U. S. NAVAL INSTITUTE, ANNAPOLIS, MD.

NOTES ON THE RADIO COMPASS

By ENSIGN BOWDEN WASHINGTON, U. S. N. R. F.

The radio compass (radio pelorus would perhaps be a better name, as it always gives relative bearing, except ashore) is a device for determining the direction of a distant radio transmitter, and, granting that this object is attained, would seem to have considerable value for both navigational and military purposes. The present type of U. S. Navy radio compasses, though a comparatively new device and one which can be subject to tremendous improvement, is, if properly operated, a thoroughly practical piece of apparatus. It has been the writer's experience that, on the ships equipped with the radio compass, generally very little use has been made of it, and, it is believed, that owing to several causes, matters not inherent to the apparatus itself, it has come rather into disfavor. The writer hopes in these few notes to point out what he believes these causes to be and their remedies. From having installed, tested and operated probably 50 radio compasses on battleships, cruisers, destroyers, chasers and ashore it is natural that one should develop considerable interest in the device, and its operation, when in competent hands, has led to the development of faith in its performance as well.

The fundamental principle of operation is exceedingly simple, as is the apparatus itself if looked upon in the proper light. It is known to any one familiar with elementary physics that if a coil of wire is passed edgewise between the poles of an electromagnet, a potential is set up in this coil, and if the circuit in which this coil is inserted is closed a current will flow. This is really the fundamental principle of the dynamo. On the other hand, if the coil is passed between the poles of the magnet in such a position that its plane is the same as that of the magnetic lines of force between the poles, the potential induced will be in opposite directions in opposite sides of the coil and no current will flow. It can be readily seen that the above is true

whether the magnet or the coil is moved; that is, passing the lines of force through the coil or the coil through the lines of force.

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The magnetic component of the electromagnetic wave spreads out from the transmitting station in concentric lines of force like ripples on the surface of a still sheet of water into which a stone has been dropped. If a coil of wire is placed with its axis horizontal and the plane of its winding in the direction of a distant transmitter it will cut" these lines of force as they pass through it, and a current will be induced in the coil. On the other hand, if the plane of the coil is at right angles to direction of the transmitter no current will result. If suitable detecting apparatus is added, as in any radio set, to make these high frequency currents audible, we have a radio compass.

In the navy radio compass a coil of a dozen or so turns is mounted upon a shaft, equipped with a handwheel and scale, in a weatherproof house above the operating room. The shaft is fitted with collector rings and brushes to lead the current to and from the coil, which is connected in series with the secondary of an ordinary radio receiver, the primary of which is not ordinarily connected to anything. As the ability to absorb energy of a coil of this sort is perhaps a thousandth part of that of a ship's antenna, an amplifier is provided which increases the strength of the signal 400 times, thus giving the device a practical range of operation. It is only necessary for the operator to tune for the proper wave-length, rotate the coil until he finds the points (two-180° apart) of minimum signal and read these points on his scale. This places the sending station on a line-our relative bearing may be either of the two readings. For those sufficiently interested, I will attempt to describe how it is possible to discover which of these readings is the desired one.

Provided, of course, that the distance to the transmitter, power of the transmitter, etc., remains the same, the strength of the signals varies, when the coil is rotated, as the sine of the angle its axis makes with the direction of the transmitter. (Hence the use of the minimum points, as the rate of change of signal strength is much greater, thus making for accuracy.) When this is plotted in polar coordinates a figure eight will result. An ordinary vertical antenna has no directional effect and its polar plot, angle of transmitter against strength of signals, is therefore a circle. If a vertical antenna is connected to the primary of the

radio compass receiver and the coupling is adjusted so that the proper amount of energy is transferred to the detector circuit, we will have a circle superimposed upon a figure eight. As the two loops of the figure eight occur with a reversal of the position of the coil, the current. of one may be regarded as plus, the other minus, with respect to the antenna circle. Thus one subtracts from the circle, the other adds, producing a "cardioid," or heartshaped curve, having one minimum which is read. This reading is not as accurate as when the coil alone is used, and is only to determine which of the two coil readings is the desired one.

It is necessary that a correction (or variation) curve be plotted at the most used wave-lengths. Variations appear to be largely due to two causes: The first, and most objectionable of these, is due to the fact that masts, wire rigging, etc., are set into oscillation by the distant transmitter and some of this energy will be reradiated, and on being picked up by the compass will give a reading which is the resultant of this energy and the energy from the transmitter. The second source of variation is the tendency of the wave to follow the ship's hull. This effect is pretty consistent, and necessitates the application of a + correction from bow to starboard beam, to astern, to port beam and from port beam to dead ahead. The variation is usually maximum at relative bearings 45°, 135°, 225° and 315°, and zero at 0°, 90°, 180° and 270°. These variations will sometimes be as much as 30°, particularly on the shorter waves, but are consistent within 1° or 2°. The accuracy of the device depends largely on the strength of the signals; i. e., the distance and power of the transmitter. In general, it may be stated that a cut should be obtainable on a battleship or shore station at a distance of 100 miles within ±5°. The inherent inaccuracy of readings of the radio compass, due to the fact that absolute minima are not obtainable with the present U. S. Navy type, is probably ±2°. This latter varies considerably with local conditions. Re-radiation from stays, etc., is always somewhat out of phase with the direct radiation when it arrives at the compass coil, and this has a tendency to dull the minima.

The older ships with military masts and considerable wire rigging are the hardest on which to obtain results. The Montana's compass was found to be absolutely useless at a wavelength of 300 meters, due probably to a backstay from the main truck to the taffrail, but was fair on 600 meters (could be read

within 3° or 4°). A great deal of this sort of thing could be eliminated by inserting insulators in all wire rigging over, say, 50 feet in length.

Little or no trouble of this sort is experienced on destroyers, the rigging being comparatively short, and cage masts seem to be less objectionable than would at first be supposed. An idea of the degree of accuracy to be obtained on destroyers may be obtained from the following:

The writer, aboard one of the new boats, when about 10 miles off Provincetown, took bearings on the Boston Radio Station and on New York Navy Yard. The former was within 1°, the error of the latter was found to be 4°.

Not long ago, the writer went aboard 17 destroyers lately returned from European waters, all equipped with radio compasses, and interviewed the commanding officers, radio officers, and electricians in charge. Most of these ships had had their apparatus installed on the other side. It was learned that only three had made any real use of the radio compass. These three ships will be referred to again as an example of what can be done. There appeared to be several reasons for this poor showing. In a great many cases the apparatus was never tried until it was actually needed. The men had therefore no practice in its use, and it failed. Also, on these same ships it had not been used for long intervals and therefore was in poor condition when needed. (The radio compass requires no more, no less, care or intelligence than the ordinary receiving set.)

On one ship the compass room had been used as a potato locker. In general, the operators had had little or no instruction in its theory or use, and seemed to believe it to be complicated and difficult to understand; when, as a matter of fact, any secondclass radio electrician who deserves his rating and is therefore fairly familiar with ordinary radio receivers should be able to master both theory and practice in perhaps two hours.

Another thing which made against the efficient working of the device was the addition by the various radio electricians of other apparatus in an effort to copy press," etc.

It was evident also in some cases that the chief electrician (radio) had belittled its value in an effort to get out of an extra watch and an extra station to look after.

It is natural that no confidence should be felt in the apparatus until it has been demonstrated to work reliably.

It is believed, however, that if the electrician on watch is required to take several bearings during each watch on stations of known position and report these bearings to the bridge to be checked it will not only afford excellent practice for the operator, but will eventually demonstrate to the navigator the value of the apparatus. If good bearings are not obtained, and the apparatus has been properly installed in the first place, it can be attributed to extreme stupidity, or lack of trying, on the part of the operator. One instance can be mentioned as an indication of this fact. After completion of a shore radio compass station, two of the permanent crew arrived and were instructed for perhaps half an hour by the writer. Upon returning about three hours later they were found to have taken probably 30 cuts on land stations along the coast, most of which checked up within 3° or 4°, the worst error being in the neighborhood of 8°.

On a coast station at Gloucester, Mass., cuts have been repeatedly made on stations as far as Cape Race and Halifax, Bermuda and Key West which were consistently accurate within 6° or 8°, and stations within 100 to 150 miles would repeat to a degree.

It may not be out of place to mention now a few of the uses to which the radio compass was put by the ships that really used it. Two enemy submarines were located. In both cases the destroyers were with convoy, and could not attack, but the fixes. obtained were afterwards verified by the British. One destroyer made port three times in fog by means of her radio compass. A torpedoed transport was found at a distance of 27 miles. Contact with other warships was established frequently. Convoys were picked up when off their rendezvous. One ship, as a check, took cuts on land stations when 150 miles out from Brest. These came within three miles of their noon sight. Too much reduction in speed from the original engine calibration was made when allowing for listening-gear drag, and when returning from abroad Nantucket Shoals Lightship was found to be abreast instead of 65 miles ahead as expected from dead reckoning.

It must be remembered that all these ships had identical apparatus, were in many cases identical ships, and had average enlisted radiomen. The only outstanding difference between the three ships whose performances are recorded above and the remaining 14 was that in each of these three ships one officer, at