"tom" wrote in message
. ..
On 2/26/2013 3:57 PM, Dave Platt wrote:
In article ,
Sal salmonella@food poisoning.org wrote:

RELATED: If you know, why do some yagis designs place the first
director
quite a bit closer to the DE than the rest of the directors' spacing?
In
other yagi designs, the spacing of the directors is relatively
consistent,
where DE-to-first-director is about equal to the spacing of the
remaining
directors. I haven't seen that difference explained ... yet. Thanks.

You might be looking at what has become known as an "optimized
wide-band" Yagi.

According to one article (http://www.naic.edu/~angel/kp4ao/ham/owa.html)
placing the first-director parasitic element very close to the
driven element, causes the antenna to be behave as if the driven
element's diameter is much larger than it is (approximating the
distance between the DE and the first director).

This has two useful effects: it increases the radiation resistance
(making the Yagi easier to match to the feedline) and increases the
bandwidth. You give up a small amount of forward gain, but this is
said to be modest and to be a worthwhile price to pay for the ease of
feeding (even a direct 50-ohm feed can be used in some cases) and the
increased bandwidth.


Some of us old time EME/weak_signal builders call the first 4 elements the
K1FO launcher.

I did a lot of optimization over the last 25 years, but I didn't change
the first 4 elements of his design much. He got it right and it worked
especially well with a T match.

tom
K0TAR


MID-TERM REPORT.

It's been a long time since I posted. Last month, in the middle of
my work, my 45 y/o son needed to come back to live with my
wife and me. We spent over a week moving him out of his
apartment, including more than two dozen cans of trash carried
to the dumpster. Bad scene.

When we had him settled, I loaded my van with raw material and my wife and I
headed to our daughter & son-in-law's house, in Livermore CA where we were
scheduled to house-sit for specific dates.

While there, I wanted to try for a remote 220 repeater, part of a linked
system that also exsts locally. It's called the Condor Connection. I have
two I can reach in San Diego; the "target repeater" for my son-in-law in
Livermore is on Mount Hamilton, east of San Jose. Blockage is the issue,
not distance.

Last month, here at home, I had earlier made a good gamma match using just
the driven element, alone. Last week, when I added the reflector and 8
directors, that match was out the window, as expected, but I got it back,
manipulating the gamma cap and the attachment point while standing on a
ladder with a 5W transceiver, a VHF/UHF VSWR meter and a screwdriver. It's
a pain in the butt to set but it was a learning experience. (I'm 70, now,
but I still value everything I learn now as much as I ever did.) Yes, I
will try everyone's favorite, a T-match. I'm not done experimenting.

In operation, I was able to bring up the Mt Hamilton repeater -- but just
barely. I had a 50W linear with a receive preamp. I needed the whole 50W
to bring up the repeater and it's response was so noisy as to be little more
than confirmation that I had "something." By swinging the antenna left and
right while the repeater was responding, I was able to infer that my antenna
hbas a fairly narrow beamwidth.

As an alternative to Condor, I arranged for my son-in-law to get into a
CALNET repeater in Stockton. I gave him my 27-element UHF beam. His 5W
FT-60R is plenty with that gain antenna. CALNET is represented down here,
so we can chat occasionally. He'll eventually upgrade, so we can use HF.
For now, linked repeaters constitute the only game in town.

I want to play more with that yagi. Having long elements (among the various
designs) it's no surprise to me that its VSWR low point (1.2:1) fell neart
222 MHz. I'll either shave the existing elements or make some new ones and
see if I can move the low-VSWR point up nearer 224. I also want to take
some pattern measurements. I described a method and I want to try it. My
plans were short-circujited by the family issues.

"Sal"