[TowerTalk] Gamma match

[Jim Lux]

Electric screwdriver?  Cheap, about 1 rev/sec, etc..
There are more surplus DC gearmotors out there than you can imagine. Check 
places like All Electronics (http://www.allcorp.com/) and so forth.

Here's some things you're going to need to consider:
1) How will you couple the motor to the cap?
2) Do you care about position feedback, or will you just use reflected 
power/VSWR to adjust
3) Do you need to move fast or slow? (or both)
4) How precise does it need to be (do you need to be able to make little 
tiny "bumps" to tune that high Q system.

For instance, I've got a screwdriver antenna on my mobile installation.  It 
takes many seconds to move from one band to another, and on low bands, very 
small bumps of the motor make a very big difference in the match, but on 
20m, I can just set the antenna by eye, because the 2:1 SWR bandwidth is 
100s of kHz.

For drive systems... a short length of rubber or vinyle hose makes a fine 
connector between motor and driven shaft, if you don't need high speeds, 
high torque, or accurate relative position between motor and load.  It will 
suck up lots of misalignment, it's easy, etc.

If you need absolute position feedback, worry about how you're going to do 
that first, then worry about the drive motor.

Stepper motor drives can be very, very simple (as simple as a 4 position 
rotary switch to apply power to the windings in sequence... turn the switch 
one way and the motor turns one way, turn the switch the other way, etc.), 
especially if you don't need a lot of speed. A 12 position rotary switch 
will nicely generate 3 step cycles (of 4 steps each) per revolution, so, 
ganged with a 24 step/rev motor, you effectively get a 2:1 ratio between 
knob and stepper output (and you could claim "electronic vernier drive"!). 
There are a lot of 200 step/rev motors around out of old stepper motor 
positioned disk drives (floppies).

Another fairly straightforward device is a radio control servo.  A simple 
555 circuit will generate the PWM control signal you need, and they are 
available in an amazing variety of torque, size, etc. And they are 
reasonably weatherproof as well.  The only real problem is that they only 
have 270 degrees of motion (great for a standard variable cap, terrible if 
you need multiple turns).


At 01:22 PM 8/22/2003 -0400, you wrote:
>I want build gamma match to match boom of my TA36 to 30M and I'd like to
>use remotely powered motorized gear box to drive shaft of the capacitor.
>The motor has to be able turn right and left and be with a gear box.
>Motor should be able to drive fair size capacitor.
>Any suggestions as what could be used here?
>I do not any stepper motor as it is kinda complicated.
>73 de Andy
>
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