Principle of Operation

The radio compass is an imti ument for observing by means of radio, the direction of a station emitting radio signals; briefly, in aerial and marine navigation, it is an instrument for taking radio bearings.

Operating Principles.—Since the radio compass is built around the loop antenna it is necessary to understand its principles of operation before undertaking a study of the other units.

The radio compass utilizes the directional receiving properties of the coil, which consists essentially of several turns of wire wound into a large coil. This coil will receive the radio signals with maximum intensity when the plane in which the coil is wound is in the line of the direction to the transmitting station. See fig. 1.

As may be observed from the diagram, the minimum is well defined, and the maximum is not, hence it is evident that the strength of the signal varies rapidly with the movement of the coil near minimum, but very slowly with the movement near the maximum.

It is for this reason that the minimum is utilized in observing • bearings. If it were not for this, there would be great advantages in taking bearings on the maximum, on account of greater audibility and thus diminishing the interference effect.

The coil or loop antenna part of the compass operates on the principle that the amount of electronotive force induced in a vertical loop of wire by an arriving electromotive wave, depends on the angle between the plan of the loop and the wave front.

MAXIMUM

COIL

MAXIMUM

FIG. 1—Illustrating directional effect of radio waves on the loop-antenna part of the radio compass.

In a rotatable coil of practical size the voltage induced by radio signal is very small. Hence for use of such small coil for radio compass purposes, it is necessary to generate a great amplification, and it was the utilization of the multistage vacuum tube amplifier which made the instrument usable for marine and aerial navigation.

As previously asserted, the radio compass utilizes the directional receiving properties of the coil type of the antenna. To further facilitate the understanding of the instrument, an analysis of the behavior from the point of view of the magnetic field is as follows:

It is a-well known fact that the radio waves emanate from the transmitting stations in all directions, very much the same as water waves on the surface of a quiet pond when a stone is dropped in. The length of a radio wave is the distance between successive crests of the wave; the number of crests passing a given point per second is the frequency, and the product of the wave length and frequency gives the velocity of the radio wave. The product of these two factors (wave length and frequency) is always about 300,000,000 meters (186,300 miles) per second. Radio waves are accompanied by a magnetic force which is horizontal and at right angles to the direction in which the waves are traveling.

As a radio wave passes a given point the magnetic force, or field strength, varies from moment to moment from a maximum in one direction through xero to a maximum in the other direction. At a given point the cycle from maximum in one direction back to maximum in the same direction is performed in a very small fractional part of a second. For a wave length of 300 meters this cycle is performed in one-millionth of a second.

If a coil of wire is held in a fixed position, such that the lines of magnetic force thread or pass through the coil and are not parallel to it, while the magnetic field varies in intensity, as is the case with that accompanying the radio wave, an electromotive force or voltage will be induced in the coil.

Since the magnetic force is horizontal and may be thought of as forming circles around the transmitting source, the compass coil when turned with its plane parallel to. the direction of the

FIG. 2—Typical Marine radio compass installation. (Courtesy Western Electric Co.)

FIG. 2—Typical Marine radio compass installation. (Courtesy Western Electric Co.)

COIL HOUSING EVQD

COIL HOUSING EVQD

BECK-

Conduit For Leads

UNI-DIRECTIONAt. ANTENNA APPROX FT UJN6

14 IN. HAND WHEEL

UNIVERSAL JOINTS

UNI-DIRECTIONAt. ANTENNA APPROX FT UJN6

CONDUIT FOR LEADS FROM COLLECTOR BRU5HES TO RECEIVER

BECK-

14 IN. HAND WHEEL

AUTOMATIC — COMPENSATOR

HANDRAIL BROKEI AWAY IN FRONT

TO SHOW -SIGHT WIRES

SIGNAL LIGHT-

CONDUIT FOR LEADS FROM COLLECTOR BRU5HES TO RECEIVER

Radio Antena Switch Diagram

UNIVERSAL JOINTS

DIAGRAM SHOWING SPACE REQUMEP. FOR RECEIVER

ANTENNA SWITCH IN RADIO ROOM-16" FROM WALL TO TOP OF SWITCH

6R0UND CONNECTION

AUTOMATIC — COMPENSATOR

SIGNAL LIGHT-

ANTENNA SWITCH IN RADIO ROOM-16" FROM WALL TO TOP OF SWITCH

6R0UND CONNECTION

FIG. 3—Kolster radio compass installation.

transmitting station is threaded by the maximum number of magnetic lines of force, and the signal heard in the telephone receivers is a maximum. When the plane of the coil is turned at right angles to the direction of the transmitting station, no magnetic lines of force thread through the coil, and therefore no voltage and no current are induced in the coil and no signal will be heard in the telephone receivers.

The Marine Radio Compass.—A radio compass frequently found on board ships of U. S. registry was developed by F. A. Kolster and is manufactured by the Federal Telegraph Co., named variously The Kolster Radio Direction Finder or The Kolster Radio Compass.

Other radio Direction Finders have been developed by the Radio-Marine Corporation of America, the Marconi Wireless Telegraph Co., etc.

By means of this device, radio bearings can be taken in dense fog, snow storms and over distances greatly beyond the horizon with an accuracy equal to that obtained with visible sight, thus eliminating one of the greatest hazards to marine navigation.

The modern method of obtaining radio compass bearings on shipboard requires the installation of radio beacons on light vessels and light houses in the vicinity of harbor entrances and other places dangerous to navigation, the exact locations of which are shown on all sailing charts.

Operation of Unit.—With reference to the electrical circuit fig. 4, the theory and operation of the unit as developed by the U. S. Bureau of Standards is as follows:

The variable condenser CV together with coil Li form the main receiving circuit which is tuned to the signaling wave length.

Connected across the condenser Ci either directly or through the potential transformer P, is the vacuum tube amplifying and detecting apparatus.

Radio Finder Circuit
FIG. 4—Circuit of Kolster radio direction finder.

As developed by the U. S. Bureau of Standards this receiver consists of a three-stage radio-frequency amplifier, a detector and a two-stage audio frequency amplifier made up as a unit with a minimum number of operating adjustments. However, the receiver may be of any standard design.

The telephone receivers are located at a distance from the magnetic compass to avoid any effect upon the compass due to the magnets within the telephone receivers.

The auxiliary circuits of the direction finder are controlled by switch S. With the switch closed to the right, the middle plates of the double condenser C4 are directly grounded.

The double condenser is utilized to bring about electrical symmetry of the coil system with respect to the earth. In other words by adjusting the middle plates of the condenser C4, to the right or left the earth connection is brought to the electrical midpoint of the coil system, and the signal received in telephones, results only from the energy directly received in coil Li.

With the switch closed to the left a small condenser C3 is connected across half of the double condenser C4, and the inductance L2 and tuning condenser C2 are inserted in the ground lead.

Under these conditions the coil system is no longer electrically symmetrical with respect to earth, and received energy enters the coil circuit L1C1 indirectly through the tuned ground circuit, of which the capacity of the complete coil system to earth forms a part.

By proper adjustment, a complete uni-directional effect can be obtained. ,

In the practical operation of the direction finder, all tuning adjustments remain set for the wave length of the signaling station. Switch S is closed.to the right when observing the line of direction of a given signaling station and to the left when it is desired to determine the sense of direction.

In other words, to determine the line of direction of a station, the coil system which is directly grounded at its electrical midpoint by throwing switch S, to the right, is rotated to the position-of critical silence at which time the plane of the coil is normal to the direction of the approach of the signaling wave.

To determine the sense of direction of the station, switch S is closed to the left and the coil rotated to the position of maximum signal intensity at which time the plane of the coil is in direction of approach of the signaling wave and pointed toward the signaling station as indicated by an index pointer for that purpose.

Bellini Radio Compass.—Another type of radio compass known as the Bellini direction finder, is shown in fig. 5.

The frame consists of the loop L and condenser Ci and C2. The vertical antenna consists of the grounded center of the loop and the two condensers Ci and C2 which are grounded through the tuning condenser C3.

Operation.—To obtain a bearing the switch S is closed and the loop tuned to the incoming signals. The false vertical effect of the loop is practically eliminated by grounding the center point of the loop through the variable resistor R. The sharpening effect of the minimum due to the elimination of the remaining vertical effect is accomplished by varying condensers Ci and C2.

To obtain the sense of true direction, the switch is opened and the vertical antenna tuned by means of condenser C3, by varying the resistance R, it is possible to obtain the conventional heart-shaped diagram.

Dependable bearings can be obtained only by precise adjustments of condenser Ci and C2. The presence of tube damping

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