Well ***Opps****. I took out my contacts already. And I read the cable
Zo as the impedance at the input end. As you can see it says 102.11-
j3.74. I am still going to search out some new RG-59/U...

-Scott, WU2X

Sorry I don't have the time to dig into this more deeply right now. I'm
sure Owen has done a great job in estimating loss, but here are some
things to think about:

1. Belden 9204, like a lot of other 75 ohm cables, has a copper-plated
steel center conductor for strength. At 3.8 MHz, depending on the copper
thickness, current might be entering the steel. If it is, the loss will
be a lot more than a simple model for solid copper would predict. I
notice that the statement at the bottom of the data you posted says
"Loss model source data frequency range 10.000 - 1000.000 MHz". You're
well below that. A good reason for a lower limit on the model would be
not accounting for current penetrating into the steel.

2. Some common RG-59 type cables have stranded center conductors and tin
plating. Both increase the loss. More importantly, stranding results in
much thinner copper for a given percentage of wire diameter of copper
cladding.

3. A logical way for a cable manufacturer to cut costs is to put a
thinner copper cladding on the center conductor. This would have no
effect on the performance at VHF and above, where the cable is most
likely to be used. So thin copper wouldn't surprise me.

The only way to really know the loss is to measure it. And this might
not be the reason for any apparent error. But it might be. As Tom said,
though, 10, or even 15% deviation from nominal isn't unusual.

Let me relate a story. Years ago, I came across a very large surplus
quantity of approximately 0.1" diameter 75 ohm cable. It was just before
Field Day, and because it looked to be in good physical condition, I
measured off 100 feet, put a couple of BNC connectors on it, and tossed
it into the pack as feedline for the 40 meter antenna. (I backpack my
gear on Field Day, so weight is a major consideration.) Field Day went
ok, but it was one of those years when we were just at the other
stations' noise level, requiring a lot of repeats, QRZs, etc. Afterward,
my FD partner was saying that all we needed was another 2 or 3 dB gain
on 40, and we'd do a lot better. I agreed. Not too long afterward, I was
measuring the impedance of a folded dipole through a half or full wave
of that 75 ohm coax (since I had a lot of it), and was getting bizarre
results. And that's when I first learned of the importance of cable loss
on impedance transformation. I had been assuming lossless cable for my
calculations of load Z given input Z, but got suspicious that loss might
play a role. When I modified by equations to account for loss, I was
surprised at how much difference even a little loss made. (As it turns
out, loss makes more difference when the load Z is far from the cable
Z0, as it was in this case, than when they're about the same.) I put
more and more loss into the formula until I got about what I expected
for load Z, given the input Z I was measuring. 4 dB at 7 MHz! A quick
measurement with the wattmeter confirmed that the cable did indeed have
that much loss. The problem was the thinness of the copper cladding on
the very small steel center conductor strands. Even though the cladding
was a substantial portion of the wire diameter, it was still very thin
because of the tiny wire diameter. At our next sked, I told my FD
partner that I'd figured out a way to get a couple more dB out of our 40
meter antenna. . .

Roy Lewallen, W7EL

Roy Lewallen wrote in news:13cid39fvvdsk28
@corp.supernews.com:

....
1. Belden 9204, like a lot of other 75 ohm cables, has a copper-plated
steel center conductor for strength. At 3.8 MHz, depending on the
copper
thickness, current might be entering the steel. If it is, the loss will
be a lot more than a simple model for solid copper would predict. I
notice that the statement at the bottom of the data you posted says
"Loss model source data frequency range 10.000 - 1000.000 MHz". You're
well below that. A good reason for a lower limit on the model would be
not accounting for current penetrating into the steel.

Roy, the reason I show the freq range on which the model is built is for
exactly the case you are discussing. It makes it clearer when the model
is an extrapolation, and confidence limits should be wider.

One of the things that I have done when doing the regressions on the
source data is to discard low frequency points that have high error wrt
the regression. This effect occurs almost intirely with CCS centre
conductor type cables.

Not all RG59 and RG6 type cables have CCS, and the worry with low cost
CCS is whether the coating is even thinner than the 9204.

I use RG6 quite a bit for ham work, and the cable I buy uses a HDC centre
conductor. I would avoid CCS for lower HF.

Back to the original problem, it would take a huge loss to deliver an
input impedance of just under 75 ohms from a 54 ohm load and a quarter
wave of nominal 75 ohm line. The Zo looks low.

Owen

PS: The quoted output from TLLC is somewhat hard to read due to the Greek
characters and some other symbols not copying to plain text.

Roy Lewallen wrote:
Sorry I don't have the time to dig into this more deeply right now. I'm
sure Owen has done a great job in estimating loss, but here are some
things to think about:

1. Belden 9204, like a lot of other 75 ohm cables, has a copper-plated
steel center conductor for strength. At 3.8 MHz, depending on the copper
thickness, current might be entering the steel. If it is, the loss will
be a lot more than a simple model for solid copper would predict. I
notice that the statement at the bottom of the data you posted says
"Loss model source data frequency range 10.000 - 1000.000 MHz". You're
well below that. A good reason for a lower limit on the model would be
not accounting for current penetrating into the steel.

2. Some common RG-59 type cables have stranded center conductors and tin
plating. Both increase the loss. More importantly, stranding results in
much thinner copper for a given percentage of wire diameter of copper
cladding.

3. A logical way for a cable manufacturer to cut costs is to put a
thinner copper cladding on the center conductor. This would have no
effect on the performance at VHF and above, where the cable is most
likely to be used. So thin copper wouldn't surprise me.

The only way to really know the loss is to measure it. And this might
not be the reason for any apparent error. But it might be. As Tom said,
though, 10, or even 15% deviation from nominal isn't unusual.

Let me relate a story. Years ago, I came across a very large surplus
quantity of approximately 0.1" diameter 75 ohm cable. It was just before
Field Day, and because it looked to be in good physical condition, I
measured off 100 feet, put a couple of BNC connectors on it, and tossed
it into the pack as feedline for the 40 meter antenna. (I backpack my
gear on Field Day, so weight is a major consideration.) Field Day went
ok, but it was one of those years when we were just at the other
stations' noise level, requiring a lot of repeats, QRZs, etc. Afterward,
my FD partner was saying that all we needed was another 2 or 3 dB gain
on 40, and we'd do a lot better. I agreed. Not too long afterward, I was
measuring the impedance of a folded dipole through a half or full wave
of that 75 ohm coax (since I had a lot of it), and was getting bizarre
results. And that's when I first learned of the importance of cable loss
on impedance transformation. I had been assuming lossless cable for my
calculations of load Z given input Z, but got suspicious that loss might
play a role. When I modified by equations to account for loss, I was
surprised at how much difference even a little loss made. (As it turns
out, loss makes more difference when the load Z is far from the cable
Z0, as it was in this case, than when they're about the same.) I put
more and more loss into the formula until I got about what I expected
for load Z, given the input Z I was measuring. 4 dB at 7 MHz! A quick
measurement with the wattmeter confirmed that the cable did indeed have
that much loss. The problem was the thinness of the copper cladding on
the very small steel center conductor strands. Even though the cladding
was a substantial portion of the wire diameter, it was still very thin
because of the tiny wire diameter. At our next sked, I told my FD
partner that I'd figured out a way to get a couple more dB out of our 40
meter antenna. . .

Roy Lewallen, W7EL

Interesting topic! I've never encountered an RG59 that had a stranded
center conductor but I'll take your word for it. I *have* seen an 80 ohm
RG59, though (Belden 8221).
Bryan WA7PRC

wrote in message
ups.com...
Well ***Opps****. I took out my contacts already. And I read the cable
Zo as the impedance at the input end. As you can see it says 102.11-
j3.74. I am still going to search out some new RG-59/U...

-Scott, WU2X

If you are going to buy new coax, might as well get something decent. I used
RG11 FOAM for a 1/4 wave transformer at 3.8 MHz, and it worked per theory.

Tam/WB2TT

Tam/WB2TT wrote:

If you are going to buy new coax, might as well get something decent. I used
RG11 FOAM for a 1/4 wave transformer at 3.8 MHz, and it worked per theory.

I think you were lucky. I've found that the velocity factor and
characteristic impedance of foam dielectric coax, even quality coax, to
vary a lot more than solid dielectric cable. Apparently they don't have
very good control over the foam density.

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

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

After looking over some of the followups to your post, I had a thought.

Addressing the errors' introduced by loss, and the possibility that
you're seeing worst case loss at a lower frequency, why don't you go
back and re-measure your matching section at 3X, 5X, 7X 3.8 mcs.?

It's the reverse of a trick I've used to cut n*1/2WL 1:1 coax runs for
VHF -- by measuring at *sub* multiple freqs.

Anyway -- it's a thought.
Jonesy
--
Marvin L Jones | jonz | W3DHJ | linux
38.24N 104.55W | @ config.com | Jonesy | OS/2
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