Rotator Cage for ALM-31

SteppIR UrbanBeam on a US Towers ALM-31 crank up tower

I love the US Towers ALM-31, but I want to put a 10′ mast up above the rotator. This would get the UrbanBeam up to 41′. Since adding that extra length would cause too much side torque on the rotator – and also stress the weld at the rotator plate. When you look at pictures of hams in EU that have crank up masts – you always see a rotator cage. These are rare in the US, and I’m kind of surprised. In fact, I can’t find anyone who makes them in the US. I did think about using a Rohn top section of a tower – upside down and with a thrust bearing, but that ends up being almost $400 – with shipping.

The tower is on the outside of my trellis, and I used to have a Spiderbeam telescoping aluminum mast that held a 2 element home brewed yagi. Because the tower and the trellis upright beam are only 10 or 12″ apart, it would be very easy to build a telescoping “rocket launcher” support. I found a special thrust bearing for such an application called a Pillow Block Bearing:

The bearing could be put at the end of an arm that comes off the Spiderbeam telescoping mast. This would ensure that the extended mast never leans sideways – which could damage the rotator. Adding a 10′ mast puts the weight right at the ALM-31 limit, so all of this is just musing – I probably won’t do it. I never knew about a Pillow Block Bearing – so this is cool.

I also thought about using this Rohn top thrust bearing section upside down as a rotator cage. That would work very well, and wouldn’t require all that crazy “scaffolding”.

This was a “what if” post. I won’t be doing any of it – but its fun to think about mechanical or electrical solutions to problems. Sometimes viable product ideas emerge . . .




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