It would be nice to have a formula and calculate the precise load and match
the rotator to antenna system, and sleep well knowing they would last forever.
Just as we can't calculate the "eternal" tower installation, it is hard to do
that with sizing up the rotator. The reason is the type of load that antennas
present. That load is dependent on weather factors, like wind, temperature,
ice, etc. Metals get brittle with subzero temperatures. Even "properly"
designed power line towers and buildings fail when things get extreme, never
mind the "puny" ham stuff.
Let me elaborate a bit on few aspects of antenna installation. I went
from
home brewed 65' tilt over tower with TH6 and 402BA on Ham-M all the way to
110' Big Bertha with pair of stacked 62' boom Razors and Telrex rotator.
Design criteria I used especially on home brewed Razor (4Quad 3Yagi
elements)
installation was that in worse case scenario, I wanted tower to be left
standing. If tower came down (5000lbs tubular 110' self-supporting mast), it
would take out house and maybe people. So I wanted that to stay up. Next
heaviest thing is the boom, it should stay there, pointed down wind, it
presents minimum wind load (my booms were telescoped/tapered, heavy wall). If
the boom folded or broke, it should not fly off, so there is guying on the
boom. If there is a choice between boom or elements flying off, bending,
folding. I preferred elements, they are cheapest/easiest to replace. If the
elements fly off, boom and tower have chance of surviving (most expensive-
dangerous thing).
Now lets look at antennas as a load. Weight or mass is important to
consider
for handling purposes and for inertia loads. Lighter, better. Due to antenna
design, (element sizes, spacing, balance) they are not always perfectly
balanced as far as wind load is concerned. There is a way to balance the
antenna by adding dummy piece of aluminum or plastic tube (element) on the
boom to achieve the symmetry for the wind load and also possible to maintain
the weight balance (if needed, insert some weights in the boom at certain
distance). There is the software to do that. We should try to keep this
balance. But then comes the freezing rain, ice and wind, which disregard our
efforts and deposits load based on the surface area of the antennas, which
does not exactly match our previous match and throws things off balance. Add
the crazy gusty winds and disaster is in making. This is why I prefer things
to fly off in order of 1- elements, 2- boom, in hope to have tower standing.
The point is, we can try to calculate and design the antenna system for our
worst case scenario, but the there is always that big unknown in the sky that
can trash our efforts (hurricane, killer freezing rain, storm).
Now looking at more peaceful scenario with "normal" loads presented to
the
rotator and the tower. Antenna system on the mast presents the axial load from
the weight of the mast/antennas and torque load made of inertia and wind load
imbalance. Axial load should be taken out by the axial bearing system, do not
(in larger antennas) rely on the rotator taking it (need to replace, etc.).
The torque load comes from wind load unbalance and inertia when starting-
accelerating-decelerating-stopping. We could divide the torque to braking
(locked) torque and turning torque. Normally they should be the same, but some
ham rotators, that have tiny gears and could be backdriven by nonsymetrical
loads, have brakes that lock the mast in position when resting. Brakes, like
in Ham-M family, have teeth, inner gear and plunger to achieve that. Ideally,
brakes should be activated and released when the mast stops, delay arrangement
is needed. Inertia load comes from mass x arm (distance), the faster we are
trying to get going, the bigger the load. The worst case scenario is the
sudden starts and stops. The stress can be many times higher and depends on
accelaration/decelaration. (Add ice on the system and things can go up by
factor of 5 - 10 times.) The best way to handle the inertia load is to
accelerate the motor, run, decelerate, stop, apply the brake if needed.
The best rotator is the one that is well overrated for the job. The best
design is the one using worm gear drive as the last stage. Worm gear is self
locking (with proper worm pitch) and properly rated it would take the worst
case load, no need for brake. The next best is the high gear ratio that is
self locking (prop pitch with original gears). The motor should be capable of
gradually accelerating and decelerating (use DC motor or suitable speed
control circuit) with possible smooth proportional control. Motor speed should
be proportional to the deflection of the control lever. This allows for smooth
control of antenna movement from few degrees to major excursions. Ideal max
speed is somewhere around 2 rpm (with proportional speed control).
The best rotator has to be overrated many times, it should not fail.
The best
commercial rotators I have seen is the Telrex worm gear one. It turned 5000lbs
110' Big Bertha tower with antennas top to bottom. It has a worm gear, has
chain reduction between it's sprocket and tower gear. There were times when my
offset stacked Razors (not balanced for wind load) could not be turned in 100
mph wind, but nothing broke. I would let the antennas go and point down the
wind to present least load.
The point of all this is that it is difficult to calculate precisely the
loads, have specs on rotators and antennas, match them and be happy. Mother
nature plays games that are hard to put numbers on. Heck, there was a case
when long beam was flipped upwards and folded over the top (who would put guy
wires on the bottom of the boom?). The wind, ice loads are unpredictable,
could reach "unreasonable" levels. The best we could do is to overrate our
systems to the best of our abilities and resources. Load situation can vary
with geographical location. If I remember correctly, Telrex designed their Big
Bertha system with stacked Christmas tree load for 1" radial ice and 100 mph
winds. That seemed to be verified in real life, W2PV system survived worse
than that (one element broke off). But then Arnold WA2HCW put 4 el. 80m KLM on
the top of his Bertha, and it snapped off (he was told not to do that). Up
North ice is the killer, down South Hurricanes are levelers. This is why I
used the approach outlined in the beginning in serious antenna installation
design. Another nice thing about the Bertha (self supporting, rotating tower,
no guy wires) is that I could do all antenna installation, hoisting it all by
myself, with block and tackle. In case of destructive winds or weather coming,
I could drop my antennas by myself within about 30 minutes and leave tower
bare, which should have survived hurricanes (like fishing pole). My
installation was in residential area on a 46x120 ft lot.
Applying the above to present commercial rotators, I would go double or
triple on ratings that manufacturers give (cheaper in long run). For serious
antenna installations, go for prop pitch or Telrex style worm gear rotors.
Design your antenna system for the minimum damage in case of extreme,
catastrophic situations or have provision for "disarming" the tower if needed
in a hurry. SP3DOI had a neat installation: He had about 120' self supporting
tower with a carriage riding up and down. Carriage had the rotator, bearings,
mast and antennas installed on it. Cable hoisted it to the top. In case of
destructive weather coming up, it was lowered down, where it could weather the
storm, ice, etc.
The point of all this is: don't rely on specs, don't save a penny here,
make
provision for extreme situations. Loads exerted on antenna installation can be
many times the worst scenario and we should count with provisions for that. We
can do our best to try to minimize the loads and factors, but we could never
be sure. If you saw the destruction from last years ice storm in NE or results
of Hurricane sweep through NP4A, we would be lucky indeed to see our tower and
rotator standing, the heck with antennas.
I hope I shed some light at this aspect of antenna installations and
some
practical ideas. I sleep well now. I have 20m GP on the roof and 160m Inverted
Vee 30' up in a tree, but I hope to get the Bertha up this year and lift the
base by about 30 ft, I miss the sucker.
GL Yuri Blanarovich
K3BU, VE3BMV
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