Posted by
Jim Brown-10 on
Feb 26, 2018; 9:31pm
URL: http://elecraft.85.s1.nabble.com/RG-8X-tp7638635p7638641.html
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:
[hidden email]
> [mailto:
[hidden email]] On Behalf Of Jim Brown
> Sent: Monday, February 26, 2018 10:11 AM
> To:
[hidden email]
> Subject: Re: [Elecraft] RG-8X
>
> On 2/26/2018 9:28 AM,
[hidden email] 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
>
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