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Aircraft Radio Transceiver

Radio Interference, II

The definition of radio interference can be described as a range of situations where the reception of radio signals is disrupted by transmissions from unauthorized users, devices emitting spurious transmissions whether or not by design or disturbances in atmospheric conditions. This could lead to pilots not receiving or understanding ATC directions and missing important information regarding their flight.

Aircraft communication radio's should be equipped with functions or technical features which help the pilot suppressing these unwanted interferences. Power line filtering and digital signal processing (DSP) in the form of noise blankers and noise suppressing should be standard issue in every airplane radio these days.

The aircraft, its equipment and the engines should also be designed with these requirements in mind so they do not emit any unwanted RF energy causing interference (think high voltage strobe lights) and possible loss of communications.



Interference sources, Internal

The cause of the interference can be either from inside the aircraft or it has its origin or influence from outside the aircraft. For the pilot it can sometimes be difficult to know/hear the difference without experience. Problem source detection can be relatively as easy if a certain interference occurs only when switching on a device or aircraft light or certain changes with engine RPM.

Strobe lights

These devices generate high voltages to ignite or flash the light at regular intervals, easy to detect should you 'hear' them. They need proper grounding of the wiring from the high voltage (HV) power supply to the light unit, ground them at one end only. Make sure the 12/24 volt wires to the power supply are thick enough to handle the expected maximum peak current, usually between 4 and 10 amps. Run the wiring on a separate bus with their own fuse.

Modern aircraft now use LED based strobe lights, the advantages are low weight and longevity of the LEDs. Although they do not use high voltages to generate the flash, these device may use a microcontroller to create the flash patterns. These should be sufficiently shielded too.

Engine ignition

Spark Plug Ignition

Spark ignited engines are inherent sources of radio interference. Sparks are wide band noise emitters, much like thunderstorm lighting is. Modern engines have good noise suppression resistors built in the spark plugs and shielded HV cables so this should not be an issue in todays aircraft anymore.
You will not find these problems with diesel engines, but as these may use a FADEC system containing a microcontroller so that sshielding is important here too.

Alternators & generators

The brushes in a generator and rectifier diodes alternators can fail for any reason or become really bad with age if not taken care of properly on time (lack of maintenance). Interference is easily recognized as this will vary with engine RPM. Read more in our article on power generation and discover how they operate.

Radio settings, DSP

DSP Processor TI

The proper use of the squelch knob (some are automatic), which blocks the white noise you hear in the audio when no station is transmitting, will prevent you from this type of interference.
Modern digital signal processing (DSP) functions as audio and IF bandwidth filtering, noise blanking and reduction for suppressing engine ignition pulses and other interference, should be switched on when needed.

Not every modern aircraft radio is equipped with DSP at this time. Today, modern HF/VHF communication radio's do have these digital signal processing functions built-in and their advantages are widely known and accepted. The next logical step is to implement DSP technology in our aircraft radio's by the manufacturers.

ANR Headsets

Active noise reducing or canceling headsets are becoming more and more common with pilots these days. These actively listen for noise (within a certain frequency band response so you will continue to hear the engine) in the cockpit with separate microphones in the headset and cancel out or reduce this, thereby improving cockpit communications and radio reception. This is of course a specialized application of DSP.


written by EAI.