[TowerTalk] Coax Quality -- Was Re: Rotor loop coax

[Jim Brown]

On 8/20/2018 8:20 AM, john at kk9a.com wrote:
> I personally do not skimp on coax or connectors however many hams do. It
> is not ridiculous to point out that all RG-213 are not equal. I operated
> from one contest station in the Caribbean where the coax shield would not
> accept solder and there were numerous connector issues. I am sure that
> there are many good brands of RG-213, all of mine is Belden 8267.

Yep. And this is what I was getting at.  Here's a great pdf on the topic 
by 4X1GE. The photo comparing two cables labeled "RG213" tell the story 
better than a thousand words.

http://www.caarc.ca/wp-caarc/wp-content/uploads/2016/03/Transmission-Lines-ver-2.pdf

Scroll down to see the result of his measurements of the two cables. 
While he didn't measure attenuation or shielding, the differences would 
certainly jump out at you if you did. I know it matters to you, and it 
does to me too.

Specifically -- skin effect causes shield resistance to increase with 
smaller shield diameter, and the cheap stuff has a smaller diameter. 
Shielding effectiveness is a function of the density, uniformity, and 
resistance of the shield at the frequency of interest. The cheap cable 
has significantly less copper (fewer strands, much smaller diameter 
strands) in the shield AND a bit less in the center conductor. Both will 
cause greater loss and the less dense/higher resistance shield also 
reduces shielding effectiveness.

As to my "making a mountain out of a molehill" and giving the original 
questioner a snow job -- in addition to my detailed tutorial response, I 
also reinforced another post in the thread recommending BuryFlex.

And some background. Before I retired about ten years ago, I designed 
and specified large sound systems, to be put out to bid by qualified 
sound contractors.  While I didn't want to limit product types to only a 
few brands, I did want good stuff. Beginning about 25 years ago, lots of 
no-name "brokered" cable invaded the market that was essentially "junk." 
To make sure my client got good stuff, my bid spec required that all 
cable carry the name of the manufacturer that published specs for the 
cable and maintained QC of that cable in their own lab.  Known good 
cable mfrs like Belden, West Penn, Gepco, and Commscope were recognized 
as qualified; others had to be documented. (I didn't specify 
transmitting coax, so didn't list vendors like Times.)

One day I ended up talking to one of these cable brokers who wanted me 
to OK their cable. I asked to speak to their chief engineer to get a 
feel for whether they were or were not "for real." They connected me to 
someone who was a salesman claiming to be an EE. It turned out that the 
only thing they measured was the cable length. When I asked about his 
technical background and where he got his degree, he responded, "IBM." :)

73, Jim K9YC