George, Short question, long answer follows --
Do to constraints at home, I no longer use open wire or ladder line feeders, but when I did use them, I found several things were true if you did not want them to radiate (and create RF in the Shack). My first rule is to use balanced antennas - off center fed antennas are famous for feedline radiation and RF in the shack. The second rule is to run the feedline away from the antenna at right angles for as great a length as you can manage, but certainly for a quarterwavelength - The feedline can pick up radiation from the antenna if this rule is not followed. Third is to run the feedline correctly - use nice gentle bends if you must change direction, support it using as few hangers as possible (if you can put the feedline under tension, you can get away with very few supports) but support it so it is stable even in the wind. Do not run it parallel to other conductors, but you may cross a conductor at right angles if necessary. The line should be spaced away from other objects by at least 3 times the spacing of the conductors. Lastly, If I could, use a true balanced tuner, link coupled is best, so if you see a Johnson Matchbox at a hamfest, get it. If you must use an unbalanced tuner, use a good balun at the output (see K9AY's info on baluns). BTW, do not assume that a 4:1 balun is the thing to use, the feedpoint impedance in the shack can vary wildly from very low to very high. 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 On 12/31/2011 11:25 AM, [email protected] wrote: > Those of you using open wire feed lines. How do you keep RF out of the > shack? 73 George/W2BPI K2/100 > ______________________________________________________________ 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
