Hi Everbody,

I have now had a look at a Shadow and here are my findings so far.

The aerial on this one is mounted on the boom central to the tailplane.

These are the readings.

Mid Range
Emergency Frequency
55 Ohms

This antenna is showing good results however the SWR graph curve is not inline with the expected.

All this said the main job at hand is the grounding/shielding of the intercom and all the earthing sequencing in the correct order. The main earth is from the battery to the engine, obviously required for the starter motor. Then from the engine to the engine mounting frame which then the bus bar is mounted on. John is having problems with RF intercom interference and this is most likely to be a major contributor as all the earths being together and finding the earthing via the shortest channel it can find.

After an explanation of how the earthing and shielding of the coms were the conclusion was to get the earth routing under control.

The earth runs straight to the engine as it should be for the Starter motor.

As you can see this is a Rotax 912. Then from the other side of the engine runs the earthing to the engine mount and the bus bar. The radio/comms earth is the yellow one pointed out.

On the left you can see the starter solenoid mounted on the earthed engine mount. At the right hand side you can see the fins of the regulator rectifier which is also earthed to the engine mount via a separate cable.

The main aircraft 12 volt supply is taken off the live side of the solenoid and feeds all instruments and radio.

*GOLDEN NUGGET* All the wiring acts like an antenna and if not shielded and earthed in the correct sequence the RF interference can run riot!

The conclusion was to re-route all the earthing so as to drain away the RF interference in the correct sequence. John was going to increase the size of the earthing cable to the bus bar and isolate the bus bar from the engine mount. The solenoid and the regulator rectifier were going to remain earthed to the engine mount. I suggest that the solenoid and regulator rectifier’s earthing be isolated and go direct to the battery as I suspect this is a major contributor to the noise that is infiltrating the system. Not only that but the this earthing is also being seen by the antenna due to the coax shielding being connected to it too!

*GOLDEN NUGGET* The earthing of the radio and the intercom should to be furthest away to the earth with regards to all the other earthing in the aircraft!

I will be adding to Johns’ post as time goes on but feel free to join the Chat Frequency follow or create a discussion. The current discussion is Ground Loops! Open to all no matter aircraft type.


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