Aircraft Radio

Introduction

There are many different types of radios systems in the world. Most are designed for very specific applications and have only the minimum number of controls needed to provide the level of flexibility needed to perform their designed task. A very general purpose receiver, such as the one pictured on the left, will have many controls so that the operator may adjust the receiver function to immediate needs. 

On the other hand, radio equipment designed for one very specialized purpose, such as the avalanche rescue beacon pictured on the right, will have very few controls. Those functions for which the operator does not have a method of control may be fixed, such as the frequency of the avalanche beacon; or they may be controlled by automation, such as the amplification (or gain) of the receiver in the avalanche beacon. Of course the automation will act to control the radio in a way consistent with its designed purpose. If one is to use a receiver in a way not intended by the designer, one must be aware of, and compensate for, the action of the automated systems. Aviation receivers have two important automatic systems, the squelch and the automatic gain or volume control.

Automatic Gain or Volume Control

It may help to think of the automatic gain control as a robot that has its own volume (gain) control that works on the signal before it gets to the pilot's volume control. The robot never shows the pilot what setting it is using. The robot turns its volume control up or down as the amount of received power decreases or increases trying to keep the signal amplitude it sends on at the same constant level. The pilot's control then increases or decreases the output volume from the one maintained by the robot. This is why a receiver is able to deal with a transmission from the control tower only a mile distant that may have a 50 or 100 watt transmitter, and the reply from an airplane with a 10 watt transmitter 25 miles away without the pilot changing the volume control. Of course there are limits to the amount of control the robot has. A very weak signal may be so weak the robot turns its volume control up to the maximum. Any reduction in power beyond that point could result in a reduction of output volume, or the loss of signal reception. A very strong signal may be so strong the robot turns its volume control down to the minimum. Any increase in power beyond that point will result in distortion of the output signal. Except at these two extremes, it is not possible to determine with any degree of certainty what the strength of the signal was before the robot applied its adjustment.

For our purposes the automatic gain control is our friend. With its aid we can use the receiver to detect quite weak signals and be confident that when a search aircraft flies into an area where the signal from an emergency transmitter may be received, we will be able to receive it. On the other hand, if we want to use the receiver to estimate the strength of the signal being received at a particular location to infer the distance to the transmitter, the automatic gain control will work very hard against us. 

Squelch

Radio communications on air band frequencies are sporadic. A long time could pass between instances when there is a transmission on the frequency. Normally this would result in the receiver producing noise which could be fatiguing and annoying to listen to. Because of this aviation receivers are equipped with a squelch circuit. The purpose of the squelch is to determine if there is a valid signal on frequency, and when there is turn on the receiver output, and when there is not turn off the receiver output. For our purposes this is a problem. An emergency transmitter signal that may otherwise be detectable may not be of the quality needed to activate or 'break' the squelch. This may result in a search aircraft flying past an emergency transmitter without detecting it. The squelch is not designed with aural search techniques in mind, so it is quite possible that the squelch may react to the signal differently at different locations. This would result in errors. Luckily in most aviation receivers the squelch circuit may be disabled, often with a 'test' switch. Consult your receiver operating manual.

In the next section we will see how the human ear interprets the signals the receiver provides.
Comments