On 12/13/2011 11:48 AM, Alan Bloom wrote: > Is that a valid assumption? I thought that much of the loss in coax is > due to the dielectric loss of the insulation. That implies that the > bifilar winding should have less loss than coax.
This is a very common misconception, and it is VERY wrong below UHF for nearly all practical transmission lines that aren't defective (for example, a wet dielectric). If you do the math, you see that below UHF, the loss is virtually ALL due to copper (taking skin effect into account for both conductors). There's an excellent paper by Frank Witt in one of the ARRL Antenna Compendiums (which Dean also edited) showing that window line exhibits significant dielectric loss at HF when it gets wet. You can see the equation for coax on datasheets for Times LMR coax types on their website, with the equation for each cable type reflecting the physical constants for that particular cable. There are two terms, one for copper loss, the other for dielectric loss. Measured data for a few cables that I've measured track those computed curves, and if you put them into a spreadsheet and plot the two terms vs frequency, you can clearly see which terms are contributing. I suspect that they are also used in Dean's TLW program. Right, Dean? 73, Jim K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
