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#11
April 6th 05, 04:29 AM
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On Tue, 05 Apr 2005 18:07:57 -0700, Roy Lewallen
wrote: K7ITM wrote: . . . But beware that you are more likely to have dielectric loss in open-wire line for a variety of reasons... . . . Yes, this is something I didn't mention and should have. My statement about the negligibility of dielectric loss below 1 - 10 GHz is strictly true only for coax with decent (common) dielectric material (e.g., PE or PTFE). When the impedance is higher, as it is with ladder line, the effect of the dielectric loss is proportionally higher. On the other hand, a good part of the ladder line field is in the air (although it's most intense directly between conductors, where any insulation typically is), which reduces the effect of loss in the dielectric. Many years ago I measured the attenuation of some common 300 ohm TV twinlead, and found that in some cases when wet its attenuation could exceed that of RG-58 coax. The extra loss is intirely due to degradation of the quality of the dielectric between conductors. See http://eznec.com/Amateur/Articles/Po...Feed_Lines.pdf. I know Wes has done similar measurements on window line and has posted the results at his web site; perhaps he'll remind us again of the URL. Su http://users.triconet.org/wesandlinda/ladder.htm Wes |
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