On 2/26/2018 10:45 AM, Tom Georgens wrote:
HI Jim
Thanks for your informed commentary
One question I have had is, if loss is dominated by resistive losses at HF,
does coax actually get more lossy with age? If so, what is the mechanism.
Does the dielectric change properties, or do the copper surfaces corrode or
break down? If the dielectric breaks down, then the impedance should
change as well
Most info on cable life deal primarily with the UV, heat, and moisture
resistance of the jacket but I have seen very little about aging of the
internal components.
Hi Tom. I don't know enough about this issue to say anything useful,
except for this. When N6RZ died something like 8 years ago, I bought a
lot of his stuff and helped his XYL, Kerry, get rid of the rest of it.
He had a lot of coax and some hard line; I bought the hard line, and
took the random lengths of coax to an NCCC meeting, telling members
where it had come from. Almost all of it was good quality stuff, some
used inside, some outside. I even found some stored in a shed. I'd guess
most of it was at least 20 years old. Everyone turned their noses up at
it, so I brought it home, made stubs out of it, and measured their
effectiveness. I didn't have a VNA then, so I used an HP generator and
HP spectrum analyzer (as an RF voltmeter), doing point by point
measurements. Most of the stub data in that Q&A are for that cable.
Since a harmonic stub depends on a low value of impedance to work, that
tells me the cables were still quite good. The only bad piece of cable I
found was one whose interior jacket was green from oxidation of the
shield. I think it had come from that shed.
Until I learn more about it, I go under the assumption that the primary
hazard is moisture. Because dielectric loss doesn't kick in until at
least UHF, I don't buy that being as a factor at HF. A few years ago, I
noticed moisture around the Polyphaser for one of my high dipoles when I
removed it for some reason. I had only recently replaced the dipole and
the coax run (Belden 8213), but hadn't done a good enough job of sealing
the coax around the center insulator. Water penetrated, came down
between the braid and dielectric, and in less than a year, had turned
the shield black. I put that length of coax on the VNA and found that
loss had increased from about 0.4 dB to about 0.45 dB.
I'm copying this to the reflector, hoping that guys like W3LPL will jump
in with more.
73, Jim
Thanks
Tom W2SC
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Jim Brown
Sent: Monday, February 26, 2018 10:11 AM
To: [email protected]
Subject: Re: [Elecraft] RG-8X
On 2/26/2018 9:28 AM, [email protected] wrote:
I believe all RG8X has a .242 diameter PVC jacket and about the same loss.
Certainly some brands are better made, I use only Belden 9258.
Not quite. First, for at least 40 years, RG-numbers are NOT a specification,
but only a generic description of cable, mostly telling us the approximate
outer diameter and the characteristic impedance (Zo).
Below about 500 MHz, loss in coax is all due to I squared R, which depends
entirely on the combined resistance of the center conductor and the shield
at the frequency of interest.
Zo depends on conductor diameters, spacing, and the dielectric material.
Coax with a foam dielectric allow the center conductor to be larger for the
same shield diameter. THAT'S why foam cables have lower loss, NOT because of
lower loss in the dielectric -- dielectric loss doesn't show up until we're
well into the UHF region. Shield resistance is reduced by a larger diameter
for two reasons -- more copper and skin effect. Many coax cables don't use
copper for center or shield or both. Loss will be greater at low frequencies
if the center is copper-coated steel, which is often done for both cost and
physical strength. Loss will also be greater if the shield uses less copper
or is made from aluminum.
As usual, Frank has asked the right questions -- there are important
"applications-related" differences between coax types, even from the same
manufacturer.
In general, it's best to use bigger coax from a trusted manufacturer, and
with the best quality shield. Larger coax has less loss. Don't buy smaller
coax because you're running low power -- our Field Day team runs QRP, and
all of our coax is RG8-size with a foam dielectric for low loss and a robust
copper braid shield!
There's a tutorial on this topic at http://k9yc.com/Coax-Stubs.pdf
73, Jim K9YC
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[email protected]
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html Message
delivered to [email protected]
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[email protected]
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [email protected]
