Coupling can still approach unity with an air core coil at RF frequencies. Iron is only needed at lower frequencies to accomplish the same thing (or close to it).
One practical reason to short out unused turns is that if they are left unshorted the coil can then become an auto-transformer. If only a small number of turns are active and the rest are left unshorted it steps up the voltage according to the square of the turns ratio. That can lead to huge RF voltages at the open end of the unused section. I've never done Q measurements with the unused turns open or shorted, so I was curious about any responses myself. I've always shorted the turns for the above reason. Ron AC7AC -----Original Message----- I had two personal replies, from K6DGW and GM4ESD, thanks guys. I think the crux is that these are air cored coils, so the coupling between turns and therefore the detrimental effect is lower. Unlike the case of say a mains transformer, where the core ensures tight coupling, so a shorted turn is much more significant. Please excuse the BW but I hope this is of interest. 73 Fraser G4BJM _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
