Thursday, February 24, 2011

Coax cables and shielding.

After looking into the minimal amount of shielding on my bargain-bin coax I decided to do a little research on the difference shielding can make to signals traveling through coax cable.

The following information is from
 So, what's the result of these differences in shielding? Engineers measure the effectiveness of a shield by what is termed "transfer impedance," which indicates the extent to which a signal outside of the cable reaches the inside. Transfer impedance is a function of frequency, so to gauge the relative effectiveness of shields, one has to know what the frequency band in question is, but at all frequencies, a precision video cable with a 95% braid and foil outperforms quad shield cable. The following table is taken (with permission of the author) from the Audio/Video Cable Installer's Pocket Guide, McGraw-Hill 2002, by Stephen Lampen, an engineer with Belden Wire and Cable. It shows the transfer impedance for various shield configurations on RG-6 type cables at various frequencies; the lower the number, the better.

My bargain-bin coax would be lucky to have 60% coverage and there was NO foil. Clearly a case of "you get what you pay for" and let the buyer beware. The foam dielectric was very soft ... it could be permanently deformed with just my fingers. I shouldn't have been surprised to have problems with high SWR after running that coax under carpet and across walkways!
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