RF shileding and Interference from steppers

While testing our existing v2 tracking box, we did a simple test of adding and removing ferrite beads on the stepper motor cables.

The results were astounding. Almost 20dB difference in noise level (measuring with a simple antenna and HackRF just outside the box) at 144Mhz.

Interestingly on 437Mhz the interference is almost not existing. Any tips for shielding our current v2 setup? Any tips on selecting ferrite cores/beads and placing? We should also probably do some testing on other bands and using different steppers (although I doubt this would change anything).

1 Like

I would assume that a strong part of emitting comes from the driver and the wires towards the motor too. The motor itself is already contained inside a metal box and what we see is the hf ringing every step. I think the v1 electronics board and bare wire can act very much as an antenna.

I am no expert in HF shielding, but try braiding the motor wires. This can already help at least a litttle bit for free.

One step up would be to use shielded cables of course or try a hack with some grounded aluminium foil and tape for isolation.

After that, it’s metal caging of the electronics (e.g. metal candy box) as well as supply rail decoupling. I successfully managed to decouple noise on a supply rail generated by a cooling fan in an audio circuit with a double LC lowpass filter (the Ls being a ferrite core from an old PC motherboard and one from a cordfree PC mouse RF transmitter, although the latter wouldn’t withstand the steppers power demand). Another option are filters used for motor noise decoupling in the RC FPV community. They either use LC filters or DC-DC transducers, both available from places like hobbyking.

1 Like

Hi Pierros,

I am not an expert, but here I have some links concerning to ferrite beads, which helped to me for avoiding the noise on my RF circuits: link1 and link2.

In addition to all recommendations above, is desirable to place the ferrite close to the element affected for the noise.

Of course, to twist the pair of cables when possible is highly desirable.


In addition to all recommendations above, is desirable to place the ferrite close to the element affected for the noise.

Thanks for these links. Also, what do you mean by the “element affected”? Would this mean to put the ferrite close to the stepper or close to the board?

@dosman, I’m sorry if I have not been clear :frowning:

In this case, we want to reduce the noise incoming from the stepper, so putting the ferrite between the board and the stepper it would be enough.

But if we assume the wire works as antenna, we can think the wire might be affected by another external noise sources, which can affect to the board too. Because of this, to put the ferrite close to the board will avoid the noise incoming form the stepper, and also, we will win an extra of shielding if a nosiy source is close to our board. Obviously, as the wire works as antenna, part of the noise generated by the stepper will be radiated to other parts of the system, this will be avoided if we put the ferrite close to the stepper.

As you see, a convenience solution is needed if only one ferrite is available…
In this case I assume the board is the most vulnerable part in the whole system, and therefore I assume the board needs the best shielding, so the ferrite close to this board.

If we want to have a fully isolated system, we shall to put two ferrite, as the follow HDMI cable I show in the picture.

I hope I have made myself clear :smile:

I am pretty sure, that the interference from the motors and boards is picked up also by the antennas. The electronics are in close proximity with the antennas and certainly inside the near field of them.

Would using longer pipes and a separate electronics boxes as we do in v3 produce lower noise levels from the motors?

I don’t believe it is getting picked up in the electronics themselves as the interference is getting picked up through the antennas… Better option here might be to line the inside of the cover with foil… I also run my motor cables through ferrites.

I am just before the final step of installing my satNOGS V3 rotator on the rooftop but I have the exact same problem. Every time the rotaror moves the noise floor is increased causing the loss of signal.

Could you please, @Pierros or @cshields advise on ferrites?? What type, how many???

Any picture of the installed ferrites would be greatly appreciated.

I did a couple of things, first I braided the stepper motor cables, and then fed them through a ferrite just big enough for the cables to fit through, like this:

I’m not sure if this is the magic fix or not but it is worth trying. Shielded USB cables would help, as well as a metal-shielded RTLSDR (like the v3 sticks at Buy RTL-SDR Dongles (RTL2832U) )

Might check this out in regards to RFI.

On rotator v3.1 we have designed a metal box to stop the interference from the electronics and the motors. We have not yet finalized, issue #53.