[antennaware] RE: Dipole 17/12 meters
Tue, 30 Jun 1998 08:51:30 -0400
From: Merv Schweigert[SMTP:email@example.com]
Sent: Monday, June 29, 1998 9:15 PM
To: Gary Breed
Subject: Re: [antennaware] RE: Dipole 17/12 meters
Gary, interested in trying the 17/12 dipole design, do you
by any chance have any info on using this as a driven
element for a yagi ? would like to try and interlace 17/12
on one boom . Appreciate any info or where to look.
73 Merv K9FD
I don't recall seeing this written up anywhere, but it will work (see W4RNL's
posting, too). Somewhere in my EZNEC files is a 2-element 17/12 Yagi on
a six-foot boom, but I couldn't find it.
The reflectors on such a beam would be exactly the same as a monoband
or trapped antenna. The 17 meter driven element would be very nearly the
same, as well. As W4RNL noted, the antenna would initially be matched
on 17 meters, then the 12 meter driven element resonator adjusted for a
match through the same matching network.
This ability to adjust the antenna impedance to work with a fixed feedline/
matching network is one of the powerful tools in the coupled-resonator
(open sleeve) design. The spacing between the fed element and the
coupled element primarily controls the resistance, while the length of
the resonator controls the reactance, with some interaction, of course.
Since a two-element beam tuned for a good gain and f/b point has a
feedpoint in the 20-28 ohm range, I have had very good luck with a 1/4
wave matching section using parallel 75/75 ohm or 50/75 ohm coax.
I use the Smith Chart to determine what the impedance is at other
frequencies throught that line section and adjust the model for the same
driving point impedance. I use a ferrite sleeve balun at the feedpoint.
Good luck and have fun.
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