When studying the requirements of marine radiotelephony, it is essential that consideration be given at the same time to both the economic and technical

1In the frequencies above 1,500 kc the following are those on which is based the exact per cent station band-width. From these points up and down the frequency scale to the next band, the percentage width is approximately per cent and the actual band-width in kilocycles is o per cent of the following frequencies: 2,000, 3,000, 4,000, 5,000, 6,000, 7000, 8,000, 9,000, 10,000, 12,000, 14,000, 16,000, 18,000, 24,000.

effect such a service will have on marine telegraphy and on air services in general.

At the present time, neither the actual needs of radiotelegraph in the marine service nor the actual requirements for high-frequency radio in aircraft are sufficiently known to enable a determination of the probable expansion of such services. Furthermore, there has been practically no experience with marine telephony for general public service.

Maritime radiotelephony may be expected to come into use to a limited extent in the immediate future, with prospect of expansion within the next three years, provided suitable provision of station-bands can be made without undue sacrifice of station-bands necessary for mobile telegraphy. It is, therefore, desirable that, based upon the experience gained with the first marine telephony circuits, a study be made of the possibilities of arranging to provide space in the mobile service bands for the expansion of this service, through coordination with mobile telegraphy and aircraft services.

Meanwhile, it is possible to formulate a tentative technical plan under which such assignments as are made to marine telephony will be so related as between themselves and to other mobile services as to result in a minimum of interference and of readjustment of frequencies as new stations come into operation. Any technical plan must take into account the following factors:

a. The frequency relations which are required between the one-way telephone channels themselves, to permit of two-way telephone operation.

b. Relation between the telephone and telegraph frequencies required for simultaneous telephone and telegraph operation from the same vessel and to generally minimize interference between telegraph and telephone. c. The carrying out of these relations within the various frequency-bands which are required for covering the wide range of distances involved in marine service.

2. Relation between the Radiotelephone Frequencies

a. If marine telephony is to be used frequencies must be available for each pair of communicating stations in each of a series of bands to permit of communication over the minimum and maximum range of distance.

b. Each telephone communication will require two channels within a given frequency-band; one for transmission from the shore and one for transmission from the ship. Two-way operation on a single channel would be possible only by the use of some switching arrangement. It is not feasible to rely upon push buttons for this purpose when connections are to be established for the public to the land-wire telephone system. Voice-operated switching devices may be expected in time to be extensively used in this service, but it would not be safe at the present time to adopt an allocation plan which would be dependent for its success upon the use of such devices upon all vessels.

c. A frequency separation of at least 2 per cent (about 20 standard stationbands) should be maintained between the frequencies used for transmitting and receiving on a given ship.

d. From an operating standpoint, it is desirable that each shore station be assigned frequencies in pairs, one for transmitting and one for receiving. Ship stations would be prepared to make contact with the shore stations on these frequencies. If specific frequencies were assigned to ship stations, it would be necessary for the shore stations to make frequent changes in frequency adjustments to connect with successive ships, with a corresponding loss of circuit time.

3. Relation between Radiotelephone and Radiotelegraph

a. In order to provide for simultaneous telephone and telegraph communication on a given ship, using frequencies in the same general frequency range, it is necessary to arrange the frequency assignments to provide for the following three relationships:

(1) A frequency separation of approximately 0.2 per cent, increasing to 0.5 per cent at the lower frequencies, between the telephone and telegraph signals received upon the ship, in order that the radio-receivers on the ship may discriminate between them.

(2) A frequency separation of at least 2 per cent between a ship transmitting frequency (either telephone or telegraph) and a frequency simultaneously received upon the same ship (either telephone or telegraph). (3) The frequency separation to be maintained between the telephone and telegraph transmitting frequencies on the ship, in order to avoid crossmodulation or interaction between them, depends upon the physical separation between the two transmitting antennas and the design of the transmitters. A frequency separation of 1 or 2 per cent may be necessary in the present art.

b. In view of the comparative ease with which a receiving set on the ship can discriminate between the two frequencies (telephone and telegraph) transmitted from the shore, the problem is facilitated by keeping the two shore transmitting frequencies close to one another. This and other considerations lead to the practical elimination of all the various possible arrangements of the four one-way channels involved, except the following two.

c. One general arrangement is to dispose the channels in a given frequency band in four blocks in the following order:

(1) Ship telegraph transmitting.

(2) Ship telegraph receiving.
(3) Ship telephone receiving.

(4) Ship telephone transmitting.

This disposition has the advantage of separating the telephone and telegraph frequencies, making the individual assignments for either independent of the other. The proximity of the two receiving frequencies is not objectionable. The proximity of the telegraph transmitting and receiving frequencies is not objectionable, except in the case of duplex operation. This may be provided for in making the specific assignments, or by making all shore station assignments in pairs (transmitting and receiving) as in the case of the telephone assignments.

This leaves as the outstanding requirement of this scheme that care be taken to insure that the frequency separation necessary for two-way talking be maintained between the sending and receiving channels which comprise a twoway circuit.

d. An alternative arrangement of the channels is the following order:

(1) Ship telephone transmitting.
(2) Ship telegraph transmitting.
(3) Ship telegraph receiving.

(4) Ship telephone receiving.

Under this arrangement the shore telephone and telegraph transmitting frequencies are grouped together in one frequency range and the ship telephone and telegraph transmitting frequencies in another range. This may have some advantage where the same apparatus is to be used for both services upon either a switch-over basis or, as may later develop, upon a superposition basis. How

ever, for the case of large vessels, where simultaneous telegraph and telephone service is to be given in the immediate future by means of separate sets, this arrangement has its limitations because of the necessity of maintaining a certain frequency separation between the two transmitters on shipboard for the purpose of preventing cross-modulation. This may considerably limit the extent to which one transmitter on shipboard may be shifted in frequency to meet a given shore station, without requiring a corresponding shift in frequency on the part of the transmitter engaged in the other service.

This shifting could be avoided by the assignment of a specific telegraph and a specific telephone transmitting frequency to the ship. This would bring about a situation where several ships, all within communication range of a given group of shore stations, would be those to which the same transmittingfrequency assignment had been made. Accordingly, these ships would have to wait in turn in order to establish communication with one shore station, while the receiving frequencies associated with the other shore stations would remain idle.

e. The above considerations lead to the conclusion that the system of allocation first discussed above is preferable in respect to providing greater independence between the frequencies employed for telephone and for telegraph transmission on board ship. The second system of allocation possesses the important advantage, however, of enabling shore stations to be assigned, in general, in the shared bands and mobile stations in the bands designated for mobile service exclusively, and, in general, would appear to cause a minimum change in existing assignments. From the standpoint of international assignments, therefore, it is considered to be the preferred arrangement for use in the more or less immediate future.

4. Frequencies for Covering the Distance Ranges

Frequency-bands, which are open for mobile services under the Washington Regulations, occur at intervals throughout the frequency spectrum, such that the necessary distance ranges can be covered. Some of these bands are designated for mobile radio service exclusively; others are shared by mobile and fixed service.

5. Fitting the Telephone Frequencies into the Mobile and Shared Bands

a. In order to avoid as far as possible changing existing mobile service assignments, it is recommended that the assignments to shore telephone stations be made at frequencies in the vicinity of the dividing line between the mobile and shared portions of the available bands, or in about that relative position in the bands which do not have such a division. The associated land receiving frequency (or ship transmitting frequency) would then be designated at a point at or near the lower frequency end of the corresponding mobile service band. b. It is recognized that there are existing assignments in the frequency ranges involved which should not be disturbed, and that the expansion of aircraft service and of marine radiotelegraph service should be adequately provided for.

c. It is recommended that pending a more complete study, such telephone assignments as are made for marine telephony be at approximately the following positions, being limited for the present to a total of 2 frequencies at each indicated position, the exact frequencies to be determined preferably by mutual