Roger,
(K7LXC -- check out notes on tower rotation below)
Difficult question. I agree that the fixture is crucial for installation,
but I stopped using it once I discovered the ladder technique. That's
because the ladder is much faster and simpler. That's especially true if you
have the motorized version. The reason is that the tower can't rotate 360
degrees unless the fixture is removed -- otherwise the motor and relay
cabinet would hit the fixture. I know that's true of the motorized
MA-770MARB and the MA-890MARB, and imagine it must be true of the MA-550MARB
as well. This means that the procedure for tilting the tower includes
hauling the fixture to the tower, lifting it in place (mine is 80 lbs!),
bolting it, removing the pulley wheel on the tower, threading the tiltover
cable through the pulley, and reinstalling the pulley wheel. Then you have
to disconnect the rotor from the tower shaft, turn the rotator so the tower
can tilt (or remove the rotor), remove two pin bolts from the tilt hinge and
remove the bolts in the upper collar. When you're done, you have to do all
this in reverse. You also have to be careful to do everything in the right
sequence. For example, if you forget to remove the pin bolts, they will be
sheared off. In my case, I also have to remove various cables and a
protective cage (see below) before I can tilt the tower. I can install and
remove the ladder in less than half the time.
It's also easier to do certain jobs on the ladder. Remember that the boom of
the antenna prevents the tower from being tilted horizontal. This means that
you may need a very high stepladder to reach the feedpoint and other areas
on the antenna. In my case, a 10-footer is required. Depending on the
technique you use, installing a large yagi may be easier using a strap-on
ladder. When we originally installed my TH-7, we attached the 5-foot mast,
stood the beam on end and mated it with the tilted-over tower. It took six
guys to do that. I suppose you could "build the beam" on the tilted tower,
but that is complicated as well. When I replaced the TH-7 with a 20M
monobander, I found it much simpler to use the ladder and a pulley/rope
assembly. It only took two of us to do that and it went pretty quickly.
Another reason I use the ladder is to adjust the @#%$! pull-down cable. It's
a major design problem with these towers. The cable stretches or loosens up
over time and either pops off the pulleys (if you have them) or gets caught
behind the main pulley brackets. It happens when the tower is fully extended
(and the cable is at its slackest) and blowing back and forth in a stiff
wind. When the cable snags, it's dangerous to raise or lower the tower. I
found that I had to remove the pulleys (recommended by U.S. Tower) and keep
that cable tight. This involves loosening the tension spring (which must be
done in stages, raising and lowering the tower at each stage), tightening
the cable at the top with a come-along (using the ladder technique), and
retensioning the spring (again, in stages, raising and lowering the tower.)
This procedure is extremely difficult and time consuming using the tilt-over
fixture. It may not be an issue on the MA-550, but it is a real problem on
the MA-770 and MA-890.
A word on the ladder techninque -- it's potentially dangerous. There are two
main ways to to it. One is to lean the ladder against the tower at an angle.
You must have a couple of strong people at the bottom to hold the ladder in
place while you scramble to the top and secure the top of the ladder to the
tower. It's wise to have a tree fixture at the top (kind of an open collar
that keeps the top of the ladder from slipping off the tower.) It's very
important to secure the ladder so that it can't twist. I use the vertical
method -- I put the extension ladder flat against the tower and strap it
every 4-6 feet from the bottom to the top. I can do this by myself. You
*must* use standard tower climbing techniques in this case -- including a
climbing belt, safety lanyard, tool bag, steel-shank shoes, hardhat, etc.
It's only 22 feet, but you can still get maimed or killed if you fall.
With all that said, there is one reason I'm glad I bought the tilt
fixture -- cable replacement. The cables on these towers must be replaced
periodically. U.S. Tower says every three years. Some say that's more
frequent than necessary (remember the LXC directive!), but even in the most
favorable environments you're looking at 5-10 years max. The tiltover
fixture is essential for dismantling the tower to restring it. In fact, U.S.
Tower says the cables can be replaced on the 550 and 770 without removing
the tower from the base. You just tilt the tower and remove each
cap/section. Even the next-to-last section on the 770 is light enough to be
handled by a small crew. Apparently, the 89-footer requires special
equipment because that section is too heavy for unassisted people to handle.
(LXC -- read this!)
Finally, let me say something about tower rotation. There are a couple of
secrets to making the U.S. Tower MARB series rotate that they don't tell
you. First, there is only one proper orientation for the rotor shelf, even
though it looks like you can fit it either way. The proper orientation is so
that it looks like an "L" from the side (not an upside-down "L".) Mine came
shipped upside down and so did a friend's. If you mount the rotor on the
plate when it's upside down, the tower will bind (the plate is made in a jig
and the angles are crucial.) Second, the small pipe that connects the rotor
to the tower drive shaft must be allowed to flex -- it's a poor-man's
U-joint. The rotor jaws must be well below the bolt used to attach the pipe,
at least 2 inches or more, or the pipe can't flex properly. This can be
complicated if there is not be sufficient room between the drive shaft and
base of the tower to lower the rotor mounting plate. I had to cut off at
least an inch from the anchor bolts to be able to lower the shelf enough
(this was recommended by Bruce at U.S. Tower -- I should point out that U.S.
Tower made an error in manufacturing my tower, such that there the gap was
2" smaller than it should have been.) Some rotors are large enough that even
the standard dimensions will present a problem. I also had to cut off part
of the pipe to get it to fit (I'm using a Tailtwister.) Note that the
aircraft nut on the bolt must not be fully tightened -- just screw it down
until it touches the pipe (it won't fall off.) Third, the hinge and pin
bolts must not be tightened so much that the hinge fixture is distorted.
That's easy to eyeball. Fourth, the upper collar must be generously lubed
with grease. Plain old automotive grease will do, or if you're in the North
use low-temp grease. Just filing it is not sufficient because the
metal-to-metal contact will eventually damage the steel tube. I'm skeptical
that filing the high spots will solve the problem on that 89-footer -- the
mast moves around a lot in that collar when the wind is blowing, so it's
hardly a precision fit (that's why the flex joint is so important.) Fifth
(and related to the last comment), the tower must be absolutely plumb and
the beam as balanced as possible. It won't work to plumb the base vertical
member -- there is an offset angle between the member and the tower.
Instead, adjust the base bolts until the mast is plumb. When the tower is
properly adjusted, you can turn it with one hand and the rotor won't bind.
I don't agree that it's better to put the rotor at the top. That adds a lot
of weight at the top, which makes the swaying worse in stiff winds. Mast
mounting is much tougher on the rotor, too. When it's at the bottom, there's
no side loading at all. Finally, it's a *heck* of a lot easier to maintain
the rotor when it's mounted at the bottom. Neither the ladder technique nor
the tilt fixture will simplify removing a rotor at the top. It is
significantly more difficult to remove a mast-mounted rotor than one that's
inside a tower.
One last word. If you have the motorized tower, be careful about dressing
the cables up the tower. Use very flexible cable (RG-213 is OK, LMR-400UF is
not) and put the standoffs opposite the motor assembly. I found it necessary
to install two additional standoffs, one above the motor and one below.
Otherwise, the cable could snake around the tower and get snagged on the
motor assembly (this happened to me, with almost disastrous results -- I
could have lost the tower.) You might get away with locating the bottom
standoff just below or above the motor, or just adding just one extra
standoff there. I also fabricated a protective cage out of hardware cloth
that keeps the cables from getting fouled in the rotor area. This is all
needed because I can't see the the tower when I'm raising and lowering it.
Hope this is helpful to all,
73, Dick WC1M
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