That's a "linearly loaded" doublet. The zig-zag provides a little more inductive reactance to help offset the capacitive reactance caused by the overall length being short. Studies suggest that the resistive losses in the wire is somewhat less using the "zig zag" than using loading coils each side of center. So the linear loaded antenna is at least somewhat more efficient.
Since that antenna is fed with a ladder line, the length is unimportant anyway, except to choose a combination of radiator and feeder length that provides a reasonable impedance on each band. If you have a wide-range tuner, that's not important either. You can skip the linear loading and simply put up a center fed wire fed with high-impedance (400 to 600 ohm), low loss line. There will be almost no loss in "gain" over a 1/2 wave radiator as long as the radiator segment is at least 1/4 wave long, end to end, plus another 1/8 wavelength in the feeders. That adds up to a total of 1/2 wave of wire counting the two wires in the feeder and the radiator. Such an antenna operated where the top is 1/4 wave long is only about 2 dB (1/3 of one S-unit) below a full size dipole in "gain". On bands were then radiator is longer, the antenna will be more efficient, even showing significant gain on the higher frequencies where the radiator is a wavelength or more long. The high impedance line is important to keep the SWR at a reasonable level to avoid unnecessary feeder losses. Note that the K1JEK antennas meet this criteria even without the linear loading. The biggest things the prefab antennas have to offer is convenience. In the case of a simple wire antenna like this, you don't need to dig around for the wire, feeder and suitable insulators. Ron AC7AC -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of David Wilburn Sent: Friday, January 04, 2008 3:05 PM To: David Yarnes Cc: Dohn; [email protected] Subject: Re: [Elecraft] dipole antenna efficiency I understand many G5RV's are getting replaced with these antennas. I have the Grampy version, that is 100' long on each side, for a total of 300' of wire on each side, all in a 100' on each side package. Have had good luck with it. Their shortest is a bit over 70'. http://www.k1jek.com/ - David Wilburn [EMAIL PROTECTED] K4DGW K2 S/N 5982 _______________________________________________ Elecraft mailing list Post to: [email protected] 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
