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Electromagnetic Compatibility

Bringing together different radio systems for control of servos and video,
it may not just work first time, this may help to get problems resolved:

This page details my learnings when I built my first rig with 35MHz aero-SERVO control and 70cm VIDEO downlink, Rig02. Whilst the rig worked OK in the workshop, field trials highlighted a series of issues whose root cause were interactions between the servo receiver and video transmitter, resulting in severe servo jitter at normal operating distances.

This type of inter-operability issue are reasonably common in electronics, however solving the problems whilst minimising weight and complexity, all in the absence of test equipment is a little tricky. There is little alternative to curing problems than experimenting, so here are a few ideas as to how to proceed.

Identify the issue

So you switch it on and it doesn't work. Here is a typical way to proceed. Successively switch-on part of the circuit to try and work out which parts are causing the problem. A good sequence is to start with the Servo RX and servos only. This is standard aero-modellers technology and should work. Remember things might work when spread out on a workbench, but not when in the close proximity needed for a rig.

DON'T use a carbon fiber rod to support the SERVO RX aerial. It conducts electricity and desenses the aerial! Use glass fiber supports instead!

The system should work jitter free with the transmitter aerial extended only 15cm and separation between TX and RX up to 30m line of sight. When doing this test the receiver aerial should be in free space, NOT lying on the ground. Add extra items (Camera, video downlink etc) and re-check performance. 

It is more than likely that the receivers performance will be adversely affected by the presence of the video downlink signal, however we don't need the full range the servo receiver offers, just a few hundred feet. This effect is called "deafening" of the receiver. 

Having done the previous tests you can identify where the problem is coming from. However the transmission mode for the interference can be either conducted through the wires connecting the various elements, or radiated through the air between the elements.

Conducted Interference

This is where interference is coupled between elements on the power supply and signal wires. To reduce these effects manufacturers put large ferrite cores onto their leads that look like "bulges" in the cable. You can check whether you have this problem on power leads by temporarily powering different elements from different batteries and seeing if the issues go away. With non-power leads you have to just experiment to see what helps, if anything!

A clamp on ferrite for experimenting

A good tool here is a clip-on Ferrite, (e.g. Maplin part number N96AB) that can be added to cables without re-wiring the cable. They are large, 26x13x13mm, and heavy, but a useful starting point. Clip over the wire that is suspect (called a turn), and if possible loop the cable through twice (1 turns). When doing this you need to put all the wires through, e.g. both the plus and minus wire of a supply should be wound through the ferrite TOGETHER.

ANOTHER tactic is to use small 0.1uF capacitors across the power supplies into each device. This can absorb RF energy that is picked up on the power supply wires.

The Ferrite should be located at the end of the cable closest to the source of the interference, this ensures that the interference is snuffed out before leaving the source, and is not changed into a radiated source as it travels down the interconnecting wire.

Choose the smallest ferrite that works

Once you have identified an improvement, remove the clip-on ferrite and see if you can get the same effect with a much smaller ferrite, say 18x6x6mm (Maplin part-number N98AB). Try to put as many turns on the ferrite as you can squeeze through the central hole, this will increase it's performance with little weight increase, and get you a lightweight solution. Unfortunately this work may involve you re-making wiring harnesses and lengthening cables.

1 turns: wind as many turns as you can

Once you get good at this you can also save weight by removing the ferrites on the cables supplied with your camera for example, and replace them if necessary with smaller ferrites.

Radiated Interference

This is caused mostly by the relatively high power TV transmitted signal interfering with the sensitive receiver, and other circuits in the rig. A good test for this is to disconnect the aerial and replace with a dummy load. Use a 50ohm resistor if you don't have a proper load, or if the aerial doesn't have a plug/socket screen the entire transmitter and aerial in a sealed metal box. The radiated signal can interfere by either being picked up on wires and turned back into conducted interference, or just simply getting into the circuit "legitimately" through the servo receiver's aerial.

Although you can re-design the servo receiver to reject the interference better there are a couple of simpler solutions for the KAPer.

  1. Make sure all metal on the rig is connected together and grounded to either servo or video ground (this is important on my rig since the grounds are switched to enable the video transmitter).
  2. Make sure the aerials are as far apart as possible. This is complicated by the fact that you also need to keep the aerials out of the sight line of the camera. I have the aerials out on horizontal booms on opposite sides of the camera.
    Using a phono socket / plug for a light wave aerial and ground-plane on a boom

    This makes the whole rig rather ungainly, but it works. The servo wire aerial sticks out to the right by ~400mm on a 2mm glass fibre rod, the remaining 400mm of the aerial dangles down. The video transmitter aerial is 300mm out on the other side on a 2mm carbon fibre rod, thus the separation is about 300mm plus the width of the rig, i.e. 500mm. Note that the picture at the top of the main Rig02 page was taken before this final TV aerial separation was added.

  3. Try different orientations, positions or types of aerial. For the 434MHz downlink I use a wave dipole with four radial ground-plane, (detailed on page 6 here) but you could try a 3 element yagi beam, there are short beam designs that suit KAP. 

Start the analysis again

Once you have done all this you need to repeat the process! Having removed some causes of interference, you may find that there are other problems that were masked by the problems you have just solved!

DON'T be tempted to use walkie-talkies, mobile phones etc to communicate between the rig and ground station when debugging problems. These electronic devices can themselves cause interference to your rig.

Unfortunately I found that there was no alternative to going round this loop several times, with a visit to a kite field or local park in-between to try-out the latest improvements. At least this work can be done on wind-free days, tying the rig to a tree and experimenting can be as productive as a full "flight" test.

Final thoughts...

Remember your "ground station" can also suffer from the problems described above, in this case between the servo transmitter and the downlink TV receiver. You can employ all the same strategies and solutions to identify the problems, fortunately weight and balance are not so important in solving these issues.


Thanks to several people on the web who exchanged e-mails with me and guided me through the above. Special mention to Terry T, Simon H, and Dennis F, thanks for listening!

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