[Elecraft] RG-8X

[Jim Brown]

On 2/26/2018 9:28 AM, john at kk9a.com 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