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Author Topic: Servo for possible antenna rotator?  (Read 7000 times)

OMK

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Servo for possible antenna rotator?
« on: March 19, 2009, 01:24:35 AM »

All you techy engineers out there, could I pick your brains for a minute?....

First off, pardon me for this being a non-boaty question -- it's more a question of how to couple a small, lightweight medium-wave loop antenna to a servo in order to have the antenna rotate through 180-degrees.
Now just in case you're wondering, a loop antenna is highly directional. Having it rotatable means it can be remotely operated to the direction of the transmitted signal. The aim is have it mounted to my vertical mast, and possibly employ a servo to take care of the rotation.
If you would be kind enough to take a look at the following link you will get a better idea of the psychical size:

http://www.amradioantennas.com/

Being that I could make the same from plastic conduit I figure the overall weight would be no more than a handful of ounces. So my questions are....

Since I have little knowledge of high-torque servos, would such a servo stand up to the rigours of the British weather and at the same time be powerful enough to rotate the loop? In other words, I don't want a gust of wind to alter the position of the loop - the servo would have to be strong enough to hold the loop at its set position. Could you recommend one which might be man enough for the job?
Secondly, bearing in mind that it would have to be mechanically rigid, would you know how I might attach the servo spline to the conduit shaft.

I would dearly appreciate any help with this.
Thanks for reading.
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FullLeatherJacket

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Re: Servo for possible antenna rotator?
« Reply #1 on: March 19, 2009, 09:18:02 AM »

PMK

The simple answer is don't, dude.
If 't were me, I'd fasten the antenna base into a suitable bearing and fit a large gear wheel to it. Fit a similar one to the servo output spline and mesh the two. That way the bearing takes all the load and leaves the servo just doing the rotating stuff.
OR
Use a worm-and-wheel arrangement on a DC motor, and a bi-directional RC switchy thingy to turn the motor - maybe with limit microswitches. Further food for thought here (top diagram):

http://www.action-electronics.co.uk/pdfs/Limit%20Switches.pdf

FLJ


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andrewh

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Re: Servo for possible antenna rotator?
« Reply #2 on: March 19, 2009, 09:31:39 AM »

PMK,

Well yes-ish :}
FLJ has hit the nail on the head - the loop needs its own bearings - you might give it a couple of light pillow blocks and bolt them to a convenient bracket.

THEN your idea can work - fit a servo inline with the loop shaft.  You may need a large servo to get the response and holding power you are looking for, and if precision is an essential  - you prolly need a digital (finer resolution too)
I guess you will need 360 travel - and the simplest way might be to use a large sail winch servo and just stay in the middle of the travel.  (or you could use resistors to reduce the travel from 3.5 turns to 1.5 - that would be a reversal of the usual :})

You could drive it with a servo tester - they are incredibly cheap and useful - and this, too can be modified to give you a 360 dial movement with degrees etc so that the dial pointer was exactly followed by the loop above :}

But if you are like me you will try it anyway with an old servo  :-))
andrew
 
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Shipmate60

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Re: Servo for possible antenna rotator?
« Reply #3 on: March 19, 2009, 09:59:15 AM »

PMK,
Why use a servo?
One of the highly geared 550 type motors will give a slow rotational speed with lots of torque and you would only need a battery and a switch.

Bob
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John W E

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Re: Servo for possible antenna rotator?
« Reply #4 on: March 19, 2009, 10:08:20 AM »

hi dude

 

have a look at this circuit it is  for a servo driver easy built from bits out of your junk box - on a bit of strip board - then if you get a cheapo MFA gearbox and motor this will do the job J.just remember to fix the feed back pot to the output shaft on the gearbox

IC 741

aye

john

bloobs

 
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Circlip

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Re: Servo for possible antenna rotator?
« Reply #5 on: March 19, 2009, 01:26:39 PM »

Another two, Ham radio aerial rotator or modify an electric car aerial unit.

  Regards   Ian.
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OMK

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Re: Servo for possible antenna rotator?
« Reply #6 on: March 22, 2009, 05:15:46 AM »

After some thought, I figgered making it more rigid by omitting that short stub, then bolting the thru-box directly to the servo horn. ServoShop has one particular device rated at 13kg, which might take care of any unwanted movement in a gust of wind.
Can anyone see anything wrong with this sketch, or anything that I might not be taking into account, before I spend any ?

Bloobs:
Many thanks for the neat circuit, but I'm keeping this as KISS as possible. This simple circuit is heap plenty good for my needs - precision/resolution with a standard servo will pretty much cater for the bandwidth/directivity of the antenna. Since the loop is most effective at both ends, I only need to have it rotate 180-degrees.
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malcolmfrary

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Re: Servo for possible antenna rotator?
« Reply #7 on: March 22, 2009, 11:50:14 AM »

Perhaps having the antenna under a radome would git rid of the wind problem and remove the need for a large servo?

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OMK

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Re: Servo for possible antenna rotator?
« Reply #8 on: March 23, 2009, 06:51:27 AM »

A radar/dome would be a good solution to the wind problem, but then things start to get even more mechanically difficult. Not only that, the diameter of the loop will be somewhere around 18 inches, meaning more weight with a rather huge-looking radome, meaning more funny looks from the neighbours. I had to fight tooth and nail for permission to erect the antenna mast. I don't wish to push my luck, so I'm hoping a skinny-looking bit of plastic flexible conduit hiding the antenna loop wiring will not cause too much concern from the powers-that be.
The 13Kg servo is, so far, the only alternative to an expensive rotator. Not toooooo large, neither. If it should fail afer a couple of English seasons, then I've only lost 28.00.
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malcolmfrary

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Re: Servo for possible antenna rotator?
« Reply #9 on: March 23, 2009, 10:27:06 AM »

Ah, neighbours.
I have some niggling doubts about the strength of the single screw holding the horn onto the servo.  Maybe better to mount the loop on a shaft alongside the servo and drive the shaft via a 1:1 pulley, or gears.  That way there is less chance of a windy gust (little known C&W singer) uprooting it and giving your neighbours even more reason for the funny looks.
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FullLeatherJacket

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Re: Servo for possible antenna rotator?
« Reply #10 on: March 23, 2009, 12:25:46 PM »

Ah, neighbours.
I have some niggling doubts about the strength of the single screw holding the horn onto the servo.  Maybe better to mount the loop on a shaft alongside the servo and drive the shaft via a 1:1 pulley, or gears.  That way there is less chance of a windy gust (little known C&W singer) uprooting it and giving your neighbours even more reason for the funny looks.

"If 't were me, I'd fasten the antenna base into a suitable bearing and fit a large gear wheel to it. Fit a similar one to the servo output spline and mesh the two. That way the bearing takes all the load and leaves the servo just doing the rotating stuff."

It's clearly not just great  minds that think alike...................  %)
FLJ
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OMK

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Re: Servo for possible antenna rotator?
« Reply #11 on: March 23, 2009, 04:05:45 PM »

"If 't were me, I'd fasten the antenna base into a suitable bearing and fit a large gear wheel to it. Fit a similar one to the servo output spline and..."

Okay, okay - I heard you the first time.  ok2
Like Malcom says, that single screw at the servo horn is the bone of contention. It's the weakest link in the chain for sure, but I was hoping that the beefy servo's output spline would be man enough to cope. Now I'm not so sure after all.
Gearings and bearings are obviously the way to go, but how?? I mean, I'm no engineer. I'm just sketching up drawings in the hope that I might strike a chord. I don't have access to gears and suchlike. Where does one buy the appropriate bits & bobs from?
I would like to stay with the servo-drive idea, simply because it's an easy matter to rotate the loop with simple electronics. In other words, an electric car aerial unit, and even Shipmate 60's suggestion are all good ideas, but maybe a tad bulky (read heavy) for a simple, lightweight antenna. I'm trying to keep it very KISS but I don't have much idea about the mechanics.

FLJ, you say to fit a couple gear wheels. But how, when, who, what, why, where........ Help pse?!
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Shipmate60

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Re: Servo for possible antenna rotator?
« Reply #12 on: March 23, 2009, 05:00:45 PM »

PMK,
Do you need feedback to where the Aeriel  is?

Geared Motors up to 3000:1 : http://www.modelflightaccessories.com/products/powerpacks.shtml

Bob
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OMK

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Re: Servo for possible antenna rotator?
« Reply #13 on: March 23, 2009, 05:29:45 PM »

Not quite sure what you mean by feedback. Being that the antenna is directional in both N/S axis means it has to be pointing to the station of interest. So rather than keep going outside to alter the direction, I can do it from the comfort of the shack just by tweaking a potentiometer, which in turn rotates the servo... if that's what you mean.

By the by, I don't wish to sound like an ingrate, but that link you sent is a dud'un - one of those 404 jobbies.
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FullLeatherJacket

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Re: Servo for possible antenna rotator?
« Reply #14 on: March 23, 2009, 06:08:03 PM »

FLJ, you say to fit a couple gear wheels. But how, when, who, what, why, where........ Help pse?!
I've PM'd you, dude.
FLJ
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malcolmfrary

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Re: Servo for possible antenna rotator?
« Reply #15 on: March 23, 2009, 06:17:33 PM »

If you think in terms of pulleys, then one should bolt to the servo horn.  You COULD build them up from circles of ply.  If, instead of a rubber band, the drive was transmitted through two cords each anchored at both ends, there would be no slipping problem to upset alignment over time.  The shaft one needs to be thick enough to not wonk about (techspeak), and then just needs a bit of bent wire to retain the wheel and ensure the drive, or it could be bolted to a "proper" wheel grub screwed to the shaft.
I am sure I have seen gears in places like Maplins, and Squires.

Not just GREAT minds FLJ  ;D, but manny others (little known Jewish saxophonist featured on most compilation albums) as well as me.
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Shipmate60

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Re: Servo for possible antenna rotator?
« Reply #16 on: March 23, 2009, 06:40:06 PM »

PMK,
The link works for me!!
Its to MFA Motors page and includes motors with a g/box of 3000:1

Feedback as in do you need to know the bearing that the Aeriel stops at remotely?

Bob
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Bryan Young

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Re: Servo for possible antenna rotator?
« Reply #17 on: March 24, 2009, 06:17:57 PM »

I'd been pondering on how to get the radar scanner on "Northumbrian" to work efficiently. No good having a straight driveshaft up through the passenger "lounge" as the windows are so big that all internal "stuff" will be visible. So the only answer was to fit the motor directly under the scanner (as it is in "real life"). But what motor? One of my fellow club members who always seems to think sort of "sideways" came up with the answer. He gave me an electric motor about 1/4" dia and 1/2" long complete with driveshaft.
Turns out to be the power behind "old-fashioned" mobile phone buzzers. At 1.5v it seems to turn at 30rpm which is about perfect. So all I need to do now is route the wiring out of the way. Thanks to Tom Butler. BY.
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Bryan Young

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Re: Servo for possible antenna rotator?
« Reply #18 on: March 24, 2009, 06:26:27 PM »

All you techy engineers out there, could I pick your brains for a minute?....

First off, pardon me for this being a non-boaty question -- it's more a question of how to couple a small, lightweight medium-wave loop antenna to a servo in order to have the antenna rotate through 180-degrees.
Now just in case you're wondering, a loop antenna is highly directional. Having it rotatable means it can be remotely operated to the direction of the transmitted signal. The aim is have it mounted to my vertical mast, and possibly employ a servo to take care of the rotation.
If you would be kind enough to take a look at the following link you will get a better idea of the psychical size:

http://www.amradioantennas.com/

Being that I could make the same from plastic conduit I figure the overall weight would be no more than a handful of ounces. So my questions are....

Since I have little knowledge of high-torque servos, would such a servo stand up to the rigours of the British weather and at the same time be powerful enough to rotate the loop? In other words, I don't want a gust of wind to alter the position of the loop - the servo would have to be strong enough to hold the loop at its set position. Could you recommend one which might be man enough for the job?
Secondly, bearing in mind that it would have to be mechanically rigid, would you know how I might attach the servo spline to the conduit shaft.

I would dearly appreciate any help with this.
Thanks for reading.
Sorry for butting in again, but may I ask why and what for do you need this sort of aerial/antenna? To me it looks like a very early Radio Direction Finder aerial. (Wrong?). I've only ever come across the double-loop system where uncorrected signal strength/direction was done via the receiver.
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John W E

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Re: Servo for possible antenna rotator?
« Reply #19 on: March 24, 2009, 07:24:35 PM »

hi ya Dude my mate

I have done a little scribble of how I would make the support & drive for your aerial antenna.   I would make it all from 1/2 inch ply and I would scavange the bits from an old scanner cum printer.    I have stripped a couple of these down myself and found excellent brass bushes/shafts/cogs etc., which would be ideal for this.    We are talking about a 'real antenna' here which has to be rotated through 180 degrees - and not something we are going to put on a model??? is it ????  {-) {-) {-) {-) {-) {-) {-)

Just an idea for your mate.

aye
Blueby
John
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OMK

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Re: Servo for possible antenna rotator?
« Reply #20 on: March 24, 2009, 08:57:43 PM »

To me it looks like a very early Radio Direction Finder aerial. (Wrong?).

Nope, not wrong, because the loop antenna is exactly that - an RDF antenna. In your case, the scanner on your Northumbrian is constantly scanning the airwaves, 360-degrees, because, in a real-life scenario, they are never sure where the next signal might be coming from. So once a signal is detected it can then be homed-on with a directional antenna.
In my case I don't need to scan through 360-degs because I pretty much know where the signals are coming from already -- I just simply point the antenna at the station of interest. And being that the loop antenna is directional in both directions means that it also has two nulls, both occurring at right angles to the ends. For instance, I have two nearby stations (Taunton and Wales), both competing for the same frequency.... which basically means their signals are just a mish-mash of interference by the time they clash at the antenna. By pointing the loop towards Wales means I can null-out the interfeing Taunton station, and vice-versa.
The same principle was used during the war when they wanted to locate, say, an enemy transmitter. With a directional antenna and a meter showing the signal strength, they could take a reading from three different locations (triangulate), then plot a postion where the three lines crossed.
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OMK

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Re: Servo for possible antenna rotator?
« Reply #21 on: March 24, 2009, 09:29:38 PM »

Feedback as in do you need to know the bearing that the Aeriel stops at remotely?

Ah, right. I hear you now.
Yes, the bearing must tally with the position of the aerial. As you probably know, a standard potentiometer usually rotates zero through 270-degs. If you take another butcher's at that above schematic you'll see that these timing components have been calculated that the the potentiometer gives a fairly linear reading throughout 180-degs (in reality the servo moves a tad more than 180-degs because of that bit of spare travel on the pot'). And bearing in mind that the loop is most liveliest at both ends means I only need care about 180-deg throw.

I logged in about an hour ago and suddenly that link now works.
Geared motors = neat suggestion, but how do you propose I get it operate over just 360-degs? The idea is to have a simple scale, showing North to South, with a dial pointer attached to the pot' spindle.  So if you turn the dial, say, 45-degrees, then so too will the antenna.
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OMK

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Re: Servo for possible antenna rotator?
« Reply #22 on: March 24, 2009, 09:48:29 PM »

hi ya Dude my mate

Hiya my mate ya dude.

I got just ONE itsy-bitsy question for you..............
The enclosure has to be weatherproof - not wood (see where this is going?).................
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malcolmfrary

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Re: Servo for possible antenna rotator?
« Reply #23 on: March 25, 2009, 11:03:49 AM »

Quote
The enclosure has to be weatherproof - not wood (see where this is going?).................

Creosote used to do a cracking job on telegraph poles which were usually wood, and outdoors.  Generous use of exterior house paint would probably work as well, and you get the choice of colour.
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