You really should find some software so that you can model your two element
rather than guess. It's not as simple as just removing an element from a
three element design. The heavy duty element schedule should hold up well in
your location. A hairpin match is pretty easy to make and adjust or you can
also design the antenna for a 50 ohm impendence by increasing the reflector
spacing so no match would be required.
73,
John
To: "Towertalk" <towertalk@contesting.com>
Subject: [TowerTalk] 20M Yagi
From: "Michael Harris" <mike.harris@horizon.co.fk>
Date: Fri, 10 Dec 2004 19:46:03 -0300
List-post: <mailto:towertalk@contesting.com>
G'day,
I'm currently putting together the material requirements for building a
three element 20M Yagi based upon the optimised design in the ARRL
"Antenna Book" 18th edition.
Falklands is a windy place, not particularly high but persistent and a
real antenna killer due to fatigue. With this in mind I am planning to
use the heavy duty element taper schedule. At present I don't have enough
thick wall 2" tube for the three element boom but I do for a two element
of either reflector/driven element or driven/director element combo. Even
two elements are better than no elements.
The latter should give a little more gain. I would assume that either of
the parasitic element lengths would remain as per the detail in the book
with just the driven element having to be retuned. Driven/director with
the greatest spacing probably least susceptible to change. Interesting
that for the optimised designs of the 18th edition the reflector is much
closer spaced to the driven element than that shown in the 13th edition
(1976).
Because of this I can only make a stab at the approximate radiation
resistance of the driven element for a two element array at somewhere in
the region of 20 ohms.
Pondering the option of "T" match but not too much information available.
Does one try and match the 20 ohms or so to 50 ohms with the "T" of go for
200 ohms and transform it down with a 4:1 balun, this could make for
rather long arms maybe. In either case I would be interested in some ball
park figures for the mechanicals, also the option to do away with the
series capacitors in the "T" arms by detuning the element on the hf side.
I have a copy of the ARRL "Yagi Antenna Classics" on order but also
thinking about getting a copy of "Physical design of Yagi Antennas" by
W6QHS.
Now if I could only get the software of the 20th edition without having to
replace my 18th edition rather earlier than is reasonable.
Regards,
Mike VP8NO
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