I used to think that dielectric losses were a significant factor until someone pointed out the attenuation data for various transmission lines between 2 and 500 MHz. The plots for all the various transmission lines are straight lines. If dielectric losses were involved, they should curve toward the higher frequencies.
When I feed a "doublet" with open wire line for all-band operation with a tuner, I use the largest conductors practicable to minimize copper losses, especially at current loops. Another factor is the impedance the feed line sees at the radiator. Most open wire line has an impedance of somewhere between 300 to 600 ohms. It the radiator is at least 1/2 wavelength long, the impedance at the feed point will range from about 50 ohms at 1/2 wavelength to perhaps 3500 ohms when the radiator is 1 wavelength total. With 50 ohm line that represents a ratio of 1:1 to 70:1. Using open wire line at, say, 450 ohms nominal, the ratio is only between 8:1 and 9:1. Ron AC7AC -----Original Message----- Jim Brown wrote: >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). Much of the confusion arises from the advertising for newer types of coax that have lower loss than a similar solid PE equivalent. The improvement is touted as being due to "low loss foam dielectric" when that simply isn't true. The reduction in loss is almost entirely due to increase in the diameter of the center conductor (because that conductor has the largest current density and hence the highest skin effect losses). The foam dielectric is merely something that *has* to be used to compensate for the thicker center conductor, in order to keep the same characteristic impedance. In all the coaxial cables we know in amateur radio, dielectric losses only begin to become important at frequencies above 1GHz. -- 73 from Ian GM3SEK http://www.ifwtech.co.uk/g3sek ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
