2.4 GHz Radio Interference

 

When I started with bigger airplanes (26% scale) I bought a world models Extra 300S with a 50cc Gas engine. I was using a Dx7 radio with an AR7100 receiver. The receiver was connected to the battery via a JR heavy duty switch to a 2000mAh battery. The battery was plugged into the battery port on the receiver. I used 10.5kg Chinese servos for the ailerons(2), elevator(1) and rudder(1). The throttle had a normal 5kg analogue servo. After the 4th flight I lost signal and the plane ended up in the ditch. SO NOW WHAT???

 

Once I got to the plane, the fuse was broken in half and the wings were full of hole. All the electronics was still intact and working. First thing I did was check the battery with my volt spy and it indicated that it was still good. I asked some of the guys at the airfield if they knew what went wrong. Everyone said flat battery.

From my racing experience I was taught to find the fault otherwise it will bite you again. So I was not satisfied with the answer I got and started to phone a few hobby shops. After speaking to 6 shops and explaining my whole story to them, I still got the same answer from all of them…….Flat battery.
Now the next thing I could think of was simulating the complete circuit on my work bench at home. I switched everything on and it was working perfectly. So I messed around a little and after 2 minutes and not finding anything wrong, I started to move both sticks in circles simultaneously. By the fourth circle one servo started to get different movements from my input. I then thought why that would happen.

 

Next logical thing was to put a voltmeter on the battery. Before any movement the reading was 6.2 Volts. By the third circular movement the volts have dropped to 4.1 volts. The cause was then that the receiver could not get enough supply from the battery for the demand of the servos. Then I had a look at the battery supply wires. The Chinese battery had a 1.5mm2 wire and the current that it could handle was 4 amp continuous and 6 amp spike. Now when I was moving all the servos at the same time each servo was asking 3amp at maximum throw each. So demand outstripped the supply and then causes interference.
After further investigation I found that the spectrum batteries has got 2mm2 wires for the normal connector as well as an ec-3 connector with 18 gauge wire. The normal wire can handle 6 amp continue and the 18 gauge can handle 15 amps. The servo wires on the Chinese servos were also much thinner that say JR servos. The Chinese’s servo had 1mm2 and JR 1.5mm2 wires.

So in the end my solution to all the issues above was to use JR servos with thicker wires so that the servos can get the required current from the receiver. The receivers also need the correct supply and the use of two batteries is the solution. One battery will plug into the battery channel and the other battery into any of the open channels except telemetry which runs at 3.3 volt.

Once I started to fly 35% and bigger I switched to the 9 channel receiver with the 2 battery cables coming straight from the receiver. The wires are 18 gauge and have ec-3 connectors on the ends.

I always try to explain the scenario above by means of an example. Say you have a big dam full of water and you want to water your garden with a normal hose pipe. The hose pipe is fine as long as you want to water one plant at a time. Now say you want to water two plants at the same time, you need two hose pipes. So now you take your one hose pipe and split it into two. Now you have half the amount of water coming from each pipe. Say you want to water 4 plants at the same time. Now you have to split your hose pipe into 4 and every plant gets a quarter of the water. So the only way to water all the plants with the same amount of water as one hose pipe, is to make the supply bigger so that all four hoses pipes can give 100% supply. The same happens to the current on the receiver. You have to make the supply wires thicker or more to supply more.