On 7/17/2011 7:54 PM, Rich WA1TRY wrote:
> Hi Roger,
Hi Rich,
Probably the easiest and most reliable would be to build a telescoping
pole out of the proper size tubes and alloy.. The smallest tube would
have to be large enough and of a strong enough alloy to minimize flexing.
This structural steel tube comes in 24' lengths and both schedule 40 and
80. Schedule 80 is getting pretty heavy per length.
Ideally DOM would be the material, but it's kinda pricey.
OTOH even only going 30 or 40 feet only takes two sections and gives a
lot of overlap. Even with DOM it's outlandishly priced, but it'd depend
on how elaborate a set up was constructed. The ham series rotators are
not designed to mount on top of a mast and hold a large tribander, but
some one on here about a year ago had built a mast with a cage that
enclosed the rotator at the top and also served as a thrust bearing. Not
all that pretty, but very effective.
I'm guessing a 3" DOM inside a 5" DOM would run about $400. Remember if
you only go up one inch that will only leave about a 1/4" clearance and
you'd not be able to use over 3/16" wire rope with that kind of
clearance. I'd think that would work for a 30 - 35 footer and there is a
big price difference between 4" and 5" DOM. Add to that the winch and
cables for maybe another $250 - $200 and we are up to $600. You still
need to come up with a base. I figure the 60 footer using 4 sections
would probably run me around $1500 - $2000 for material including
structural steel for the base. IOW everything except the rotator and
antennas.
I've been thinking of building a rotating, fold over, crank up mast
starting with a 3" center section, but going up about 60 feet. I'd use 4
sections which would give substantial overlap. Rotating the whole mast
really complicates things as the mast needs a top and bottom bearing on
the base support. These can be sleeve bearings. The mast has to be
mounted high enough to take a rotator underneath, and the telescoping
sections must be keyed into each other to prevent them rotating inside
one another, yet still allow the sections to move linearly. You also
need enough space between nested sections to allow for a cable of
sufficient strength to safely raise and lower the thing. Also "IF need
be" you can add a slip ring for guys although I'd hope to make it strong
enough not to need them.
I'm also working on a motorized crank up tower. With the crank up pole,
if you make it strong enough to minimize the movement, it's going to be
strong enough, but the tower comes with a whole different set of
engineering requirements. In addition I'd want a good 5 to 10 foot of
overlap between sections. That means for a given height the tower is
going to be very heavy. Even heavier than a commercial crank up and
certainly not any cheaper. I'd prefer to use a latching mechanism for
each section so they would hold the sections in place rather than the
entire thing hanging on the cables all the time . OTOH I don't think
anyone in their right mind would climb a "crank up" that was up and
supported only by the cables. although I know it's been done. Certainly
with the proper precautions (blocking) when the tower is up and *stuck*.
That's not a problem with the mast and there is no way to climb it. BUT
stop and think how heavy a tube(s) would be needed to hold a big
tribander at 60 feet without wobbling around in the wind.
>
> Oh I know where you are coming from. I agree. Someone mentioned (at
> a recent club meeting) that he once saw someone selling a kit for that
> purpose. I'm just more interested in the concept and such and if anyone
> had seen anything like that. Maybe the fact that none of us has seen or
> heard of such a thing in the last few years is a clue about whether it
> works or not. :-)
With out going into the details that might give some, ideas, as a
concept, I'd not think it'd be difficult. You'd need guides/wear pads,
cables, and a winch. In this case the fold over would likely make much
more sense. Even the old 25G fold over had the back of the tower
reinforced IIRC.
>
> I regret getting rid of my old EZWAY 55 foot crank up tilt over tower
> years ago.
Now that I'm older I wish I had put up a big crank up instead of the 10'
45G, or maybe a couple of 70 or 80 footers instead of the one big tower.
> Anyway, after that club meeting a bunch of us were
> discussing this odd-ball tower and much of what you alluded to came up.
> Currently I have a home made tilt over Rohn 25. It's 30 feet tall and
> has about 18 feet of mast coming out of the tower's top thrust bearing.
> I made a tilt over fixture similar to that of the commercial crank up
> and tilt over systems. It works fine. And I raise and lower the whole
> tower with a 1/2 inch cordless drill. So there is no electric motors
> etc. When I want to lower it I use a 5/8" socket instead of the winch
> handle and that provides the power to drive a worm gear winch.
Years ago, my late cousin made a 60' fold over out of 2 or 2 1/2" pipe
reinforced with a welded truss arrangement of 1/2 rod. This think held a
5 or 6 L 6-meter yagi plus rotator in a cage at the top. I don't know
how many people wondered how he stood that much pipe so small, on end.
IIRC It stood up to some mighty strong winds associated with thunder
storms along with quite a few Michigan winters, but one summer the gust
front from a line of thunderstorms took it down. If it'd been
structural steel tube with the same truss arrangement it'd likely still
be up.
73
Roger (K8RI)
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