Posted by
Jim Brown-10 on
Feb 26, 2018; 6:10pm
URL: http://elecraft.85.s1.nabble.com/RG-8X-tp7638635p7638636.html
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.pdf73, Jim K9YC
______________________________________________________________
Elecraft mailing list
Home:
http://mailman.qth.net/mailman/listinfo/elecraftHelp:
http://mailman.qth.net/mmfaq.htmPost: mailto:
[hidden email]
This list hosted by:
http://www.qsl.netPlease help support this email list:
http://www.qsl.net/donate.htmlMessage delivered to
[hidden email]