Greg,
I didn't respond because I figured that you would get plenty of responses
and they would include what I consider to be the best combination. You
didn't get that combination in your responses but, to your credit, you hit
upon it yourself.
I like the 40/10 and 20/15 combinations and here is why. The windloads and
mechanical loads are relatively well balanced for the two towers by
combining the largest and smallest antennas on one tower and the two medium
size antennas on the second tower.
And the electrical interactions are relatively smaller this way. Here are
the arguments against various combinations and then the arguments in support
of the proposed combination.
1) 15 meters is a 3rd harmonic of 40 and putting them together is the worst
possible scenario, although with loaded 40s this may be mitigated somewhat.
(see 4 below).
2) 15 and 10 meters are close enough together in frequency (percentage wise)
that 15 meter directors look like 10 meter reflectors, it makes for bad
interactions unless one does a dual-band single-boom beam with 15 at the
rear and 10m at the front. (There is an old design doing this in CQ, I
think, from the late 50s or early 60s)
So the above rules out 40/15 and 15/10 combinations which leaves 20/15 and
40/10.
Arguments in support of this combination.
1) 15 meters is not harmonically related to 20 and is far enough away in
frequency (percentage wise) to minimize their interactions.
2) The 10 meter beam is so much shorter than the 40 it is transparent to
the 40.
3) The 10 can be mounted relatively easily more than a 1/4 wave above the
40 helping to reduce the coupling effects, I would suggest trying to get it
10 ft or more above the 40 if possible (at least 8ft).
4) If you are planning on a loaded 40 meter beam rather than a full size
one then their may be no resonances within the 10 meter band, since the
loading tends to pull the harmonic resonance quite a ways away from the
actual (mathematical) harmonic, meaning it is still a big piece of conductor
but may not resonant within or too near the 10M band.
There are two ways that nearby conductors can affect the performance of a
tuned beam. 1) if the nearby conductors are resonant then they can be
anywhere in the near field and have a large effect on the antenna performance
2) if the nearby conductors are non resonant then their effect comes from
capacitively coupling to the elements in the beam and detuning the elements
themselves. This requires much closer proximity than the case with the
resonant nearby conductors. This is relevant for conductive supports for
side guys in long boom beams. If the side guy supports are not placed too
close to one of the elements they can be made of conductive material as long
as they are kept to only a few feet in length. This can be modeled with
interesting results in YO (and confirmed in NEC) by adding an extra director
to the design and making it very short. It has virtually no effect as long
as the spacing is a couple of feet and the length is also just a few feet
(at HF of course).
footnote: Propagation might tend to favor putting 15 and 10 on the same
stack because they are generally open in the same directions as well as 40
and 20 on the other stack which can also often be open in the same
directions. But for the above interactions this is possibly the best
combination. But with the 40/10 and 20/15 combination you have combined on
the 40/10 tower the two bands that are open at opposite times, allowing the
10M beam to be rotated where ever you want during the day and the 40M beam
to control the direction at night. That only leaves 20/15 to fight it out
for best directions, and then only during the day time since 15 is usually
dead at night and 20 may well be open. And during the daytime 20 and 15 are
usually open the same direction, although not as closely linked in
propagation direction as the 15/10 combo. You might consider doing a ring
rotator on at least the 20/15 combo so that they can be pointed in different
directions. The 10/40 can wait for independent direction control for a
couple of more years until 10 gets good enough to worry about.
Hope this helps.
73 John W0UN
John Brosnahan
La Salle Research Corp 24115 WCR 40 La Salle, CO 80645 USA
voice 970-284-6602 fax 970-284-0979 email broz@csn.net
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