On Fri, 02 Jul 2004 17:21:57 -0400, Mike S wrote: >My intent was to correct an obvious error
So was mine. >According to Verizon, over 20% of US subscriber loops are more than >18,000 feet long. The balance of your post continues to demonstrate a >lack of familiarity with real world audio transmission lines. For instance, >you're apparently unaware that loading coils are inserted on a long >subscriber loops to offset the parallel capacitance. I'm not unaware of it all. That technique is practical BECAUSE the line can be viewed as a lumped capacitance. 18,000 ft is still less than a tenth of a wavelength at 3 kHz in 0.66 Vf cable. I learned about such techniques in an EE circuit analysis course around 1962. Then they were called "peaking circuits" when applied to the outputs of active electronics. They form a resonance with a broad peak near or above the end of the desired response limit, then roll off more sharply than they otherwise would due to the capacitance alone because of the resonance. If you have doubt about the characteristic impedance of audio cable, try measuring some. Or look in the printed copy of a Belden catalog -- the one that is a half-inch thick (if you can find one). You will find the characteristic impedance listed for some of their twisted pair cables. They are all in the 60-80 ohm range. Or look at the some of the excellent applications notes and tutorials on the Jensen Transformers website. You will also find Vf (velocity factor) listed for some cable types. Gepco (a good small cable mfr based in Chicago) also lists this data for some products. http://www.jensentransformers.com/ Jensen still makes repeat coils like the old 111C used by Ma Bell (except that their coils have better specs than the original, thanks to the greater precision of modern manufacturing and computer-aided design). This thread is, indeed, rather far off topic, and this will be my last post to the list on it. My apologies to any who might have been bothered by the length of it. Jim Brown K9YC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
