Having had both of our commercial antenna mounts fail, I decided to make something that could survive a direct hit. It turns out that the new mounts are utterly rigid, which means the next failure point will be either the antenna mast or its base structure. We’ve occasionally dropped the bikes and when the antenna hits something on the way down, the mount is not the thing that bends…
Incidentally, the Nashbar 5-LED blinky white light aimed rearward seems to push motorists over another few feet to the left. Nobody quite knows what we are from a distance, but they do notice that something is up ahead. That’s just about as good as it gets; we tend to not ride in the wee hours of the morning when bike lights just give drunks an aiming point.
The overall structure is a 2-inch square aluminum extrusion, with a hole in the top that matches the right-angle SO-239 base connector salvaged from the Diamond mount and a 1/2″ nylon stiffener plate in the middle. A pair of relentlessly square circumferential clamps attach it firmly to the top seatback rail. A coaxial cable pigtail ensures that the antenna base makes good electrical contact with the seat. I’m not convinced the bike makes a good counterpoise, so we’re now using dual-band antennas that are half-wave on VHF.
Stainless-steel hardware holds everything together, as I’m sick and tired of rust.
Not having a huge drill, I helix-milled the SO-239 hole, then reached down through the box to drill the hole for the plastic block retainer screw. Flip the box in the vise, drill four holes for the clamps (I love manual CNC for that sort of thing), manually deburr the holes, and it’s done.
The block of plastic is a tight slip fit inside the box extrusion, with slightly rounded corners to suit. I milled the slot across the top to a slip fit around the SO-239 connector.
The two clamps were the most intricate part of the project and got the most benefit from CNC.
The clamp hole must have exactly the same diameter as the seat top tube. I helix-milled the hole to an ordinary 5/8″; I have trouble drilling holes that large precisely in the right spot with the proper final diameter. Milling takes longer, but the results are much better.
Helix-mill the other block while you have the position set up, then flip and reclamp to drill the pair of holes that match the box extrusion. Drill 10-32 clearance (#9) all the way through.
Bandsaw the blocks in half, paying some attention to getting the cut exactly along the midline, then flycut the cut edge to make it nice & shiny & even. That should result in 1 or 2 mm of slit between the blocks when they’re clamped around the seat rail.
Break those relentlessly sharp edges & corners with a file.
I finagled the dimensions so a 1-1/2″ socket-head cap screw would have just enough reach to fill a nut, with washers under the screw and nut. Your mileage may vary; I’ve gotten reasonably good at cutting screws to length.
Normally, you tap one side of each clamp for the screws, but in this situation I didn’t see much point in doing that: the box must attach firmly to the clamps and I was going to need some nuts in there anyway.
With all those parts in hand, assembly is straightforward. Secure the SO-239 with its own thin nut, screw the plastic block in place, hold the clamps around the seat bar, poke the cap screws through, dab some Loctite on the threads, install nuts, and tighten everything. That all goes much easier with four hands!
The grounding braid fits into a huge solderless connector that must have been made with this application in mind. It originally fit a 1/2″ lug, but with enough meat that I could gingerly file it out to 5/8″ to fit the SO-239 inside the aluminum extrusion. I’ve had those connectors for years without knowing what they were for!
I eventually came up with a simpler and even more ruthlessly rugged mount that’ll appear in my column in the Autumn 2009 Digital Machinist. More on that later… [Update: There]
The Smell of Molten Projects in the Morning 
I am very interested in your antenna mount for my recumbent trike. I am not that handy
with metal work and wondered if you sell your finished design?
73,
Bill W2WHC
That one was a labor of love, built from available materials. In retrospect, I’d never do it that way again: too many fiddly parts.
I must write up the simpler design after Christmas, but here’s the general idea: two 1/2″ aluminum plates separated by about 3 mm, with a hole (actually, two trenches) milled to fit the seat crossbar and four clamping screws through both plates. Another hole for the SO-239 mobile antenna cable socket. Done!
I’ll send a picture by email to give you an idea of what it looks like. You might be able to find a cranky guy with a basement shop to build it; being able to see & feel your bike before cutting metal would be very helpful!