433Mhz transmitter interference when using long cable, only on Arduino Due
I think I found the problem. It seems that there's an anomaly with the Due where when a pinmode is set as OUTPUT, it sets the pin as HIGH by default, whereas with the other Arduinos it's set as LOW. This forum page is discussing the problem: http://forum.arduino.cc/index.php?topic=154869.0

Likewise this page suggests the problem is a bug with the wiring_digital.c file: http://forum.arduino.cc/index.php?topic=185230.0

In RCSwitch.cpp, this is the enableTransmit code:

void RCSwitch::enableTransmit(int nTransmitterPin) {
  this-&gt;nTransmitterPin = nTransmitterPin;
  pinMode(this-&gt;nTransmitterPin, OUTPUT);
}


It seems the code sets the pin as OUTPUT, which would cause an anomaly on the Due alone.

I solved the problem by calling the enableTransmit function just before sending a signal, and then calling disableTransmit() immediately afterwards. Can anyone think of a cleaner/better solution?

Out of curiosity, can anyone suggest what a standard transmitter might be sending when the data line is constantly held at HIGH?