[NLRS] Tuning 222 yagi [solved]

[Brent Casavant]

Hello again,

Well, it's probably bad form to respond to your own posting, but
I'm fairly excited.  I got the beam tuned!

I seem to have stumbled upon an unorthodox tuning method.  Well,
unorthodox in that I haven't seen this described anywhere before,
but it's worked for me with two seperate Yagis (222 and 70cm).

I use a T-match on my driven elements because that's what I know
how to build, and I'm getting good enough at it that I even like
how it turns out cosmetically.  Also, this seems to be what is
detailed in the K1FO designs in the Antenna Book.  I build my
T-match out of an appropriate gauge wire (12 for 70cm, 10 for 222)
and a hand-bent piece of aluminum to serve as the strap between
the matching section and the driven element.  I don't use any
capacitors in this design, though I understand that some people
get better tuning results if they insert capacitors at various
points in the matching section.

To tune, I start with the matching section cut fairly long (just
a few centimeters short of the driven element).  I then move
the connecting straps around until I minimize the SWR.  Usually
the SWR at this point can't be driven below about 2:1 or higher.
Also, the straps almost invariably end up very near the center
of the driven element (i.e. near the boom).

Then I begin snipping off sections at the end of the matching
section.  I start with about 2-2.5 centimeters at a time, and
reduce this amount as the antenna begins getting a better SWR.
I only snip off wire from one of the two ends of the T-match at
a time.  Usually this starts dropping the SWR fairly rapidly,
and it's not at all hard to get the SWR below 2:1.  After any
significant drop in SWR via clipping, I take a bit of time to
readjust the straps to minimize SWR.  If I'm still not happy
with the SWR at that point, I resume some of the snipping.
I seem to be able to eventually drive the SWR at the frequency
of interest down below 1.5:1 (usually in the neighborhood
of 1.3:1), and have adequate SWR bandwidth to work the rest
of the band below 2:1.

The whole process usually takes less than a half hour, and
gives fairly good results.  One outstanding problem I have
is that the SWR doesn't match up as well as I'd like across
the entire band, but I suspect that's more due to the design
of the Yagi (designed via computer with QuickYagi) than a
problem with the matching system.

So, I don't know if any of this is helpful, or just stunningly
obvious and trivial.  But I do know that it seems to work, is
reproducable, and makes a fairly quick job out of a problem that
otherwise seems to take days and results in the use of words
my mother doesn't know (and I don't mean "capacitance" :).

If anyone has any comments on this tuning method (ranging from
"cool" to "you got lucky kid -- you're about to destroy your
radio"), I'd sure appreciate them.

I'll talk to you this weekend on 222 from the N0RPM limited multi-op
station in Woodbury.

Brent
KD5EMB

P.S.  Thanks again to everyone who has given me suggestions, pointers,
signal reports, and just plain encouragement while getting all this
222 equipment going.  Now the fun really begins!

-- 
Brent Casavant            http://www.angeltread.org/
44.9067N 93.0558N 907F    -.- -.. ..... . -- -...