IBCrazy's DIY antenna tracker!!!
There are many tools to aid in flying FPV, but nothing compares to the antenna tracker. This device actually follows an airplane in flight to keep a directional antenna locked on target. This allows you to greatly increase your range without having to ever aim your antenna. It does that for you!
This tracker is unlike any other out there. Other trackers use GPS coordinates to determine Azimuth (heading) to direct the antennas beam. This tracker uses raw signal strength. The great thing about using signal strength is that it is un affected by multipath interference and you can fly it without an OSD. To obtain a lock, simply turn it on. That’s it!
I encourage anyone who has fair soldering skills and some electronics knowledge to try this. I will assist anyone who might need help. Feel free to post your questions here.
EDIT: This tracker has far exceeded my expectations. Watch the Demo video:
First a thanks for those who helped me
Before I get into the tutorial, I want to take a second to thank God for this. This project would have never even been conceived if it weren’t for a little help from Him. Everything fell into place just as if it were scripted.
I also want to thank the numerous members of RCGroups who have supported me in this effort. Whether you simply cheered me on and wished me luck, or offered advice and constructive criticisms, I want to thank you for taking an interest in projects like this. Without your support, I may never have made this happen.
Martin Y – He actually came up with the idea (Based on my diversity controller)
Daniel Wee – For support in technical areas of video systems
Mr. RC CAM and dpcav.com – For technical support on the RF modules
So Alex, how much for you to DIY and then send it to me? :D Seriously. ;)
The tracker works best with moderate directional antennas with small or no side lobes. Antennas such as the BiQuad and double BiQuad are great for this. The Cantenna is also a good choice and can be fabricated in under 5 minutes.
Yagis and Patch antennas are ok, but can cause problems due to sidelobe radiation. While these will work, they are more susceptible to mistracking.
Building the Tracker – Making the control board
The first thing to do is to make the control board. How you solder it is up to you. Below is a components list. All of these parts are common parts that can be bought from www.Jameco.com, www.Alliedelec.com, or even Ebay.
1- PCB board (5X7 cm, .1" hole spacing works well)
1 - uA 7805 voltage regulator
1 – LM393 voltage comparator
1 – LM358 OpAmp
2 – 555 timer (NE555N or similar – Optionally, you can use a single 556 timer as well)
2 – 10uF electrolytic capacitor
1 - 4.7uF Capacitor
1 – 1 uF capacitor
2 – 100K potentiometers
1 – 1 MegOhm potentiometer
1 – 270K resistor
2 – 22K resistors
6 – 15K resistors
2 – 2K resistor
1 – 390 ohm resistor
2 – 1N4148 Diode
1 – TO-220 heatsink (for the voltage regulator)
1 – Standard size servo (the slower and deeper gearing, the better)
3 – standard RX units (or just two and one spare module) SUNSKY and BOB TX’s WILL NOT WORK! YOU NEED A STANDARD RX SUCH AS THE ONES FROM WWW.NGHOBBIES.COM , WWW.DPCAV.COM OR WWW.READYMADERC.COM
EDIT: Updated with a revised schematic that limits the oscillations a bit better. I also updated the parts list to reflect this.
Building the tracker – The RSSI pin
With the control board built you now need to find the RSSI pin on your receivers. To make this system work you must have to very similar if not identical receiver modules. Open up your receiver and locate the module inside. Remove the securing nut to remove it and inspect it. You must have two identical modules (check the part# on the sticker). I highly recommend Comtech modules which can be bought from www.dpcav.com . The Comtech modules are a drop fit replacement for the typical module that is in these RX units. If you already have a Comtech module, consider yourself lucky, these are the best I have used.
On the module along the edge you will see a row of pins. Locating the RSSI pins on your receiver modules is not difficult. If you don’t know where they are, you can easily find them by checking your voltages on each pin and turn on and off your video TX and see which one changes. It’s usually pin #8 if you have a Comtech module, but I’ve also seen pin #6 on other models. The Comtech scales from 0 to 5V, others scale inversely from 3.5 to 1V.
Once you have found the RSSI pin, solder a wire to it on two of your RX units. These will be used to determine the position of your airplane. The third module will be your receiving module.
EDIT: I found out I had the wrong pin wiring on the Comtech modules. I attached corrected photos. Thank you Mr. RC CAM for catching this!!
Building the tracker - Modifying the drive servo
The next thing you must do is hack a servo. luckily, this isn't hard. I recommend a standard size sail or winch servo, but standard RC car servos work ok. The slower and stronger the servo, the better. Remove the back of the servo and locate the potentiometer. It is directly underneath the main drive spline of the servo. Remove the potentiometer from the servo. Solder a wire to the circuit board where the center pin of the servo was connected. Now reassemble your servo without the potentiometer. Your wire will now determine it’s position.
Now remove the final drive gear and cut out the stop tab. This will allow it to rotate 360 degrees. Reassemble the servo.
Finally, take a servo horn and screw an airplane wheel to it. I used a 2-1/2” wheel. In general, the smaller the wheel (that is bigger than the servo) is better. You will want a good hard wheel. Foam wheels are no good.
Building the tracker – Building the turn table
The next thing you need to do is build a turn table. I made mine from a sheet of ¼” plywood and a few 1X2’s. My frame measures 24”X24”. I marked and drilled a hole in the very center to fit a ¼” bolt through. I also made an identical lower frame for the tracker to spin on.
Now you need to add the wheels. I used a 3 wheel design to keep all wheels in contact at all times. 4 wheels is easier to rotate, but you will need to be sure all of the wheel lie in the same plane. Note the orientation of the wheels. The axles point directly toward the center hole. This allows the turntable to spin.
Secure your servo with the drive wheel to the base. Now take a tight fitting bolt and run it through the center hole so that it sticks out the bottom below your wheels. Now place the turn table on top of the lower frame passing the bolt through the center of the lower frame.
Now you need to mount your antennas. I made a stand with Velcro. You will want to face your tracking antennas between 60 and 110 degrees apart. Mine are 90 degrees. You will want to put these out of the way of your RX video antenna. I placed my video RX antenna in the front of the tracker and the tracking antennas in the rear.
Calibrating the tracker
Centering the tracker is a one-time set and forget operation. There are two ways to do this.
The most accurate way:
Turn on your VTX and your tracker with the servo disconnected. Aim both of your tracking antennas directly at the transmitter at fairly close range. Now measure the voltage at the output of the RSSI buffer circuit. Adjust the potentiometer so that the voltages of both are equal. You cannot exceed 9V or the LM358 will saturate. For Comtech modules 5-8 volts is good at close range. For other modules, with inverse scaling, 2V works well. I have Comtech modules, so I set mine to 7.5 volts. Your tracker is now centered.
The quick and dirty way:
Turn on the tracker and your VTX. Turn the 100K potentiometers until the tracker is aligned on your airplane. This will be a very fine adjustment.
Determining rotation duty cycle;
This one must be done in flight. Fly your airplane regular POV and check (or have a friend check) to see how close your tracker is following your plane. If your tracker is constantly behind and playing catch up, turn back the 500K potentiometer. You want to have the potentiometer set for as little oscillation as possible, but still be able to follow the airplane sufficiently fast.
That’s it! You are ready for an FPV flight. Good luck and have fun!
This section is for answers to questions asked and will be updated frequently.
Q: Will you build one for me?
A: No. But I will sell you all of the parts to make one and give you plenty of support to build it. Between my full time job as an electrical engineer, building my custom antennas, and R&D work, I have almost no time. I have even been cutting sleeping out of my schedule.
Q: Is this difficult to build?
A: Define difficult. If you have some soldering skills and can interpret an electrical schematic, then I would think you would have no problem building one.
Q: How long does it take to build?
A: It depends on your skills. I built and tested mine in just a couple of evenings. If you are an adequate solderer and are good with wood working, you can build this in 4-6 hours.
Q: Why do you use a gain controller on the buffer rather than a simple shorted buffer?
A: The main reason is to desensitize the RSSI delay and limit the oscillation. However a straight buffer (replacing the 4 resistors with a shorted wire) would work great as well.
Q: Is the delay timer really necessary?
A: Not really, but it's a nice option. The servo will be under constant stress most likely higher than it was designed. The delay timer helps limit the servo wear and also extends battery life.
Q: Is the RSSI buffer necessary?
A: In a perfect World, no, but in the real World, yes.
Q: My tracker spins all the way around and never seems to get a lock
A: Chances are your RSSI wires are reversed, try swapping them. If that doesn't work, check your RSSI buffer circuit to be sure the voltage isn't above 9V.
Q: I am having trouble getting the tracker to center, what should I do?
A: If you are not having much luck, try moving outside to an open environment. You can also maneuver your directing antennas some or just simply turn your video antenna to compensate.
Q: My tracker works great at some ranges, but others it gets off center.
A: I had this problem when using the standard modules. When I switched to Comtech modules, the problem was solved. It appears most modules do not maintain equal scaling, but the Comtech does.
Q: Are you MacGyver?
A: Nope. I have no mullet and have only been on TV once when I was 9 years old. I was on the news because the police had shut down my brother's and my lemonade stand due to lack of a permit!
You are awesome. Hauntme13 should incorporate your design into his 4 way diversity system. I'm going to link to your post on his post. This would be the ultimate setup!
Execellent work Alex.
Incredibly inventive and impressive! Excellent! :)
well I want one. looks like I will build one ... well once i get everything else working :)
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