Jeff - First off, most 2M antennas will load OK on 70cm, it being the
third harmonic. Yagis will usually present a reasonable load to 50
Ohms, but, and this is a big but, the pattern will be split in the
forward direction and you will get a null or minimum in the direction
of the maximum on 2M, so you need to off point by 45 degrees or so.
This is not an optimum solution, but it works after a fashion. It is
probably best with short boom 2M antennas. I don't recommend this as a
permanent solution to your dual band antenna requirements, but if all
you want to do is get on and make noise, it will suffice. Similarly, a
2 meter halo, horizontal loop, or Big Wheel should load OK on 70 cm.
As some one has said, interlacing 432 and 144 antennas can be
problematic as the 144 elements are resonant on 432 and tend to couple
power from the 70 cm to the two meter elements, which skews the
radiation pattern to the side.
One way around this is to put the 432 MHz Yagi ahead of the Two meter
antenna instead of interlacing them. If the 70 cm reflector is placed
far enough ahead of the two meter director, interaction will be at a
minimum. The drawback of this is that the boom is longer than it
would be if the antennas were interlaced. Feed the Yagis with separate
feedlines or you can combine them with a diplexer at the antenna.
For homebrew versions, see WA5VJB's work on such Yagis for LEO
SatelliteB LEO use:
< http://www.wa5vjb.com/references/Cheap%20Antennas-LEOs.pdf >
You can use the same concept for his antennas cut to the weak signal
portion of the band:
< http://www.wa5vjb.com/yagi-pdf/cheapyagi.pdf >
He also has a simple diplexer in the first reference.
THe late W4RNL has a similar antenna on his web pages:
< http://www.cebik.com/content/a10/vhf/2-70.html >
You will need to register to get access to the site now that Jack
Stone has annexed the web pages after L. B.'s death, but it is still
free.
You can put the close coupling to positive use by placing the 432
element at the right distance to the 2 meter element so that you get
mutual coupling for specific impedances like you do between 2 coupled
parallel L-C resonators. DK7ZB has some designs:
< http://www.dk7zb.fox28.de/start1.htm >
I would stick with the 2/2 or 2/3 element designs until you are
certain that you can make the concept work. These spacings may have
tight toleranes.
Not quite portable in the sense you want, but N6NB has a interlaced
triband Quad he uses for roving:
< http://commfaculty.fullerton.edu/woverbeck/vhfquad.htm >
There are plans for log periodics around as well, but they tend to
have lower gain for the same bandwidth. A 2M Log Periodic can be made
to work on 70 cm, but there is the skewing of the pattern problem. I
think the Elk has this problem. If you want to make a short boom two
meter log periodic that will probably also load on 70 cm with the
split pattern, here is a good one from October 2001 QST:
< http://www.arrl.org/members-only/tis/info/pdf/0110042.pdf >
That link is for ARRL members only.
It should have performance similar to or slightly better than the Elk.
Tennadyne makes some log periodics for VHF, as does KMA, but again,
you end up with a fairly long boom and lots of elements for the gain
you get. They probably are not easily portable like you want either.
Other than the log periodics, as well as the Cushcraft and the Arrow
antenna, which both have the orthogonal polarization on the two bands,
I don;t know of any commercial solutions.
Let us know what you decide to do. - Duffey
--
KK6MC
James Duffey
Cedar Crest NM
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