ANTENNA
ARTICLE BY WILLIAM B. SEABREEZE, W3IY (SK)
Edited and Posted by Paul Bock, K4MSG
Hamilton, Virginia May 14, 2011
Although not a member of LARG (he was not a
"club joiner"), Bill Seabreeze, W3IY, was well known
to many of our members and some of us even had the privilege of working with
Bill in the "real world" of electronic design & engineering. He was not only a fine gentleman and a good
friend to those of us who knew him, he was also one of
the best VHF/UHF/Microwave amateur operators on the air and was always willing
to lend his time and expertise to anyone interested in improving his or her
station. Professionally, he was the best
hands-on RF design engineer I've ever known.
Sadly, Bill passed away in September, 2005, after
a long and courageous battle with cancer. He is sorely missed not only by those who knew him personally but also
by the legions of operators who worked him in VHF Contests yet never met him
face-to-face. Bill's favorite motto,
"Listen for the weak ones", is timeless advice for all of us who enjoy
operation above 50 MHz.
|
This article on antenna design for VHF/UHF "Rover" operations was located on the Internet and is posted as a tribute to Bill. I have edited Bill's article by incorporating his antenna pattern graphs into the body of it and writing the above short tribute. The technical content is still relevant; in fact, I was "googling" the topic of Yagi spacing at 144 and 432 MHz and this article popped up! It may be found here: http://www.w1rt.us/w3iy/stacking.htm. The article bears no copyright information but is republished here in whole and with full credit to WILLIAM B. SEABREEZE, W3IY (SK). It is still relevant today. |
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Antenna Application Note 47899.11.
(by Bill Seabreeze, W3IY)
Many rumors and misconceptions exist in the
amateur radio world about stacking VHF/UHF antennas on the same mast. I have
heard all sorts of warnings about screwing up the performance of my yagis by mounting them in a compact fashion on my rover
masts. I have been modeling antennas using NEC2, and I am here to tell you that
you can really get away with alot, and still maintain
good antenna performance. For example, I am considering adding a medium-sized
2m beam (12-15ft length) on my rear mast, just below the 432 yagi. I wanted to know how close together these yagis could get before the 432 gain went to hell. Well, I
found that I could safely stack them, at a distance of only 2ft, and only lose
about 0.5dB of forward gain!! ...and the pattern is still very good! This is a
pleasant surprise. I always suspected this, from previous experiments, but I
wanted some computer modeling to really convince me it was OK...well...guess
what...IT IS...JUST DO IT! I have always been pleasantly surprised, when I have
stacked yagis close together. You don't need to worry
about that old ARRL VHF handbook recommendation of
"half-the-boom-length" of the upper yagi.
Ed Tilton was a great guy, but he was also pretty conservative about antenna
stacking (and he didn't have computer modeling like we have today).
Below is a pattern of an 18el yagi of my own design, optimized for forward gain in free
space.
Here is a pattern of the same 18-element yagi stacked 24 inches above a 12 ft long 144 MHz yagi. You will
notice only about 0.5dB degradation! Not too bad, considering the extra
rover-ERP-squirt you will get!
I plan to do a lot more studies along these
lines, and I will publish my findings here, if there is any interest.
For example, I am going to model my front
mast, which has a 6-meter halo on top, a 5el 2-meter yagi,
and a 222 MHz yagi, all pretty close together.
Loop yagis are off-limits...unless I can model them as rods...which makes sense. They require more exotic 3D EM
software (and hardware) than I have access to.
Loop yagis are
wonderful...it's just that nobody has yet completely optimized them, and they
are typically 1 or more dB below the performance of an equivalent rod-type yagi. They do have other advantages, though, like being
easy to get working, and being more tolerant of construction anomalies. Watch
out for low hanging tree branches, however...loop yagis
are much harder to repair if you hit trees with them, which will probably happen
if you do a lot of roving.
Many 1296 yagi
designs are fine, but then get screwed up when the unsuspecting builder applies
the wrong boom correction factor to the element lengths...beware! You cannot
use the accepted DL6WU/G3SEK boom correction factors for booms greater than
0.5in or so at 1296 MHz. A strange non-linearity
occurs with these electrically large booms, and the correction factor actually
shrinks to zero at around 1.0in boom diameter at 1296. The effect of the boom
changes from a shunt inductance at the boom center to a capactive
end-loading effect as the booms grows larger than 0.5in, and at some point
around 1.0in diameter, these opposing effects cancel each other completely. I
have verified this using a network analyzer, and I
hope to find time to generate some useable graphs on the subject. I would be
greatly interested in hearing from anyone with experience using these larger
booms at 903, 1296 MHz, or above.
You don't want to spend a weekend drilling
holes, and filing elements just to find out you built a dummy load.
If you have the time and inclination,
consider building several prototypes with fewer elements, to verify your
element correction factor, before embarking on the 144 element long-long
microwave yagi. If you can measure gain vs frequency, you should be able to assess the validity of
your element lengths. It can be very hard to match a 1296 yagi,
even when the element lengths are correct. The DL6WU yagi
design algorithm seems to produce a good match to a folded dipole, from my
experience. Even folded dipoles can be difficult to build without excessive
parasitic reactances at 1296 MHz.
If you want to read more about antenna modeling...go to www.cebik.com
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