Track Satellites With A 2-axis Antenna Positioner

Ham radio operators are curious beasts. They’ll go to great lengths to make that critical contact, and making sure their directional antennas are pointing the right way can be a big part of punching through. Of course there are commercial antenna rotators out there, but hams also like to build their own gear, like this Raspberry Pi-controlled 2-axis rotator.

[wilho]’s main motivation for this build seems to have been the sad state of the art in commercial 2-axis rotators, which seems firmly mired in the 90s. Eschewing the analog pot sensors on DC brushed motors that seem to dominate the COTS market, [wilho] went with steppers and stout gearboxes for the moving gear. Feedback on the axes comes from 10-bit absolute encoders, and an MPU9250 9-axis IMU makes sure he knows exactly where the antenna is pointing with respect to both compass heading and elevation. A mast-mounted Rasp Pi controls everything and talks through a REST API to custom software that can return the antenna to custom set-points or track the moon, satellites, or the ISS. It’s a very impressive bit of kit that’s sure to drive your home-owners association bonkers.

For another 2-axis antenna positioner, check out 2015 Hackaday Prize finalist SATNOGS.

22 thoughts on “Track Satellites With A 2-axis Antenna Positioner

  1. A Rasp Pi, PWM stepper drivers, digital everything. I’m amazed he’s not killed by QRM, yet he claims 70 EME contacts. At 2M, even. Must have done an exceptional job at shielding everything.

    1. I was wondering about that, the IMU must be on the antenna itself, sending digital signals down the wire. I can only presume he switches it all off once he’s got the antenna where he wants it.

    2. Digital signaling doesn’t mean automatic QRM… as long as the mean it rate doesn’t get near your operating frequency(or a harmonic of it), you’ll be fine. Plus you’re right, shielding is also key and not that difficult of a thing to accomplish…

  2. “…sad state of the art in commercial 2-axis rotators, which seems firmly mired in the 90s.”

    Boy, you got that right. This is a market that is ripe for someone to jump in and own it with affordable modern products.

  3. I just finished an antenna rotator powered by step motors and was expecting EMI problems, but I checked it with a spectrum analyzer in the 2M and 70CM bands and it was clean. Also put an antenna in place and hooked it up to my radio and found not birdies.

  4. Ham radio operators are curious beasts. They’ll go to great lengths to trash anyone who doesn’t have good enough (read expensive) gear, anyone of a different race or political alignment, and anyone who hasn’t been licensed long enough. Between what I read online and what I picked up on SDR I have never noped right out of a hobby so fast. Maybe I’ll try again if I move to a different part of the country.

    1. That’s really sad, and not entirely inaccurate. My gripe is also with all the talk about expensive off-the-shelf gear – Marconi would be spinning in his grave: just build something already, or encourage people to self-build some filters for their cheap Baefengs instead of telling them they’re worthless junk.

      Luckily, my local ham radio club isn’t too bad for this sort of thing, but some of the larger clubs in the region certainly seem to have a lot of members doing the opposite of promoting the hobby or enhancing the art.

    2. Interesting. . . I’ve been licensed since 2008 and have been on several HF bands and VHF/UHF. That has not been my experience at all. I have found most people on the air to be quite helpful, kind, and inclusive. I’ve only heard lids on the air a couple of times out of many hours of listening (and a bit of talking, too).

  5. I am not familiar with the combination of stepper motors and rotary encoders. Is this a common configuration, i.e. using steppers in a closed-loop control system? Does it have to do with the reduction gearing?

    1. Any stepper is prone to losing steps if the acceleration is to fast or the static load too heavy. The simple approach is to use beefier motors that weigh a lot more.

      A closed loop system counteracts that, and one with steppers is conceptually a lot simpler than using DC motors because of the great braking capability of the steppers. It reduces high amplitude oscillations a lot.

      Using brushless or brushed motors requires significantly more skills in the pod control department, but the results are also significantly more impressive in terms of weight savings, overall speed…

      It’s always a trade-off. In the industry they usually prefer beefier motors and less complexity, hence less chance of failure because uptime is critical.

      At the other end of the spectrum are camera stabilising gimbals, where low weight and fast speed is paramount.

      Ham radio antennas are somewhere in between. Occasional downtime is ok and speed is moderately important since some satellite passes go across the whole sky in 10 minutes. Having an antenna easily transportable means low weight is an improvement.

      1. Thanks for taking the time to reply. The points you make sound quite reasonable. I just thought steppers were only used for non-accuracy critical applications in an open-loop configuration and for all other cases, where accurate positioning is essential, closed-loop AC/DC servos are the only way to go.

        In this particular case, however, he’s also using a worm-drive gearbox which means that holding position should not require motor braking. You just cut the power and the thing stays put. Plus, the 60:1 reduction ratio should probably provide enough resolution to make any errors due to missed pulses or gearbox backlash insignificant for this application – unless of course the motors are really weak for this kind of load, even with this gearbox, in which case closing the loop becomes a necessity.

        Maybe after getting all this HaD coverage, he will update his page with more information on the control system.

  6. I haven’t got around to playing much with satellite but sounds interesting. I did save the one reply about using a different antenna system without having to use a rotator. But if you do want to build a rotator this group has one you can build with 3D printed parts etc.

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