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Re-rebuilding a Recumbent Antenna Mount

Antenna Mount

Antenna Mount

Quite a while ago, I built this slab mount to hold an amateur radio antenna on our daughter’s Tour Easy. It worked fine until the bike blew over and whacked the antenna whip against something solid, at which point the mast cracked.

The antenna screws into an ordinary panel-mount UHF connector secured to the bottom of the slab, with a hole through the slab just large enough to accept the antenna mast. That put all the mechanical stress on the slab, not the connector.

Modified antenna mounting plate

Modified antenna mounting plate

Alas, the new antenna had a slightly different mast outside diameter, so I machined a new adapter to clamp the connector atop the slab. The antenna screws down into the adapter against a brass washer, again keeping the strain on the fitting.

I recently found the commercial mobile antenna cable that I’d been meaning to use on her bike, which required Yet Another Modification to that slab. It turns out that the UHF connector on the cable expects to be secured to sheet metal found in a car body, rather than a half-inch aluminum plate: the threads aren’t long enough!

So I machined circular recesses on the top and bottom to hold the mounting nut and washer, respectively, with 2 mm of aluminum remaining in the middle of the slab.

Milling top recess

Milling top recess

The recesses are just fractionally larger than the nut & washer, so most of the stress gets transmitted directly to the slab. Even in the high-vibration bicycle environment, I think there’s enough meat in there to prevent fatigue fractures.

Milling bottom recess

Milling bottom recess

I recycled a G-Code routine I’d written to chew out circular recesses. It does a bit of gratuitous (for this application, anyway) spiraling in toward the center, but got the job done without my having to think too much.

The bottom view shows the washer in action. The recess is deep enough that the cable just barely clears the slab.

Modified mounting plate - bottom

Modified mounting plate - bottom

The top view shows the recessed mounting nut. The nut has an O-ring around the connector threads, but the water will probably drain out through the four through-holes left over from the old panel-mount connector.

Modified mounting plate

Modified mounting plate

I turned the top nut down as far as I could with a wrench & (ugh) needle-nose pliers, then tightened the bottom nut about 1/3 turns with a wrench.

You’re not supposed to notice the crispy edges on the PVC bushing holding the reflector to the antenna mast. The high setting on that heat gun is a real toaster…

The G-Code is over there.

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  1. #1 by randomdreams on 2010-05-18 - 11:55

    That’s a beautiful piece of work. I presume you’re also responsible for milling a name into the billet? That’s gorgeous.

    Hey, you’d enjoy this. My toaster oven died — it still ovened but refused to toast. I’d push the ‘toast’ button in and it would pop right back out. I opened it up, and saw two contacter points the button triggered, and a solenoid that the button touched when it pushed back. I cleaned the faces of the points but that did nothing. The solenoid had continuity across it. One end of the coil went to ground, the other to some nameless transistor controlled by a timer chip. I crossed my fingers, clipped out the transistor, and soldered in a 2n2222 that was lying in a crevice in the electronics table, and hey presto it worked. It took less time to fix than it would have to drive to some store and buy a new one. Electronics FTW!

    • #2 by Ed on 2010-05-18 - 15:17

      milling a name into the billet

      That’s an example of having all my mistakes cancel out simultaneously.

      I had to flycut the surface anyway, which left me with a known-flat expanse covered with nice swirls. So I dug out TrueTypeTracer from the EMC2 project, found a cute font, got the G-Code scaled right, ran a conical cutter with enough lube to have it not burn up, and then had the pattern come out centered on the spot marked X!

      My tail was thumping against the chair for joy by the time that was done.

      some nameless transistor

      These days, it could be a three-pin microcontroller…

      Good work!

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