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Old Nov 01, 2010, 10:40 AM
Engineer for Christ
IBCrazy's Avatar
Amherst, VA
Joined Jun 2006
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IBCrazy's homebrew antenna tracker

Well it's about time I posted this. I am designing my own 360 degree antenna tracker. There will be no tilt as I feel that polarization is more important than aiming the antenna vertically. Besides, my diversity system worked great like this.

Unlike every other antenna tracker out there this one requires no GPS or OSD! It works by comparing the RSSI of two recievers and moves a modified servo to rotate a turn table. There will be a tutorial to follow after I have tested this unit

How it works:

Two RX antennas (BiQuads) will be aimed approximately 45 degrees apart. When the unit is turned on, the RSSIs of each VRX will be compared. The antenna tracker will turn until the RSSIs are equal (or close to it). This will place the transmitter (and therefore the airplane) right between the beams of the antennas.

To center the airplane, the potentiometers are adjusted. Once set, it is set for life. The tracker should take matter of seconds to get a lock on the plane.

The tracker requires one servo to be hacked and modified for continuous rotation and fitted with a small drive wheel. This is as simple as cutting out the potentiometer (the part that tells the servo it's position) and removing the small limit tabs on the main gear and screwing a model airplane wheel to a control horn. The center pin on the potentiometer is instead fed by the antenna tracker's signal comparator circuit and the wheel will drive a turn table.

The 555 timer generates a servo center signal and gives the servo a reference position just as your airplane RX would. When the signal comparator goes high, the servo will try to rotate to re center on it's new position. If the comparator goes low, the servo will rotate in the other direction. However, the potentiometer is now gone, thus, the servo will continuously rotate until the comparator gives it a neutral indication. Thus the tracker becomes one big servo.

-Alex
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Last edited by IBCrazy; Nov 12, 2010 at 06:53 AM. Reason: Added pics
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Old Nov 01, 2010, 10:46 AM
Future-proof EVERYTHING
DarkHeli's Avatar
NY
Joined Jul 2008
762 Posts
First Post!

Looks great, the idea seems like it works well, but what would happen if one of the RX's would crap out for whatever reason?


Anyway, love the tutorials. Thanks a lot IB!

I may try this one out after more testing is done.
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Old Nov 01, 2010, 10:55 AM
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Amherst, VA
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Trials and mishaps

I spent the better part of 4 hours trying to get an H-bridge circuit to work on my prototyping board which was already crowed with several other projects. After blowing up 3 H-bridges in less than 20 minutes, and never even getting a motor to even run, I decided I needed to go a different route.

Still determined to put my Tamiya planetary gearbox to good use, I decided to try and design my own H-bridge. BAD IDEA! Besides being overly complex, I could not keep the four transistors from shorting each other out when the signal comparator gave a neutral indication. I tried using inverters, buffers and even considered several different logic gates and a relay circuit.

Finally, I gave up the hope of using an H-bridge...

Then I considered a servo. Immediately I opened up one of the cheapest RC car servos I could find. I knew I needed this thing to go 360 degrees, so I had to remove the stop tab. I also needed to hack it's electronics... but how?

The answer was simple: Give it an artificial position reference and feed it rotations on it's potentiometer. So I cut out the pot and hooked up my signal comparator and made a quick pulse signal with a 555 timer. To my amazment, the servo behaved perfectly! It rotated continuously in either direction.

I then tested it by giving my signal comparator an artificial RSSI signal. The servo would drive back and forth as I changed the RSSI indication. I had it working!!!

Pictures will follow

-Alex
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Old Nov 01, 2010, 11:05 AM
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Charles Town, WV
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Awesome work!
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Old Nov 01, 2010, 11:05 AM
Engineer for Christ
IBCrazy's Avatar
Amherst, VA
Joined Jun 2006
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Quote:
Originally Posted by DarkHeli View Post
Looks great, the idea seems like it works well, but what would happen if one of the RX's would crap out for whatever reason?
The same thing that would happen if your RX went dead anyother time. You'd be in trouble... Head home fast... really fast... and hit the RTH switch if you have one.

As I go through this, suggestions are appreciated. This is an experiment. Input from other modelers can help me develop the best solution. I am trying to make this simple enough that most electronics hobbyists can do it. Of course, I don't expect anyne to bale to replicate my control board. I have this habit of making everything excessively tight...

-Alex
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Old Nov 01, 2010, 11:09 AM
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Cant wait for the tutorial... n1 Alex.
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Old Nov 01, 2010, 11:30 AM
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lovegroove's Avatar
Rhone Alps, France
Joined Aug 2008
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I know that using RSSI seems like the easiest way to achieve antenna tracking, but I don't think this is going to work otherwise a commercial version would be available already. The reason that I am sceptical is because of how up and down RSSI can be and with multi-path fading as well, I expect that you will just get it oscillating backwards and forwards, not knowing which way to turn.

I really hope you get this to work and I will be following your build though.
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Old Nov 01, 2010, 11:35 AM
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Quote:
Originally Posted by lovegroove View Post
I know that using RSSI seems like the easiest way to achieve antenna tracking, but I don't think this is going to work otherwise a commercial version would be available already. The reason that I am sceptical is because of how up and down RSSI can be and with multi-path fading as well, I expect that you will just get it oscillating backwards and forwards, not knowing which way to turn.

I really hope you get this to work and I will be following your build though.
+1, multi-path fading, reflections, local strong RF sources that affect RSSI, etc. etc.

Cheers,

Sander.
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Old Nov 01, 2010, 12:46 PM
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Brigham City, Utah
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Amazingly simple, can't wait to see.
Do you think it will need any firmware to make it behave?
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Old Nov 01, 2010, 01:36 PM
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Canada, BC, Victoria
Joined Feb 2008
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Excellent work IB! This is a really good idea to track the RSSI of 2x RX's. I can't wait to see it in action. Be sure to post a video once complete!
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Old Nov 01, 2010, 01:48 PM
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Quote:
Originally Posted by ssassen View Post
+1, multi-path fading, reflections, local strong RF sources that affect RSSI, etc. etc.

Cheers,

Sander.
What about a little bit of filtering of the signal to remove any short term variations in the RSSI due to multipathing, etc.?
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Old Nov 01, 2010, 01:55 PM
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Quote:
Originally Posted by twinturbostang View Post
What about a little bit of filtering of the signal to remove any short term variations in the RSSI due to multipathing, etc.?
A video Vsync lock circuit would work for this.
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Old Nov 01, 2010, 02:22 PM
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The problem with RSSI is that you need two closely matched receivers. RSSI circuit isn't a very well specified thing usually. In particular the linearity and coefficiency may not be consistent. In such a case, RSSI may not work that well. Fortunately, the application may not call for critical selection of the best signal and as long as it picks a more or less usable signal, the end user won't know how well the unit is performing.

A second reason why RSSI isn't good is that it is non-discriminatory. If you have a strong Wi-Fi killing the signal in one of the receivers, you may actually get even higher RSSI because RSSI is just signal strength, not video quality. Vsync evaluation is slightly better but coming out of the sync detector, it can be hard to distinguish between a good signal and a poorer one if both are still good enough to supply the sync.

The solution is to either hand pick matched receivers (not just levels, but linearity and coefficient), or to have a different approach either as primary or a supplementary scheme for the signal analysis.

Daniel
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Old Nov 01, 2010, 02:25 PM
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Quote:
Originally Posted by twinturbostang View Post
What about a little bit of filtering of the signal to remove any short term variations in the RSSI due to multipathing, etc.?
Already done. That's what the RC circuit out of the buffer is for. You can add resistance and/or capacitance to delay the reactions. The problem is mitigating overshoot when you do that. It will be a balancing act.

The unit will rotate fairly slowly so a little bit of searching is to be expected, but not much. The diversity controller I made worked very well based on RSSI, so I hope this will perform the same.

-Alex
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Old Nov 01, 2010, 02:28 PM
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I was suggesting to use Vsync just to lock on video then enable output and let the RSSI do the switching between rx's. Like stereo lock on radio's
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