On Sat, 10 Mar 2012, Jim Brown wrote: > On 3/10/2012 5:14 PM, Hisashi T Fujinaka wrote: >> I think the advantage (not magic) is that you get less loss in ladder >> line than you do through the dielectric of coax. > > FALSE! For all practical purposes, there is NO dielectric loss in coax > below about 500 MHz, where it just BEGINS to show up. Virtually all the > loss in any dry transmission line at HF and VHF is due to simple Ohm's > Law in the copper (including the increase in resistance due to skin > effect). It's all I squared R loss. > > The advantage of open wire line is solely the result of their higher > IMPEDANCE, which means that for a given power level, the CURRENT is much > less. For the same reason, 75 ohm coax has a bit less loss than 50 ohm > coax for the same conductor size. So-called WINDOW line loses this > advantage when it gets wet, because then it DOES have dielectric loss. > > BTW -- when you're analyzing these things, you must consider the heat > produced in a common mode choke when the antenna has a lot of imbalance. > An off-center fed antenna has a LOT of imbalance.
I think you're wrong here. The high SWRs generated along the feedline are extremely high voltages (with low currents) and there is significant loss in the dielectric of the coax. The reason that ladder line is better is because of the air in the dielectric. The best is the old school ladder line with the ceramic insulators. There is very little I squared R loss because there is very low current. That's why people see better performance with ladder line and think it's magic. -- Hisashi T Fujinaka - ht...@twofifty.com BSEE(6/86) + BSChem(3/95) + BAEnglish(8/95) + MSCS(8/03) + $2.50 = latte ______________________________________________________________ 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
