Roy - Your comment made me double check my work. I was using ON4UN's
software and I was entering the estimated loss at my design frequency.
Then I realized when I cranked up the loss (to see the effect clearly)
that it wasn't doing anything with impedance along the line. And then
I visited Owen's link for the second time and plugged in the details
from my cable. I must have done something wrong the first time -
because let me tell you it was dead nuts what I saw (Zo 75.01-j1.45
). Case closed!

-Scott, WU2X

Parameters
Transmission Line Belden 9204 (RG-59/U)
Code B9204
Data source Belden
Frequency 3.800 MHz
Length 42.700 feet
Zload 54.00+j0.00
Yload 0.018519+j0.000000
Results
Zo 75.01-j1.45
Velocity Factor 0.660
Length 89.94 °, 0.250
Line Loss (matched) 0.269 dB
Line Loss 0.279 dB
Efficiency 93.77%
Zin 102.11-j3.74
Yin 0.009780+j0.000359
, , RL, VSWR (source end) 1.53e-1-j8.49e-3, 0.153 -3.2°, 16.3dB,
1.36
, , RL, VSWR (load end) -1.63e-1+j9.38e-3, 0.163 176.7°, 15.7dB,
1.39
7.24e-4+j3.68e-2
k1, k2 3.19e-6, 1.79e-11
Loss model source data frequency range 10.000 MHz - 1000.000 MHz
Correlation coefficient (r) 0.999661

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

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
*** Killfiling google posts: http://jonz.net/ng.htm

Owen Duffy wrote in
:

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.

I have added a 'No symbols in output' checkbox to the TLLC input form to
allow selection of a more ASCII friendly output format.

Using that switch on Scotts problem yields:

RF Transmission Line Loss Calculator / Enhanced
Parameters
Transmission Line Belden 9204 (RG-59/U)
Code B9204
Data source Belden
Frequency 3.800 MHz
Length 0.250 wavelengths
Zload 54.00+j0.00 ohms
Yload 0.018519+j0.000000 ohms
Results
Zo 75.01-j1.45 ohms
Velocity Factor 0.660
Length 90.00 deg, 0.250 wl, 13.024 m
Line Loss (matched) 0.269 dB
Line Loss 0.279 dB
Efficiency 93.77%
Zin 102.10-j3.81 ohms
Yin 0.009780+j0.000365 ohms
Gamma, rhotheta, RL, VSWR (source end) 1.53e-1-j8.82e-3, 0.153-3.3
deg, 16.3dB, 1.36
Gamma, rhotheta, RL, VSWR (load end) -1.63e-1+j9.38e-3, 0.163176.7
deg, 15.7dB, 1.39
gamma 1.24e-1+j6.28e+0
k1, k2 5.46e-4, 3.06e-9
Loss model source data frequency range 10.000 MHz - 1000.000 MHz
Correlation coefficient (r) 0.999661

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

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.

For what it's worth, I looked up the specs on the Carol C5785 that is
locally available at Home Depot here in the States. It's quad-shield
RG-6 and they list the losses down to 1MHz

1MHz .26dB/100ft
10MHz .81dB/100ft
50MHz 1.46dB/100ft

According to your calculator for RG-6/U it should be
..19
..6
1.37

As a percentage difference in dB (boy that's a bad unit) it's actually
a good bit more loss at 1 and 10MHz, but in a practical sense it's
probably pretty negligible.

So unless you're going 1000 feet to transmitting antennas on mid to
low HF, I doubt it's a worry. I don't know what the price
differential is between CCS and hard drawn but i do know that cable
with about the same loss as RG-213 that costs 12 cents a foot is
pretty attractive.

As far as the original post, I picked my stuff up and made a bunch of
twelfth-wave transformers for it and that seemed to work out fine, but
I guess they're less sensitive if you put one at each end because they
would tend to match to whatever cable you're making the transformers
out of and they're very broadband too.

As far as velocity factor goes, I measured mine before I started
cutting (I built XFMRS for 2m and 70cm so I had to be sort of
accurate. It worked).

73,
Dan