The beauty of a doublet is that the length of the radiator and the length of the feed line is completely unimportant if you have a wide-range tuner and the radiator is at least 1/2 wave end-to-end.
With modern tuners -especially most "automatic" types - the feeder can present an impedance outside their matching range. That requires adjusting the feed line length to find something that will provide a match on all bands. Such an antenna should work as well or better than a dipole at equivalent height all frequencies at which it is at least 1/2 wavelength long. If good open wire line with a moderate impedance in the 450-600 ohm range is used, the line losses are very low thanks to the moderate SWR. Typically the SWR on any frequency will be less than 10:1, which won't result in significant losses if the conductors are of a decent size. Extremely high currents can flow at some points along the line, and large conductors help minimize the ohmic losses in the line. For that reason, 300 ohm "twinlead" or 450 ohm "ladder line" works, but has larger-than-necessary losses because of the small diameter conductors those lines use. Whenever possible, I use a #12 or larger wire for my open wire lines. At frequencies where the antenna is considerably longer than 1/2 wavelength, it shows some gain over a dipole with narrower but stronger lobes. It will work well down to where the antenna is only 1/4 wavelength end-to-end, showing only a small loss compared to a full size dipole. The problem there is that the SWR on the feeder jumps up as the impedance drops quickly at that size. Ron AC7AC -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Phil LaMarche Sent: Tuesday, July 14, 2009 9:32 AM To: 'dw'; 'Elecraft_List' Subject: Re: [Elecraft] The use of a doublet DX Engineering shared an antenna with me that I put up and it works wonderful on all bands with a tuner. 120 ft and fed with 90 ft of 300 ohm line into a 1:1 balun of theirs and into the tuner with RE 213. Tuner is a Palstar AT2K. The feed line length must be 90 feet. YES, it really works. Phil ______________________________________________________________ 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
