There are several multiband VHF designs out there. WA5VJB has a two band one
that he suggests for LEO satellite use, but you could just as easily use the
designs for the low end of the band:
< http://www.wa5vjb.com/references/Cheap%20Antennas-LEOs.pdf >
He also has a design for a simple duplexer. No need to have the sections on the
same board, put the appropriate diplexer section at the appropriate feedpoint
and feed with a single feedline.
He puts the 432 MHz antenna in front of the 144 MHz. This works out fine. There
is a lot of interaction if you interlace the two as 432 MHz is the third
harmonic or 144 and you get some 3/2 wave pattern from the 2M beam unless you
separate them. Putting the 432 MHz in front puts the 144 MHz three half wave
potential interaction behind the beam, where with even modest separation, it is
pretty much down by the front to back ratio.
W4RNL, now unfortunately SK, proposed a similar design:
< http://www.cebik.com/content/a10/vhf/2-70.html >
with a three element 2M Yagi beam behind a 4 element 432 MHz Yagi beam. I
hesitate to suggest the above link, as you need to register for the site, which
puts you on an e-mailing list (some call it SPAM) from Antennex. But the value
of the information may be worth more than the inconvenience of the canned meat.
DK7ZB has used closely coupled resonators, he calls them open sleeve
principles, a la Hy-gain, K9AY, and Force-12, to design 2m/70cm duoband designs
and a triband 50/70/144 MHz design. In these cases the elements are interlaced,
but done in such a fashion that the interaction is used to advantage:
< http://www.qsl.net/dk7zb/start1.htm >
One feed line and no diplexers. Spacing is critical though, and the higher band
suffers when the number of elements increases.
Wayne's triband quad:
< http://commfaculty.fullerton.edu/woverbeck/vhfquad.htm >
is a reasonable design that can be reproduced and there are at least 11 of them
in use. It still has 3 feed points, so you need three feed lines or a triplexer.
And there are several VHF log periodic designs floating around. The Elk is a
144 MHz log periodic with the elements swept forward so that the pattern is
still pretty much on axis at 432 MHz. KMA makes commercial log periodics for
VHF, including a 2M to 0.7M version. I think that the rovers who have tried log
periodics have not been particularly pleased with the performance to length
ratio though.
Multiband antennas are nearly almost always compromises, even more so at VHF,
but you don't give up much in performance if you stick to two bands that are
spaced not too far apart and rather small beams. The 2 element/2 element and 2
element/3 element (144/432) designs of Dk7ZB have performance that is pretty
close to the separate beams with the same number of elements (not the same
length though) with a single feed line. You can go to longer antennas, but the
higher band performance suffers and other than the fact that the boom length is
already there and you only need one feed line, for longer 432 MHz antennas
lower gain is the compromise, along with the requirement for careful spacing.
You need to decide what you want to give up. With the log periodic you end up
with a lot of metal you don't need in the air, and if you take all that tubing
and make 2 or 3 separate well designed Yagi antennas you end up with better
gain on all the bands. All this for only the price of one or two additional
feed lines or a diplexer/triplexer.
For the two antennas one behind the other on one boom, you give up the higher
gain that goes with the longer boom length that one would have if one did not
share the boom.
If you are using separate feed points or feed lines for each band, there isn't
much point in putting them all on a single boom although it may be more
convenient in a rover; stacking them on a short mast is not much of a problem
and the mechanics of that are probably less problematic than dealing with a
long boom that does not balance well because of elements on one end that are 9
times the size of elements on the other end, probably requires a heftier rotor,
and has large elements on one end that can act as a weather vane if you do
figure out how to balance the boom.
Having said all this, I have designed and modeled, but not yet built a 2
element 6M/3 element 2Meter beam and a 2 element 144/3element 222/4 element 432
beam fed with single feedline, and yes I realize there can be a big difference
in what NEC spits out and what is measured on a finished product. I hope to get
to build these designs this spring. Now these antennas won't generate a big gun
signal for the home station, but they should be a big step up for the guy who
is running horizontal loops and they should also be small enough for rovers to
use in motion and also a significant step up from a loop for rovers. I came to
the conclusion that trying to design a single antenna that covers both 50 MHz
and 432 MHz is difficult, whether or not you include two meters.
A 144 MHz beam can be put into service on 432 MHz if it is off pointed 45
degrees or so. The gain will be less than a well (or even mediocre) Yagi design
on 432 MHz of the same length, but it will usually work.
There are other tricks one can employ. The metallic boom of a 2 meter beam, or
even a 70 cm beam could be gamma matched (or tee matched) and used as a dipole
on 6M for instance. The two meter elements would just add a bit of capacitive
loading. One would need to isolate the two meter feed line from 6 meters, but
this kind of thing is done on the low bands when shunt feeding towers. The peak
in the 6M pattern is not in the same direction as the 2 meter antenna, which
may or may not be a problem. Probably not for most operations, but certainly an
impediment in running the bands in a contest with a distance station.
I think that the Arrow and Cushcraft beams with the elements in different
planes are not starters, unless one mounts them so that the elements are +/- 45
degrees from vertical. And then you have lost 3 dB or so.
I suspect that the real reason we have not seen more of the multiband antennas
mentioned above used is that they are not for the most part commercially
available. As to why the WA5VJB antennas are not more widely used, I suspect
that is a matter that people really think that they are too good to be true,
something so simple and effective can't possibly work. So they don't try it.
This is too long, but I didn't have time to make it shorter. - Duffey
--
KK6MC
James Duffey
Cedar Crest NM
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