The problem is that big antennas have a lot of wind load
and a lot of momentum. With any backlash at all, they
can bang back and forth using the element flexing to store
energy. This violent jerking, which is hard to analyze
using conventional engineering techniques, is what tears
The prop pitch has a track record of not breaking under
these conditions. Conventional gearmotors from the
"Motorbook" may or may not be able to handle this. The
torque ratings on them only tell you if they can turn
the antenna. If I was going to experiment with those,
I would want to get something with gears of comparable
size to the prop-pitch. Look at the gears on K7NV's site.
Pretty big, and there are three of them to share the load.
Another thing you might look at are those ubiquitous 12VDC
winches that you put on your SUV to pull it out of the mud.
They mention 8000 lbs, although it isn't clear if that is
the pulling force, or the vehicle weight (on a limited grade).
Anyway, you could perhaps modify one of those winches to
turn an antenna. Getting the 12VDC (at "too many" amps)
up to the rotor is going to be another problem.
> -----Original Message-----
> From: email@example.com
> [mailto:firstname.lastname@example.org]On Behalf Of Jim Lux
> Sent: Wednesday, January 24, 2007 2:01 PM
> To: towerTALK@contesting.com
> Subject: [TowerTalk] rotors, controllers, etc.
> I'm not likely to be buying a rotator for myself soon (being more of
> a phased array kind of guy), but, it seems that for the $1300 or more
> for a top of the line ham market rotor, you could do better by buying
> a controller like the Green Heron (for $550) and fairly off the shelf
> motor/gearbox combinations from Grainger or McMaster Carr ($400 or
> so). Sure, there's the advantage of "it's already been packaged and
> integrated", but if you're up in the category where you're twisting
> something big enough (like a MonstIR) to need the top of the line
> rotator, you've probably already done a fair amount of system
> engineering and already into the project for several tens of
> thousands of dollars. A few hundred bucks for some fabrication of
> the needed mounting brackets (if they don't already exist from the
> mfrs), and you've got something that is not only going to be
> bulletproof, but uses cheap and readily available replacement parts
> (standard sized motors are available everywhere, over the counter).
> Or is this just a sort of historical thing.. people started with
> cobbling something together from surplus (like my grandfather's (the
> original W6RMK) prop pitch rotators with polarity sensitive relays in
> a bridge circuit) then TV antenna rotators that were juiced up a bit,
> (all those AC split phase motor designs), etc. Certainly, if you go
> out and buy a Ham N or a T2X or a whatever, there's probably
> off-the-shelf bracketry to bolt it to just about every tower made and
> every mast used. And, of course, you can order it from the same
> place you buy your radio, coax, and antenna, which is nice. But
> maybe not. There are a fair number of questions on this reflector
> along the lines of "how do I fit a model X rotator in a model Y tower?"
> Maybe it's the relatively recent availability of programmable
> controllers (like the Green Heron)? While the mechanical fabrication
> issues are pretty straight forward, rolling your own controller is a
> bit of a chore (feedback loops, hunting, damping, brakes, all that
> stuff), and maybe what you you get with the fancy rotator/controllers
> is that someone has already done that work, so what you're really
> paying for is the controller, not the mechanical part. But these
> days, a programmable motion controller with a ethernet or RS232
> interface is a cheap and readily available thing. The Green Heron is
> a great example of a "ham tuned" UI, but there's a raft of simple
> motion control widgets out there with comparable capacity (especially
> if you're going only computer control and don't need a front panel).
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