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.

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

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

" wrote in
oups.com:

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


Dan, You didn't say which of the RG6 cables you used. The figures you
quote are very similar to Belden 1189A. In respect of 1189A, note that
the regression model is based on data points from 55MHz to 1000MHz. That
is either because that is what Belden supplied, or it could be that I
excised some low frequency data points that were a bad fit to the model.

Beware of results where the estimate is an extrapolation. (The frequency
range is red when the estimate is an extraplation.)

Owen

The RG-59/U I am using does in fact have a stranded center conductor.
I am using RG-59/U because I am feeding a vertical (quad) loop on the
vertical side and I am running the coax away at a 90 degree angle. The
lighter/smaller coax is more suited for this arrangement. I didn't
use RG6 (which I actually have) because the reducers for the PL259
and ferrite beads I have don't fit this size coax. I figure for 3.8Mhz
that loss wasn't such a big issue...in addition to the 1/4WL section
of 75 ohm coax is only another 25 feet of 50 Ohm coax to the shack.

-Scott, WU2X



On Aug 21, 1:15 am, Owen Duffy wrote:
" wrote groups.com:



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

Dan, You didn't say which of the RG6 cables you used. The figures you
quote are very similar to Belden 1189A. In respect of 1189A, note that
the regression model is based on data points from 55MHz to 1000MHz. That
is either because that is what Belden supplied, or it could be that I
excised some low frequency data points that were a bad fit to the model.

Beware of results where the estimate is an extrapolation. (The frequency
range is red when the estimate is an extraplation.)

Owen

wrote in message
oups.com...
The RG-59/U I am using does in fact have a stranded center conductor.
I am using RG-59/U because I am feeding a vertical (quad) loop on the
vertical side and I am running the coax away at a 90 degree angle. The
lighter/smaller coax is more suited for this arrangement. I didn't
use RG6 (which I actually have) because the reducers for the PL259
and ferrite beads I have don't fit this size coax. I figure for 3.8Mhz
that loss wasn't such a big issue...in addition to the 1/4WL section
of 75 ohm coax is only another 25 feet of 50 Ohm coax to the shack.

-Scott, WU2X


You might also want to take a look at feeding one of the bottom corners.
This will give both vertical and horizontal polarization. I found it worked
great for DX; a dog for working locals, but probably no worse than vertical
p. I subsequently added a 1:1 current balun, but don't think it did anything
relative to the original setup, which had the center conductor connected to
the vertical wire.

Tam/WB2TT

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