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

On Sun, 19 Aug 2007 16:28:32 -0000, wrote:

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

Hi Scott,

Unless both ends are completely shielded (which is to ask: Is the load
a shielded load?), then you need to choke things.

A quick test would be to take the load end of the line and coil it 8 -
12 turns around a 6 inch cylinder. Does this change your impedance
reading? You will need this choke for your antenna anyway, so plan on
something like it.

73's
Richard Clark, KB7QHC

Richard,

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.

-Scott, WU2X




On Aug 19, 1:20 pm, Richard Clark wrote:
On Sun, 19 Aug 2007 16:28:32 -0000, wrote:
However my actual measurement with both the MFJ259B and LP100 showed
an impedance of 74 + j0 Ohms.

Hi Scott,

Unless both ends are completely shielded (which is to ask: Is the load
a shielded load?), then you need to choke things.

A quick test would be to take the load end of the line and coil it 8 -
12 turns around a 6 inch cylinder. Does this change your impedance
reading? You will need this choke for your antenna anyway, so plan on
something like it.

73's
Richard Clark, KB7QHC

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.

How long is the coax (in feet)? Are you sure you didn't actually cut
a half wavelength?

Tor
N4OGW

How long is the coax (in feet)? Are you sure you didn't actually cut
a half wavelength?

Which wouldn't make sense either, then you should get 54 ohms.

Tor
N4OGW

wrote in news:1187540912.064738.6170
@g4g2000hsf.googlegroups.com:

Scott,

The inconsistency you describe seems to lie in one or more of your
instrument, the coax, or your test setup.

I am not sure whether you are solving a coax problem, or validating the
LP100.

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

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

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.

Back to the loss thing, the line loss calculator at
http://www.vk1od.net/tl/tllc.php uses a more sophisticated line model
than any other calculators I have seen, and contains data for a hundred
or so of the most common commercial lines. It also calculates the
expected impedance transformation for the line.

For example, it suggests that a 6m length of Belden B9204 (RG59/U type)
o/c line at 3.8MHz has an input Z of 0.84-j84.91.

BTW, have you stripped a bit of cable down to see visually if appears
physically sound. Common problems I have encountered have included
unevenly woven braid, unevenly laid strands in centre conductor (though
yours should be solid), un-centred centre conductor, air inclusions in
the dielectric.

As I said, I am not sure that the LP100 has been validated and the focus
is entirely on the particular coax.

Owen

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

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

Most of the important points have been well covered, but let me add a
general observation:

Loss will skew any measurement toward the characteristic impedance of
the cable. This is intuitively evident, since if the cable is lossy
enough, you'll see its characteristic impedance at the input regardless
of what's connected to the other end.

So when you say you're seeing 74 ohms when you should be seeing 105, I'd
bet without further evidence that the cable has more loss than your
model thinks it has.

Roy Lewallen, W7EL

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