you calculated is for
75 ohm line with no loss, adding a dB loss doesn't change things all
that much, certainly not enough by itself to account for your reading.

Seems like with your impedance analyzer, you could pretty easily find
the impedance and the loss of your line, and use those in your
formulas to see if the performance is what you expect. Have you
measured the line with the far end shorted and with it open? Assuming
accurate measurements, the line impedance will be the square root of
the product of those two measured values.

Then there's also the possibility that your impedance measuring device
isn't all that accurate at some impedances....

Cheers,
Tom