Yes. I bought some gearhead motors from All Electronics
(www.allelectronics.com). You can buy one to provide the rotational speed
you need directly, or you can gear them up or down further with inexpensive
cog belt and pulleys from Small Parts Inc. My Jennings cap requires about
24 turns lock to lock and the drive motor runs at about 25 RPM, giving
complete travel in about a minute.
For reasonably close limit control, you can couple the cap to the drive
motor with a short length of 1/4" threaded shaft from the hardware store.
On that shaft, you mount a nut and prevent it from turning with the shaft,
while still allowing it to travel on the shaft. A metal bracket with a flat
surface that is close to the nut will do this just fine. Now, as the shaft
turns, the nut will travel up and down the shaft and you can use
microswitches, such as those from Cherry, and mount two of them so that they
are activated when touched by the nut as the limit of the cap is reached.
Obviously, you need to allow for a bit of a margin of error here. If you
absolutely need to be able to crank the cap down to minimum capacitance,
this isn't the right approach to use.
Anyway, one drive motor lead is connected to the source of drive voltage,
and the other is connected to that source through two rectifier diodes that
are wired in parallel and connected to the other voltage source lead. The
N/C contact on each microswitch is connected in series with the appropriate
diode so that when the switch is activated, the diode is disconnected. That
removes the drive voltage, while maintaining the path to the motor for a
reverse voltage for reversing the DC motor. You do that for each limit
switch.
For approximate position sensing, I use a 1 microamp meter in the shack
controlled by a 10 turn precision pot that's driven off the capacitor shaft
via a cogged belt/pulley reduction scheme. For precise positioning, I use a
noise bridge (Palomar Tuner Tuner) connected into the transceiver's separate
receive antenna line. You turn on the bridge, and move the motor switch
back and forth until the noise is minimized. At that point, you're tuned
right on the button.
K8AC
----- Original Message -----
From: "Dennis Ashworth" <K7FL@arrl.net>
To: <towertalk@contesting.com>
Sent: Saturday, July 26, 2003 12:14 PM
Subject: [TowerTalk] Motor drives for vacuum caps
>
> Has anyone discovered a reliable scheme for remote adjustment of vacuum
> variables? I have two antennas with vacuum variables at the base and would
> very much like to tweak them from the shack. I'd need the ability to
> reverse direction and ensure I didn't go beyond the acceptable travel of
> the capacitor (maybe some limit switch or turn counter arrangement?). I'd
> prefer to stay away from LED/encoder wheels, etc in my high RF
environment,
> but that may ultimately prove the best solution.
>
> Ideas welcome!
>
> Thanks,
> Dennis, K7FL
>
> _______________________________________________
>
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Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
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