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PostPosted: Sun May 27, 2012 11:36 am 
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Location: Saskatoon
With all those gears, you'll need to be concerned about backlash:
qrp-gaijin wrote:
Assuming bandwidth of the antenna is 5-10 kHz at 7 MHz (low losses), then probably 1/10th of a degree rotational resolution would be desirable for fine tuning.

1/10th degree is very fine, but you may be able to mitigate this by using a spring on the variable capacitor to preload it.


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PostPosted: Sun May 27, 2012 2:18 pm 
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Bob Weaver wrote:
With all those gears, you'll need to be concerned about backlash:
qrp-gaijin wrote:
Assuming bandwidth of the antenna is 5-10 kHz at 7 MHz (low losses), then probably 1/10th of a degree rotational resolution would be desirable for fine tuning.

1/10th degree is very fine, but you may be able to mitigate this by using a spring on the variable capacitor to preload it.

"Backlash" just means that the shaft will not start turning immediately when I apply power, and conversely will continue turning briefly after I cut power - right? It seems like only a minor inconvenience that I could work around by observing the spin-up/spin-down times and keeping them in mind as I apply power and cut power (letting the motor "coast" into its final position after cutting power). It seems to be a common issue in the remotely-tuned magnetic loop antennas that I've read about, and people just seem to live with it.

I'm having trouble visualizing the "spring" that you mentioned as a possible solution, and how the spring would address the "backlash" issue (if indeed I've understood correctly what you meant by "backlash"). Could you elaborate?


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PostPosted: Sun May 27, 2012 3:23 pm 
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http://en.wikipedia.org/wiki/Backlash_%28engineering%29#Anti-backlash_designs

73,

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PostPosted: Mon May 28, 2012 3:46 pm 
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Location: N 64º 41' E 21º 14'
look for "worm-gear" it is used in sextants, quintants and the like and usually have less than archminute backlash, that is 1/60th of a degree, and common ratios are among others, 1:40, 1:45 1:60, 1:90, 1:180.
there are certainly any other combination conceivable, I have seen one as high as 1/1440 in real life, add that to a stepper motor and you can dial in pretty much anything, check bottom of this page to name one place wormgear is used.
http://www.surplussales.com/Variables/A ... rVar1.html


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PostPosted: Sun Jun 17, 2012 2:43 am 
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qrp-gaijin wrote:
I found this neat-looking geared-down DC motor:
[...]
Small, cheap ($10), and low-RPM (30 RPM). This is about the lowest-RPM geared motor I've been able to find cheaply (and locally available).


I bought this motor via mail-order. Right after it arrived I noticed on the back of the box that the manufacturer makes a lower-speed model that goes down to 9 RPM. That's available both in Japan via amazon.co.jp and elsewhere via amazon.com:

http://www.amazon.com/Mini-Motor-Speed- ... miya+70189

I now have this model on order as well.

Also, yesterday I bought some 2m-long wooden dowels with the intent of building a sturdy frame for my strip-based loop antenna. The capacitor will be at the top, with a long shaft made of ABS extending down to the motor at the bottom (to prevent motor control cables from running next to the capacitor, which is apparently a bad thing). Hopefully with a sturdy frame to hold the loop shape constant, and a sturdily-mounted remote-controlled capacitor, I may finally be able to get the antenna to tune up by moving the antenna around and/or squashing the coupling loop.


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PostPosted: Sun Jun 17, 2012 7:53 am 
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Location: N 64º 41' E 21º 14'
That ABS shaft will act as a spring making it harder than necessary to set frequency.
this as the parts have friction or as this phenomenon is sometimes refer to sticktion
it means that friction on parts with high precision tend to have higher friction at beginning of movement that reduces suddenly as the oil film develops.
this is what makes low speed motors seem to go from 0RPM to 50rpm with a jump than can be backed of down to 10rpm and sometimes less, this is reason I keep the propaganda up for worm-gears and stepper motor.
if stepper and worm-gear is used you could easily get things down to 1/60 of a degree and have 99% repeatability and stepper and gears... can be had as unit in a metal box.


Last edited by OErjan on Sun Jun 17, 2012 9:12 am, edited 1 time in total.

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PostPosted: Sun Jun 17, 2012 9:09 am 
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OErjan wrote:
That ABS shaft will act as a spring making it harder than necessary to set frequency.

Yeah, I was worried about that too. In the hardware store I was twisting all the shafts they had on display to find one with minimal twisting springiness. It came down to a choice of either a shaft made of ABS or a shaft made of acrylic. With no data on dielectric loss or springiness I went by gut feeling and bought the ABS shaft.

What material would you suggest for a shaft? Wood is about the only other material that was available and seemed applicable.I assume a metal shaft would be inadvisable since the purpose of the shaft is to keep the capacitor clear of any disturbing metal (like the motor, control circuitry, and cables).


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PostPosted: Sun Jun 17, 2012 9:24 am 
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Location: N 64º 41' E 21º 14'
Sorry for replying so late, post above it somehow got severely curtailed so i ammended it, ABS will work fine if thick enough but you need to be aware of the shortcoming-
as i say above it wil add to the fact that repeatability (finding same tuning place) is almost impossible with your setup, ofcource it should still be possible to get within decent SWR,

Actually even a Relay and a grasshopper escapement mechanism (as used in clocks ) can be used to tune a remote antenna, each time relay is activated the tongue activates the device and allows wheel to click one sprocket, that is 100%repeatable but sadly only goes one direction so if you get it 1 position wrong you have to go a ful turn minus one position :-(


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PostPosted: Sun Jun 17, 2012 9:27 am 
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Location: N 64º 41' E 21º 14'
just came to think of something if one makes such a relay device and use one relay to controll 1/16th of a turn and another for fine adjustment it could still be really fast.


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PostPosted: Sun Jun 17, 2012 12:56 pm 
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OErjan wrote:
it means that friction on parts with high precision tend to have higher friction at beginning of movement that reduces suddenly as the oil film develops.
this is what makes low speed motors seem to go from 0RPM to 50rpm with a jump than can be backed of down to 10rpm and sometimes less, this is reason I keep the propaganda up for worm-gears and stepper motor.
if stepper and worm-gear is used you could easily get things down to 1/60 of a degree and have 99% repeatability and stepper and gears... can be had as unit in a metal box.

Sounds good, except for the fact that the worm gear variable capacitor you linked to earlier (http://www.surplussales.com/Variables/A ... rVar1.html) costs $239, whereas the small motor I linked to costs $10!

For my purposes I'm not overly concerned about repeatability (I'm still not even sure of a magnetic loop antenna will work for me with my concrete building and 2nd-floor balcony), so a little occasional jumpiness in tuning is acceptable as long as I can move the capacitor slowly enough to get a low SWR.


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PostPosted: Sun Jun 17, 2012 9:47 pm 
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Location: Australia
Would extreme tuning accuracy be necessary when tuning the antenna circuit? It would depend on the achievable Q of that circuit?
(ie. if it has a "relatively" wide bandwidth then "near enough" should be "good enough"?)

........................


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PostPosted: Sun Jun 17, 2012 9:59 pm 
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Location: Sonoma County, CA
You might try this idea for a capacitor drive using the power unit from a motorized potentiometer:

http://www.ad5x.com/images/Articles/SlowMotor.pdf

Rich

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PostPosted: Sun Jun 17, 2012 10:08 pm 
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Tiny HF loops like qrp-gaijin's building have extremely high Q. The radiation resistance is very low, due to the small diameter, so resistive losses must also be kept very low. The result is a pretty sharp tuned circuit. Somehow, MFJ manages to pull it off. Their tuning cap has no end stop, so you can just keep going 'round and 'round. I can visualize a cam arrangement, or a lever, either moved by a fine-pitch lead screw, but I sure wouldn't want to try building such a contraption. The good news is, most modern transceivers will work fine into a 1.5 : 1 SWR, so you don't have to be dead on.

73,

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PostPosted: Sat Jun 30, 2012 3:06 pm 
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Here's what I came up with for the remote-controlled capacitor for my magnetic loop antenna.

The full view:
Image

We have a thick dowel rod serving as the main mast. At the bottom is mounted a wooden plate with a geared motor attached to it:
Image

The motor shaft is inserted into a long hollow rod made of ABS plasic. Two self-tapping screws are screwed into opposite sides of the plastic rod to grip the motor shaft.

At the top of the main mast the butterfly capacitor is mounted on a protruding wooden plank:
Image

The shaft of the variable capacitor goes through a hole in the plank and is inserted into the other end of the long plastic rod (driven by the motor). A single self-tapping screw presses the variable capacitor shaft against the inside of the plastic rod, thus gripping it.

The protruding wooden plank has a small round curve cut into it where it meets the main mast, and the main mast has a small notch cut into it to accomodate the plank's curved indentation. Copious wood glue holds the plank in place. Some additional right-angle brackets or similar would be good to prevent the plank from collapsing.
Image

Above the capacitor, a large plastic hook is screwed to the main support mast, which will bear the weight of the loop conductor (to avoid placing strain on the capacitor).

The motor is designed to run off of 3V, but it runs fine off of a 1.5V AAA cell with enough torque to turn the motor, and at a leisurely speed (rated for 9 RPM at 3V). Probably the low speed is sufficient for tuning without resorting to PWM, simply by using a double-pole switch to turn on or off the supply voltage in both polarities.

There is a bit of eccentricity on the plastic rod's rotation but it doesn't impact the operation at all. There is some backlash when reversing motor direction. I don't care about this because I'm not looking for repeatable absolute positioning, only for relative positioning (to increase or decrease capacitance until SWR is acceptable).

So why did I do it this way?

1. The capacitor is at the top, far away from the lossy ground or the metal tripod that will support the main mast. Keeping the capacitor away from nearby lossy or conductive materials is important to preserve balance, reduce losses, and ensure the antenna is tunable to a low SWR.

2. The motor and its voltage-control lines are far away from the capacitor. One report I read indicated that the mere presence of a disconnected wire hanging near the capacitor was sufficient to make the loop untunable (no low SWR could be attained). In that case, removing the wire fixed the problem. Again: we need to keep conductive materials away from the capacitor.

3. The feed loop will be at the bottom of the mast and the bottom of the loop, opposite the capacitor, which eliminates the need to run the feedline through the middle of the loop as would be needed if the capacitor were at the bottom and the feed loop at the top. Again, loop balance is improved by keeping the loop interior free of wires.

Next step is to come up with a low-loss mechanical connection to conect the butterfly cap to the loop conductor. A mechanical connection will introduce resistive loss, but will allow swapping out loop conductors easily to allow trying out differently-sized loops. The MFJ small loop tuners use a mechanical (wing nut) connection to allow swappable loop radiators, and they seem to work okay, so the losses may not be insurmountable. For the loop conductor I still think copper foil tape is the most practical material.

Looking at the finished motorized capacitor assembly, it looks pretty simple to build, but it was slow going for me, to make sure everything lined up reasonably well. I'm pretty happy with the result, and feel like this time around, I might at last have a good chance at making a good loop.


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PostPosted: Sat Jun 30, 2012 3:43 pm 
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Nice work! Looking forward to seeing how it works in Transmit mode

73,

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