The following commentary is from the founder of TENNADYNE and while are presented on the TENNADYNE web site may or may not reflect the views of TENNADYNE. Then again, maybe they do......


About That Coax.............

Back in 1980, when I was doing a Marketing Manager gig at Times Wire & Cable (now Times Microwave Systems), I came across some serious technical disinformation about coaxial cable that was put out by a small, rival coax manufacturer. I must admit that I saw a bit of red when I saw their press release, and so I set out to create a definitive document on the subject. At that time, there was a terrific engineer at Times by the name of Ken Smith who had long been working on the shielding properties of coax, and the resultant paper was mostly his work. It was called Coaxial Cable - The Neglected Link and was published in the April, 1981, issue of QST magazine, should you want to go back for further information.

Within the past week or so, I saw comments in a small pulp ham mag, published on the West Coast, in their January, 2005, issue, where yet again I saw a totally inaccurate comment about coax, this time that "......coaxial cable doesn't radiate....." How inaccurate the writer is, small wonder he tries to hide behind a nom de plume. The more sorry side is that the writer never did address the question posed by the reader, I subsequently sent the reader some information that answered his query.

The fact is, ALL COAXIAL CABLES RADIATE, to one extent or another. I learned very early that a double shielded coax cable, or even better, the Times LMR series or the more expensive hardlines, improves the performance of a good directional antenna considerably, especially when signal levels are high. It's a bi-directional circuit, this radiation from/into coax, signals egress and ingress as well. I could go on and on about this subject, but will avoid that here by simply suggesting your viewing of the aforementioned QST article (copies are available from TENNADYNE), but I will offer you the following table of information.

Relative isolation characteristics of RG-59/U coaxial cable vs. percentage and type of braid coverage/shielding. (While the cable used for testing was RG-59/U, the same basic characteristics apply to all coaxial cables)

Shield

Relative Isolation 20M (dB)

Losses in dB per 100 ft 15 MHz

150 MHz

 
         
         
40% bare copper 17 1.72 5.55 Common use on replacement auto antennas
51% bare copper 18 1.72 5.55  
59% bare copper 26 1.39 4.51  
79% bare copper 34 1.13 3.67 Then commonly used on CB antennas
98% bare copper 52 0.98 3.20 Most MIL cables
96%/96% bare copper 83 1.01 3.31 Like RG214 & RG217
96% tinned copper braid over a bonded aluminum laminated tape 90 n/a - high velocity PE   Times LMR series
Solid aluminum sheath 282 0.89 2.91 Common CATV & commercial use
         

NOTE: The isolation capabilities of coaxial cable at 20 meters is roughly 10 times greater than at 2 meters.


There, in a nutshell, you have it. There is a difference and that difference is why I've used at least a double shielded coax from my directional antenna #1 onward.

 

Chuck Brainard - KA1PM
Founder of TENNADYNE

 


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