Many people are under the impression that radio receivers, while being tuned to specific frequencies, are "brick wall" devices that will "hear" nothing but their tuned frequencies. This is absolutely not true.

Ever drive your vehicle near a broadcast FM station while having your radio tuned to a different station? Chances are you experienced interference from the "off frequency" local radio station. This situation is even worse with AM broadcast receivers and stations.

Model ignition pulses are generally in the frequency range of the human voice audio range. Radio signals do not begin to radiate effectively until they reach 15 KHz (15,000 cycles per second). However, at close range, such as the interior of a model airplane and even large model airplanes, electromagnetic energy can radiate far enough to induced voltages in the receiver, battery, switch harness and servo leads.

Our old systems (non SS) had problems much more frequently than the SS systems because the encoding data rate of our transmitters is right in the audio range of the human voice, meaning it was of a relatively low frequency that was serendipitously close to the same data frequency used in the older radio systems. It is my suspicion that the coupling occurred inductively, much like how a transformer functions.

The relatively narrow bandwidth of the system data stream of the older systems made it much easier to couple to the internal wiring than the much broader data rate used by the 2.4 GHz SS systems. While not immune to this type of ignition interference, it takes considerably more energy (field strength) to interfere with the SS system than the old single frequency radio systems. Also, the SS system data rate is significantly higher than the old R/C system data rate, meaning that it is not a close match between the ignition interference and the SS systems data rate.

No radio system in the consumers' hands will have brick wall filtering. It is simply too expensive to employ in our systems at an acceptable price. Meaning that we still have to pay attention to installing our ignition systems and our R/C systems carefully and we must employ good engineering practice. Once again, there are no absolutes.


Ed Cregger