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 www.bluejeanscable.com:
 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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