While I cannot offer much as to how a lower antenna will be protected from
lightning, I can offer some suggestions on constructing an "around the tank"
antenna. In this type of array, phasing of the antennas is NOT important
unless one is relying on electrical line lengths to split the power. With
antennas spaced far apart as on a water tower, the phase relationship
between the mobile and each antenna is always changing, so by phasing the
lines to coordinate the launch phase doesn't make any difference as to how
the array works. Years ago I was involved in designing a master antenna
system to go on the side of a building, and we used 60 degree beamwidth
panels center mounted on each wall, fed via a four way power splitter. An
antenna manufacturer modeled it for us, (they built a scale model of the
building AND the antennas and tested at a scaled higher frequency) and their
conclusion was that it was a very complex pattern with many nulls, HOWEVER
most of the nulls would interact with traditional mobile (spatial fading)
flutter producing a pattern that was essentially omni. Well, kind of square
actually because of the 60 degree antennas. The mobile flutter would
interact with the interference pattern of the antennas reducing the nulls!
Further, they moved the antennas to the corners so that two were mounted at
right angles at opposite corners of the building (4 antennas total) and the
pattern was nearly identical. You will have to take the hit of a four way
power split (6 dB) on the gain. That means if you used 4 3 element Yagis (6
dBd gain, 90 deg BW nominal) you'd end up with about a 0 dBd gain omni. If
you used 5 element Yagis (9 dBd gain, 60 deg BW), you'd have about 3 dBd
gain, but pattern wouldn't be quite as omni. Now, we never actually built
the thing, so I have no first hand on the air experience. Not having to
worry about phasing and, being able to mount the antennas in pairs may make
it worth giving it a try. If you do, please report back.--Mike, WV2ZOW
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