I have often had a voltage loop at the rig end of my open wire feed line and have never experienced a problem with stray RF, even though a neon bulb lying near the ATU blinked as I sent CW.
HOWEVER - I do use a homebrew link-coupled ATU with open wire line. It lacks the dual-differential tuning capacitors that Johnson used to adjust virtual coil taps for the feed line in the Matchbox, but mine has a much wider matching range using actual taps on the coil. (The Johnson Matchbox is only rated to match balanced lines with impedances up to 1200 ohms). Link coupling has a number of advantages including a high order of suppression of stray signals since there is no direct electrical connection between the antenna and the rig. Everything coming in (or going out) has to pass magnetically across the link coupling and through the tuned circuit. That's especially handy if you live near a broadcast station. And of course, you can use the same tuner with unbalanced lines (e.g. coax). Just connect the shield to the center of the coil and tap the center conductor out to either side. WZ5Q has a great example of a beautifully link coupled tuner at: http://www.wz5q.net/index/shack_data/tuna.htm It's bigger and neater than mine but the circuit is the same right down to the RF current sensors. Ron AC7AC -----Original Message----- If you do encounter a high impedance feedpoint on any band, that will place a high RF voltage point at the shack end - add or subtract some feedline to bring the feedpoint impedance down. If you do not understand how the feedpoint impedance changes with the length, take a look at the Antenna article on my website www.w3fpr.com 73, Don W3FPR ______________________________________________________________ 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
