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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 21st Jun 2018, 4:07 pm   #1
G6Tanuki
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Default Small motor gearbox.

I'm building a 14MHz magnetic-loop antenna (the loop itself will use 22mm water pipe and soldered 135-degree elbows so it's more an octagon than a circular loop).
Tuning will be by a 30pF 'butterfly' capacitor [I recently picked up a nice Eddystone high-voltage one at a rally] - a 'butterfly' type capacitor's necessary because a transmitting loop involves significant circulating currents which don't play well with traditional spindle-wipers in capacitors.

Problem is, how to rotate the capacitor to resonate the loop? I've got a number of small DC motors but need a gearbox to reduce the 3000-or-so RPM of the motor down to something more like 3 or 4RPM. Initially I thought about clock gears but they don't look robust enough.

Any suggestions? A 2-stage worm-drive might be an option to get the desired reduction ratio - I've got an old car windscreen-wiper motor here that has a nice single-stage worm drive.
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Old 21st Jun 2018, 4:35 pm   #2
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Default Re: Small motor gearbox.

There's a satellite dish positioner being offered below...
(No conection with the seller)
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Old 21st Jun 2018, 5:24 pm   #3
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Default Re: Small motor gearbox.

Just a couple of small Achtungs!

I looked into water pipe octagonal loops and it turned out that the solder joints added up to about the radiation resistance. It looked a lot better if bent curves replaced as many 135s as possible. Or maybe parallel turns of several runs of microbore? Or lightning conductor strip?

Sliding contacts in variable capacitors are bad news too. A butterfly arrangement avoids them. For best efficiency have the plates brazed to the pipes and have a rotor with the moving plates brazed on.

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Old 21st Jun 2018, 5:49 pm   #4
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Default Re: Small motor gearbox.

I would use small bore heating pipe.
It comes in 10mm or 6mm in coils of 3 or 10m.
You should be able to uncoil it to the correct diameter right out of the packet.
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Old 21st Jun 2018, 6:01 pm   #5
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Default Re: Small motor gearbox.

Quote:
Originally Posted by Radio Wrangler View Post
Just a couple of small Achtungs!

I looked into water pipe octagonal loops and it turned out that the solder joints added up to about the radiation resistance. It looked a lot better if bent curves replaced as many 135s as possible. Or maybe parallel turns of several runs of microbore? Or lightning conductor strip?

Sliding contacts in variable capacitors are bad news too. A butterfly arrangement avoids them. For best efficiency have the plates brazed to the pipes and have a rotor with the moving plates brazed on.

David
Yes I'm quite aware of the loop-resistance issues: having experimented in the past with using bending-springs and sand-filling on plumbing projects I'm confident that I couldn't produce a bent-pipe loop without it being a total nightmare.
So I'll silver-solder the 135s. I've seen a design too where after silver-soldring the joints a tubular length of braid from coax was slipped over each joint and then soldered with ordinary 60/40 to provide even lower resistance.

I've got access to an infrared thermal-sensing camera (the sort of thing they use to video buildings as part of energy-efficiency surveys) that I can point at the finished loop while I squirt a couple of hundred Watts of carrier into it, in order to spot any potential high-resistance points. Whatever, I think I can do a better job than a couple of the 'mag loop' antennas I've seen targetted at the amateur market which use sections of flat aluminium strip about an inch wide, held into an octagon by small bolts and wingnuts.

Somewhere [in the attic] I've got a couple of US-Military-funded design studies from the Vietnam era relating to portable loops working between 5 and 10MHz; they seem to suggest using the largest-bore tubing possible is a good way to offset the losses in the joints needed to make the things portable.

A friend has suggested that since the amount of angular rotation needed for the tuning-cap is just 90 degrees, a lever coupled to a greenhouse ventilator motor might be a possible option. I'm wanting to have an arrangement with a sprung centre-off toggle switch that I can 'blip' to the left or the right to tune HF or LF. Elsewhere on the Web I've seen it said that a bit of 'hash' from the motor can be useful because it provides a consistent signal you can use to peak the loop-tuning on receive, before you actually transmit. Rather akin to the old days of crystal-sets incorporating a buzzer to let you find the sensitive spot with your cat's whisker.
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Old 21st Jun 2018, 6:27 pm   #6
kellys_eye
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Default Re: Small motor gearbox.

I've got a small maxon motor and gearbox (I'll post back with the reduction ratio but it's either 60:1 or 500:1 IIRC) with a nominal voltage of 18V - so suitable for lower voltage/speeds if required.

Masses of torque! Too big? or too small?

Yours for the cost of postage.

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Old 21st Jun 2018, 6:37 pm   #7
G6Tanuki
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Default Re: Small motor gearbox.

Quote:
Originally Posted by kellys_eye View Post
I've got a small maxon motor and gearbox (I'll post back with the reduction ratio but it's either 60:1 or 500:1 IIRC) with a nominal voltage of 18V - so suitable for lower voltage/speeds if required.

Masses of torque! Too big? or too small?

Yours for the cost of postage.

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Oh!! I'm definitely interested - that could be just the thing!!
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Old 21st Jun 2018, 7:15 pm   #8
kellys_eye
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Default Re: Small motor gearbox.

Here's a picture of the actual motor.

18V and 60:1 reduction. You absolutely can't stop it rotating with your fingers but the no-load speed at 12V is around 50rpm.

Seem like I gave the 500:1 reduction motors away to someone building a robot!

Still, if you want it and can drive it using a PWM speed controller, I reckon it could shift your antenna quite nicely!
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Old 21st Jun 2018, 7:53 pm   #9
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Default Re: Small motor gearbox.

It looks perfect for the job.

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Old 21st Jun 2018, 8:08 pm   #10
Ed_Dinning
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Default Re: Small motor gearbox.

Hi, how about a stepper motor, typically 200 steps/ turn and easy to control/step at a low voltage.
If permanently energised at a reduced voltage will lock rotor in position.
Usually sufficient torque for a direct drive so no gearbox needed.


Ed
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