A Directional Device for Guiding Pilots to Airports
AERIAL EQUIPMENT on the Imperial Airways liner Hengist. The loop aerial used for direction-finding radio is mounted above the nose of the fuselage. The aerial is used also for homing radio in conjunction with the trailing aerial. When the machine is in flight the trailing aerial is lowered from a tube projecting below the fuselage. This tube is seen directly below the door and behind the pitot-static tube of the air speed indicator.
With homing radio the pilot is able to fly directly towards any radio station in the worst of weather conditions. It is unnecessary for him to know his exact whereabouts when beginning to “home” on the station, although he must know the situation of the station on which he is homing.
Among the advantages of the system is the fact that two-way transmissions are not required, and even broadcasting stations may be used. When the transmitting station is at an aerodrome, or closely adjacent, homing radio provides pilots with a means of finding that aerodrome. An instance of a transmitting station suitably near to an aerodrome for guiding pilots to that aerodrome is provided by the omnidirectional radio beacon at Mitcham (Surrey), which began to work at the beginning of 1938. This transmitter is approximately two and a half miles from Croydon Airport and is intended specially for homing aircraft and pilots wishing to obtain their bearing in relation to Croydon.
Because homing radio does not require a transmitter in the aeroplane, it can prove especially valuable for light aircraft where every pound of additional weight is of importance. The system is generally used on wavelengths of between 250 and 2,000 metres, the range of wavelengths being governed by the radio receiver to which the homing apparatus is attached. Most broadcasting stations transmit within this band of wavelengths; there should therefore be no lack of stations on which a pilot may home in any circumstances, provided he is able to identify the particular station that he wants.
One important advantage of homing radio is the speed with which it enables a pilot to get on to a suitable course. After having taken bearings with ordinary direction-finding methods, a pilot has to work out the bearing of the course he desires to follow, and this process also has to be repeated frequently if the pilot wishes to find out if he is still flying on his intended course. With homing radio he has a continuous indication of the correctness of the course he is following.
This feature of a continuous indication which shows immediately any change in the course of the machine has led to the use of homing radio in the Hegenberger system of radio-assisted blind approach. This is an American system which enables a pilot to bring his machine down to an aerodrome ready to make a landing when visibility is less than 100 feet.
Homing radio, apart from its other advantages, may be regarded as a valuable stand-by in the event of the normal magnetic compass becoming faulty. This was one of its points that appealed to John Grierson in his 1933-34 exploration of the Arctic air route from Great Britain to America.
The flights made by Grierson were among the first to demonstrate the value of homing radio. His loop aerials were large ones built into the wings of his machines, a method of construction made possible because both were bi-planes.
The type of frame aerial normally used in aircraft consists of one or more turns of wire arranged in a large loop, and insulated from the machine. The turns of wire are carried in a protecting casing and resemble a child’s small hoop. The overall diameter of the Marconi loop is 13 inches. Although the outside of the fuselage is the usual place for the loop to be mounted - on metal machines this is a necessity because screening has to be avoided - provision is often made, as in the Empire flying boats, for it to be retracted into the fuselage when not in use. The plane of the loop aerial is arranged vertically and, because of the special properties of frame aerials, reception from a given station is strongest when the plane of the loop lies directly along a line joining the loop aerial and station. When the plane of the loop is at right angles to this line, no reception, or virtually none, is obtained because there is then no “pick-up” by the aerial.
Between these two positions of maximum and minimum, signals vary in strength according to the angle of the loop in relation to the direction of the transmitting station. An important point, however, is that the position of zero reception is much more clearly defined than that of maximum. As the loop aerial is steadily turned away from maximum, signal strength gradually becomes less, but as the zero reception position is approached, the rate at which signals are reduced becomes greater and greater. Thus it is an easy matter to decide exactly what is the zero position.
To find the bearing of a station in relation to the receiver, the loop is turned to its zero reception position. The operator then knows that a line at right angles to the plane of the loop will run directly to the transmitter, but he does not know to which side of the loop aerial the transmitter is situated. If the zero position of the loop were east to west, the transmitter might be north or south of the receiver.
RECEIVER FOR DIRECTION-FINDING RADIO which works on a loop aerial and provides facilities for homing radio. With this receiver, which is a Marconi Type 5062, both aural and visual indications for homing radio can be provided.
In many instances the pilot of an aeroplane will know within 180 degrees his aeroplane’s position in relation to the radio station on which he is using the direction-finding apparatus. If he does not know this, however, and if there is another station suitably situated, by taking the bearings of both transmitting stations he is able to ascertain the exact directions along which the stations lie. Moreover, by plotting the bearings of the two stations on a map the pilot, or his navigator, could determine his locality by noting where the bearing lines intersected. Generally, however, for such a purpose bearings would be taken on three stations, so that a small triangle of error would denote the approximate position of the aircraft. From the foregoing, which describes the normal direction-finding methods used in an aeroplane with a rotatable frame aerial, it might be thought that homing could be achieved merely by keeping the machine on the path of zero reception. But in these circumstances, if the transmitter ceased radiating, the pilot might imagine that he -was still dead on his course when he might be many degrees off it. Further, when weak reception indicated a slight divergence from the correct course, a new bearing would have to be taken to ascertain in which direction the divergence had occurred. Neither of these considerations can arise with homing radio, which gives an instantaneous indication of divergence from course, and shows whether the divergence is to port or to starboard.
There are four items in a homing radio installation: the loop aerial, the trailing or fixed aerial, the aerial switch and the receiver. The receiver and the trailing or fixed aerial may be the ones used for normal radio reception of messages or weather forecasts. In the following explanation it is assumed that a trailing aerial is used, but a fixed one would serve equally well.
The loop aerial is permanently fixed at right angles to the aeroplane’s fore-and-aft axis when homing is the only type of directional radio to be used. When the loop aerial is used for ordinary direction-finding as well and is rotatable, it is set at right angles to the fore-and-aft axis before homing is begun.
The trailing aerial, because of its non-directional properties, will continuously pick up signals from the station on which homing is being conducted. If the pickup of the loop aerial is added to that of the trailing aerial, the strength of reception will increase more and more as the aeroplane gets farther off its course. When the aeroplane is on its course the loop aerial will be at right angles to the direction of the transmitting station, and therefore at its zero reception position, and there will be no change in signal strength when the two aerials are connected to the receiver together.
Effects of Deviation
It is also possible, by connecting the loop aerial to the receiver in the opposite “sense”, to make its pick-up oppose that of the trailing aerial and so weaken signals more and more as the machine gets farther off its course. For the purpose of finding out whether the machine is off course to port or starboard, three arrangements of the aerials are required: the loop aerial alone, the trailing aerial plus the loop aerial, and the trailing aerial plus the loop aerial connected in the opposite sense. These three positions must be rapidly available, and this is achieved by the aerial switch, which has three positions, each giving one of the three aerial arrangements.
The accompanying diagram on this page illustrates what happens in various circumstances when the switch is moved through its three positions. In the top line are indicated the effects obtained when the aeroplane is properly on its course. With the switch central and the loop aerial only in use, at zero position in relation to the station, nothing is heard. When the switch is to the left, the strength of signals is governed by the trailing aerial, because the frame, which is now also connected up, has no effect. Similarly, with the switch in the opposite position and the loop aerial connected in the opposite sense, signals remain at just the same strength.
HOMING RADIO: how the position of an aircraft affects reception.
If the aeroplane deviates off its course to the right, as in the second line of the diagram, the effects will be different. The loop aerial is partly turned towards the station and signals will therefore be heard with the switch in the central position. Of the two outside positions the left will be the louder. As in one position the loop’s pick-up is added and in the other subtracted, the overall difference between the left and right positions of the switch is equal to double the strength of the loop aerial’s pick-up.
In the third line of the diagram it is supposed that the aeroplane has deviated off its course to the left. The indication with the switch central will be the same as when the deviation was to the right, assuming the degree of deviation to be equal in the two instances. But when the switch is turned to the left the effect is opposite to what it was when the machine deviated to the right.
It might be thought that, with the switch to the left, the effect would be the same in the pilot’s earphones whether deviation was to the left or right, because the loop aerial and trailing aerial are connected in the same sense in both instances. If, however, the angle of the loop aerial in relation to the transmitting station is studied in each sketch showing deviation, it will be seen that when deviation is to the left signals are coming from the right-hand side of the machine, and when deviation is to the right the signals arrive from the left-hand side of the machine. Thus they arrive at opposite sides of the loop aerial; this induces the currents in the aerial winding in opposite senses. The same effect is achieved, therefore, as when the switch is moved from left to right. It is this feature which enables the pilot to ascertain when necessary whether the transmitting station is ahead of or behind him. On the homing apparatus there is a control which enables the operator to cut down the strength of reception from the trailing aerial without affecting that from the frame. The object of this is to enable the strength of reception from the trailing aerial to be made approximately equal to that from the frame, so that the pilot may easily note differences between strengths when the switch is left and when it is right. If the trailing aerial strength were, say, ten times as great as that from the loop aerial, then it would be difficult to notice any difference between adding and subtracting the loop aerial’s strength.
The pilot wishing to home on a station first tunes in that station with the switch either to the left or right. He then swatches backwards and forwards from left to right and notes whether there is any difference in strength. Should signals prove louder with the switch to the left, he simply turns the machine a little to the left until a balance is obtained. A corresponding movement to the right is made if the converse holds good. He then obtains a final check on his course by putting the switch central, when the absence of reception will prove he is heading in the right direction. All that he needs to do then is to make an occasional check to verify that he is holding his course properly. Should he hear signals with the switch central and not be able to tell any difference between left and right positions, then he cuts down volume from the trailing aerial.
Should the pilot suspect that he is flying away from the station instead of towards it, all he has to do is to turn left off his course and move the switch from left to right. Signals should be stronger to the right. If they are stronger to the left the station is behind the aeroplane.
EFFECT OF SIDE WIND on an aeroplane homing on a radio station.
Fig. A shows the path the wind would normally cause the aeroplane to follow;
Fig. B illustrates how offsetting the loop aerial could make allowance for the drift caused by the wind.
In the Marconi apparatus a visual indicator may be added to the homing radio, and then, visual, aural or both systems of course checking may be used. The dial-faces on the diagram above indicate the positions which the needle of the visual indicator would take up in the three instances reviewed.
The visual indicator consists of a meter on which the needle remains central while the machine is on its course. As soon as the machine deviates to left or to right, this fact is made visible by the needle going to one side or the other and remaining there until the necessary correction in the machine’s course has been made.
Operation of the visual indicator is achieved by means of a commutator switch worked by an electric motor. This switch rapidly reverses the sense of the connexions between loop aerial and trailing aerial in the same way as the manually operated switch. After amplification and suitable treatment, the signals pass through another section of the commutator switch, which rapidly switches them from one side of the meter to the other. The meter needle cannot respond to the rapid changes of the commutator switch; it remains central when the machine is on its course and the pull due to the signals is equal from either side. Should the machine deviate, the pull becomes greater on one side of the needle, which moves over to indicate the deviation.
When the visual method of homing is used, the machine is always first set on the right course by means of the aural indications. If there is no wind blowing, or if the wind is entirely a head or tail wind, then a homing aircraft will fly in a straight line to the transmitting station. Should the wind be coming from the side, however, it will cause the machine to follow a path similar to that in Fig. A above.
Allowance for Drift
The reason for this is apparent. The machine is pointing towards the station the whole time, but the side wind is continually carrying it out of its path, so that the pilot continually has to take up a new direction to fly straight towards his object.
When the direction and strength of the wind are known to the pilot, he can allow for drift if the machine has a rotatable loop aerial. The effect is shown in Fig. B above. Although the machine is pointing away from the desired direction, the strength of the wind keeps it back so that its path is along the vertical line. If the loop aerial is turned clockwise from its original position, shown dotted, it is brought again at right angles to the direction of the station and normal homing can be used. A fixed frame scheme has been devised to allow for drift by means of a variable resistance which unbalances reception, so that the pilot will automatically set his machine with the loop aerial at an angle to the direction of the station on which he is homing. As with most systems of directional radio, homing radio suffers from night effect; this is a phenomenon which, in the absence of sunlight, may at times make bearings inaccurate or difficult to obtain, though at other times bearings are normal.
Should a pilot take an ordinary type of bearing when night effect is present, he might be badly misled as to the correct course to follow. With homing radio, as the course of the machine is being continually checked, it is unlikely that night effect will cause more than a temporary deviation from the desired course.
In the Standard Telephones and Cables homing radio gear the loop aerial is carried inside a streamlined casing to reduce its wind resistance. Moreover, in this make of apparatus a device termed a “Crash Limiter” is incorporated. This is a device for reducing the noises in the earphones due to atmospheric electricity, and its object is to make operation more comfortable for the pilot.
LIGHT-WEIGHT HOMING RADIO RECEIVER having a small-diameter loop aerial. This is enclosed in the streamlined casing shown to the right. This receiver is particularly suitable for light aircraft because of its compactness and weight of only about 10 lb.