I'm working on a quadcopter project. The quadcopter will be quite autonomous, but sometimes I'll need to send some messages to it. So I built a simple, one-way RF communication system based on two simple 433 MHz modules.

It works, but when the propellers of the quadcopter approach the take-off speed it begins having trouble in receiving the RF messages, and after a certain speed it stops working.

I suppose this is due to the electrical noise produced by the four little brushed motors. Yesterday I increased a little the transmitter's voltage, and that increased a little the minimum antenna-antenna distance with the motors on, but that is far from enough.

What else could I do to increase the transmitter's power?

Thanks in advance


P.S.: Here are my RF modules:

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  • You might like to read my investigation into those modules (hackingmajenkoblog.wordpress.com/2016/11/18/…) and then rethink using them - instead using something more reliable.
    – Majenko
    Dec 2 '16 at 15:26
  • You can add an antenna to the ANT hole. Just add some copper wire. You'd have to experiment with the length of it, to get the best range.
    – Gerben
    Dec 2 '16 at 19:33
  • thanks Majenko, I read your investigation. My modules are better built than yours and my rf system is very simple (I only need to send a few dozen of bits per second (and not every second), without any transmission sopeed requirement, andalways in the same direction), but of coure you're right, they're worth 99p. I will change them, but I can't get them in the next few weeks, so for the moment I would like to improve the existing system... Dec 2 '16 at 20:28
  • You need something like LoRa modules. I have attached very nice 1/4 wave antennas with radials to these modules, and they still couldn't receive well... Polarity even became an issue. Got 25 meters without obstructions, but the slightest obstruction (leaves even) cut the signal off. Look into LoRa. I will now read @Majenko's blog.
    – user400344
    Dec 4 '16 at 14:39
  • No one uses those for micro aircraft, as 2.4 GHz solutions are both more reliable and lighter weight. But the minute your quadcopter makes it off the ground, you'll probably discover that you don't have a working flight stabilization firmware. Spend some time on rcgroups where people who have actually done this (mostly in reflashing cheap toys with custom firmware) discuss the issues. Jan 2 '17 at 17:46

As has been mentioned adding an antenna to the "ANT" pin on the transmitter will help. You need to get the length right though, but it's easy to calculate.

Best is to use single-core insulated wire (0.6mm is a good choice).

To calculate the right length you first need to know your frequency. Then you convert that to the wavelength, and then cut your wire to a quarter of that length.

Wavelength is c/f where c is the speed of light (in m/s), and f is the frequency.

So for 433MHz you have a wavelength of (299792458/433000000) 0.69m.

A quarter of that is 0.173m (17.3cm), so cut your wire to that and solder it on.

For 315MHz the wavelength is (299,792,458/315000000) 0.95m, so an antenna length of 0.238m or 23.8cm.

This is known (unsurprisingly) as a quarter wavelength antenna and is the most efficient length for a simple wire antenna.

Whether it will be strong enough to penetrate the noise from the motors is anyone's guess.

You can also add small capacitors (0.1µF ceramic) across the terminals of the motor and between the terminals and the body, to absorb some of the EMI which may help to allow more signal through - as shown here.

  • Tanks again, @Majenko. I already have an antenna, but it's like twice the length you wrote here; i'll cut it. But I just read the post mentioned by Gerben (here): the author says that I sould subtract the coil soldered in the module (I have a coil which is in fact the first part of the external antenna) to this lenght, does this make sense? And what about other types of antenna? Is it possible to connect some kind of more efficient antenna to such a simple rf module? Dec 2 '16 at 23:14
  • It is always possible to attach other antennas, yes. A Yagi array (TV antenna) will increase gain at the cost of omnidirection. You should be tackling the motor noise first though. Don't try to overpower it, eliminate it.
    – Majenko
    Dec 2 '16 at 23:15
  • But the antenna port requires desoldering the coil. What range and punchiness did you get with your modules? Partnumbers/link?
    – user400344
    Dec 4 '16 at 14:41

I have been using RF modules for different projects that involves using motors. And I have never experienced something like that. Since the motors a significant quantity of current to take off, I recommend using another power supply for the electronic part of the drone and use the actual battery just for the motors.

  • I can't add a second battery (the quadcopter needs to be very lightweight), but the electronic power supply is protected from the electrical noise of the motors. I think the problem is the electromagnetical noise: if it is too strong the rf signal is "hidden" in the noise and the receiver can't detect it... (is that correct?) Dec 2 '16 at 20:22

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