Coax recomendations
 

Ralph Mowery wrote:

Roy what kind of coax do you recommend along the same type ? I soon plan on
putting up several beams, tribander for the low bands, 6 meter, 2 meter and
a 432 antenna. Should be running about 125 feet or so. While I know
hardline would be best for the VHF , I don't want to (can't spend the money,
easy to install for the rotator) so I want to stay with one of the
9913/lmr400 types. I have had a piece of 9913 up for about 10 years and it
seems to be ok, no water I can tell, I still would like to go with a solid
foam type instead of the hollow core .

Just looking at the specks and advertising can sometimes be deceiving.

Sorry, I'm not the best person to ask. A lot of the other folks here
have a lot more experience than I do with applications like yours.
Ruggedness and other physical properties can easily be more important
than the technical specifications of a cable under idealized conditions,
so I'd listen to people who have used cable for an extended period of
time in similar circumstances to yours.

As far as loss goes, you're bump up against the laws of physics. Below a
few GHz, dielectric loss in common coaxial cables is negligible. The
shield has much larger surface area than the center conductor, so the
loss ends up being dictated mainly by the diameter of the center
conductor -- the larger it is for a given cable Z0, the lower the loss.
Assuming a fixed Z0, the only ways you can make the center conductor
larger are to increase the cable outer diameter, or reduce the effective
dielectric constant of the insulation between center conductor and
shield. The effective dielectric constant is reduced by using foamed
dielectric coax, or even more by using mainly air dielectric, with a
minimal amount of insulation for physical support. Consequently, for a
given Z0 and outer diameter, and otherwise similar construction, foamed
dielectric cable has less loss than solid dielectric cable, because its
center conductor is larger. Mostly-air dielectric cable has less loss
yet for the same reason. (In the microwave region, dielectric loss
becomes significant, so there's a further loss reduction when the
dielectric density is reduced.)

A stranded center conductor or braided shield increase the loss
somewhat. It's difficult to find definitive data on just how much,
probably because of the number of variables involved, like shield weave
tightness and frequency. But the amount of increased loss can be quite
significant, judging by the actual loss of typical coax vs. the loss
predicted by the assumption of a solid center conductor and perfect
shield. Ian, G3SEK just posted a possible explanation of an additional
loss mechanism for aluminum-foil shielded cable like BuryFlex -- tearing
of the foil shield due to bending too sharply.

But I think amateurs often get too hung up on small amounts of cable
loss. It's admittedly sometimes very important, but not in most cases.
Cost, convenience, ease of use, connector type, and physical ruggedness
might well be much more important criteria. And again, there are a lot
of folks here who have a lot more relevant experience than I do, so you
should listen carefully to their advice.

I do fully agree with your last statement.

Roy Lewallen, W7EL

Coax recomendations
 

On Sun, 11 Dec 2005 15:29:20 -0700, Wes Stewart
wrote:


Importing their data into Dan's (AC6LA) "bestfit.xls" spreadsheet
shows very poor correlation with theoretical k1, k2 coefficients. I'm
observing something similar on some RG-142 that I am measuring with my
N2PK VNA. The '142 has a silver-coated, copper-plated steel center
conductor and I believe that the skin depth encompasses the steel at
lower frequencies.

I noted when I imported Davis' Bury Flex data to tllce that the
regression model correlation coefficient was lower at 0.9918 than most
other data which tends to come in better than 0.997 and mostly 0.998
or better.

One of the reasons that some lines show a very good fit to the model
(eg 5 nines) is that the measurement data was fitted to a model, and
the published figures are from the model, not the original
measurements, and the error in deriving a model from the published
figures is principally caused by rounding of the published figures.

A possible explanation of Greg's observation of low Zo is that the
foam is more dense than intended, increasing C, lowering Zo, and
increasing loss at higher frequencies.

Owen
--

Coax recomendations
 

Owen Duffy wrote:

Very well done Greg.

Hopefully, in the fullness of time, you will roll all this information
into the web page.

I have read articles on issues with control of the foaming of
dielectric during manufacture, it seems more difficult to control than
all of the aspects of conventional solid PE dielectric line, so it
leaves one wondering if that may be the cause of the apparently low Zo
(~43 ohms) that you observed. If so, is it a batch to batch variation,
variable along the line (although your charts don't suggest that),
aging, temperature, migration of the centre conductor on the roll,
etc?

Well done OM.


I've measured a lot of cable over the years, and have found considerable
variation of velocity factor from batch to batch of otherwise identical
cable from the same manufacturer, as Owen's articles imply. Assuming
that the physical dimensions of the cable stay the same, Z0 will also
vary with foam density.

The piece of BuryFlex I have measures right at 80% velocity factor, and
very close to 50 ohms. I believe I'm within a percent or two on VF, and
an ohm for Z0.

Roy Lewallen, W7EL

Coax recomendations
 

Wes Stewart wrote:
. . .
Importing their data into Dan's (AC6LA) "bestfit.xls" spreadsheet
shows very poor correlation with theoretical k1, k2 coefficients. I'm
observing something similar on some RG-142 that I am measuring with my
N2PK VNA. The '142 has a silver-coated, copper-plated steel center
conductor and I believe that the skin depth encompasses the steel at
lower frequencies.
. . .

I've definitely seen this in RG-174 and some similar diameter 75 ohm
cable, at 7 MHz. The problem with those cables is that the center
conductor is made of very fine strands of Copperweld. While the fraction
of copper relative to the wire diamter is large, the actual copper
thickness is small due to the very small diameter wire, allowing current
to penetrate into the steel at lower frequencies.

I've also seen the effect in the time domain when using RG-174 type
cable but with solid silver-plated Copperweld center conductor. The
increased loss at low frequency actually improves the step response
somewhat because of the disproportionately higher loss at lower frequency.

Roy Lewallen, W7EL

Coax recomendations
 

Good work, Greg. It's refreshing to see that some people, at least, are
still willing and able to do this rather than unquestioningly accept
manufacturer's data. Even if the data turn out to be accurate, the
process is truly educational.

The varying SWR while terminated with 50 ohms is consistent with the 45
ohm Z0 you observed. My piece of BuryFlex is right at 50 ohms, but I'm
not too surprised at this amount of variation given the foamed
dielectric. I measured 80% velocity factor, so if our respective
measurements are good, the velocity factor of your piece should be
around 72%.

Roy Lewallen, W7EL

Coax recomendations
 

That's a guarantee??

Dave

Charlie wrote:

At the bottom of this page the ** footnote
1. http://www.davisrf.com/ham1/coax.htm

"note that Bury-Flex has a 20+ year abrasive resistant jacket of PE"

--

Charlie

"Dave Holford" wrote in message
...


Charlie wrote:

Wes Stewart said:
I find it curious that Andrew cable is seen so often at these events
and yet I've -never- even seen a piece of Davis cable, dispite the
claims that miles of it are in commercial use.

It might be that the BuryFlex is still in use and has not been removed
from
service as the Heliax you cite has been. After all Davis BuryFlex 9914
does
have a warranted - 20 year service life -

Is there any other coax with such a warranty?
---

Charlie

I don't seem to be able to find any mention of a warranty on their site -
can
you point it out for me?

Dave

Coax recomendations
 

It is NOT brand loyalty, but the published data by GM for the past xx+ years
shows that their cars are best so I'll continue to drive GM, despite what
independent testers may say. Besides which my son-in-law sells them so they
must be best.

Dave

Charlie wrote:

Owen it is NOT brand loyalty. It is the publicly published data by RF Davis
(for the past 10+ years) vs. Roy's one-shot test setup some years ago. I'm
not out to discredit Roy...I even shook his hand once at a Hamfest.

--

Charlie

"Owen Duffy" wrote in message
...
On Sat, 10 Dec 2005 17:15:44 -0800, Roy Lewallen
wrote:

If no other reader of this newsgroup has some of this cable and the
ability to test it, I'll see if I can arrange for someone else to make
measurements and post results.

Roy, this seems a case of brand loyalty vs objective measurement and
evaluation (albeit on a single sample).

You will change your mind based on more measurement data, either
strengthening your existing opionion or changing it.

Those with brand loyalty already know all they need to know, life is
comfortable.

You are talking different languages, and the only way there will be
agreement is if you capitulate (which would be unprincipled in the
absence of evidence).

Owen

I remember the fierce debates over 9913, there were nearly as many
words written about how fabulous it was, as there were words written
on how to keep water out of it.
--

Coax recomendations
 

Got your dig Roy.....kinda sad.....

--

Charlie


"Roy Lewallen" wrote in message
...
Good work, Greg. It's refreshing to see that some people, at least, are
still willing and able to do this rather than unquestioningly accept
manufacturer's data. Even if the data turn out to be accurate, the process
is truly educational.

The varying SWR while terminated with 50 ohms is consistent with the 45
ohm Z0 you observed. My piece of BuryFlex is right at 50 ohms, but I'm not
too surprised at this amount of variation given the foamed dielectric. I
measured 80% velocity factor, so if our respective measurements are good,
the velocity factor of your piece should be around 72%.

Roy Lewallen, W7EL

Coax recomendations
 

Roy Lewallen, W7EL, wrote:

I measured 80% velocity factor, so if our respective
measurements are good, the velocity factor of your piece should be
around 72%.

Ok, my curiosity got the best of me, and I decided to
to measure the velocity factor. My VNA software has
a "distance to fault" feature, and I "worked backwards",
which is to say that I measured a length of the
BuryFlex with a tape measure (27' 2"), and adjusted
the VF on the distance to fault tool until I obtained the
same physical length. The far end of the cable was
terminated with an open circuit.

I happened to start with the measurement frequency set
to 1 MHz. Lo and behold, the VF needed to compute the
same physical length was 72%, as you suggested.

My own understanding is that VF should be constant with
respect to frequency, so I decided to vary the test frequency.
I should have left well enough alone.

I picked 1, 2, 4, 8, 16, and 32 MHz. For those frequencies,
I measured the following VFs:

1 MHz = 72%
2 MHz = 73%
4 MHz = 75%
8 MHz = 80%
16 MHz = 79%
32 MHz = 79%

I'm rounding the VF to integer values, since I don't think that any more accuracy
can be claimed in this setup.


Since that result was a little surprising, I grabbed some mini 8 (8X)
that was nearby, about 51.25 feet. The published VF is 78%, and
I measured the following, at the same test frequencies:

1 MHz = 78%
2 MHz = 78%
4 MHz = 79%
8 MHz = 79%
16 MHz = 80%
32 MHz = 80%

With this cable, the VF appeared much more constant across the
1 to 32 MHz range.

Is there an explanation that fits with my measurements?

Greg, W8WWV

Coax recomendations
 

I don't recall -anyone- using the word "guarantee". Please post that
reference.....TY

--

Charlie


"Dave Holford" wrote in message
...
That's a guarantee??

Dave

Charlie wrote:

At the bottom of this page the ** footnote
1. http://www.davisrf.com/ham1/coax.htm

"note that Bury-Flex has a 20+ year abrasive resistant jacket of PE"

--

Charlie

"Dave Holford" wrote in message
...


Charlie wrote:

Wes Stewart said:
I find it curious that Andrew cable is seen so often at these events
and yet I've -never- even seen a piece of Davis cable, dispite the
claims that miles of it are in commercial use.

It might be that the BuryFlex is still in use and has not been removed
from
service as the Heliax you cite has been. After all Davis BuryFlex 9914
does
have a warranted - 20 year service life -

Is there any other coax with such a warranty?
---

Charlie

I don't seem to be able to find any mention of a warranty on their
site -
can
you point it out for me?

Dave