Radio Amateur News, August, 1919, pages 58-59:
THE AUTO RADIOPHONE
By A. H. GREBE
Expert Radio Constructor
THE present stage of radio telephone development has placed this form of communication on such a highly practicable plane that its rapid adoption for many useful purposes is only a matter of a very little time.
On aircraft it is now considered indispensable and because of the extreme simplicity of operation it will be used in many places on land over spaces which cannot be economically spanned by wires.
Having experimented considerably with vacuum tube radio telephones during the past few years, and being impressed by the adaptability of this type of radio communication to small antennae, the writer desired to make some tests with a radio telephone equipment installed in a motor car.
At first it was decided to use a flat loop as the radiating member, but this was abandoned in favor of a four-wire flat top antenna, used in conjunction with the frame and body of the car as a counterpiece. It was found best to depend on efficient radiation of the transmitted energy and sufficiently amplified incoming signals, than to sacrifice radiated energy in favor of the advantages of the loop for receiving.
The antenna system was constructed along portable lines, the supporting masts being fitted with socket joints for assembly and attachment to the car frame. When not in use the entire antenna system was slung under the running board on hooks provided for this purpose. The antenna wire was the same as used on aircraft and the non-kinking characteristics of braided wire made it more suitable than other kinds.
The transmitter consisted of a panel and cabinet assembly which included the vacuum tube mounting, chock coils, oscillating circuits and modulating system. Meters were provided for indicating the filament current, modulator and oscillator tube space currents and the radiated energy. The oscillating circuit was so arranged as to be controlled by means of a tickler coupling and the dial for indicating the position of this coil was provided on the panel. Filament current was obtained from a storage battery located back of the seat, and this battery supplied the current for operating a small dynamotor, as well as the vacuum tubes in the receiver. The microphone transmitter was mounted on a convenient handle and arranged with a plug which was inserted thru the front of the panel. Another plug and jack was provided for connecting a hand telegraph key for buzzer modulation. A switch control on the panel enabled the changing of wavelengths; it was found that a wavelength of 150 meters gave the best results.
The receiver consisted of a variometer type of regenerative receiver with two stages of audio frequency amplification and for this particular purpose was altered so that the antenna was directly cut in the grid circuit of the detector tube. This was necessary owing to the fact that the receiver was designed for use with the usual amateur antenna, and the wavelength obtained with the car was therefore below the requirement. The three stages of tubes were operated by a telephone plug, which controlled the filament and transformer circuits. Signals from ship stations and land stations within one hundred and fifty miles radius were copied without any difficulty.
An interesting feature of reception occurred when other motor cars were operating nearby. The discharges at the spark plugs were very plainly heard upon the approach of a car and continued until the latter had gone a considerable distance. This same feature caused considerable difficulty in receiving on the motor car radio station itself and was not entirely over come by shielding the ignition wires.
Owing to the fact that all the tests were conducted on a laboratory basis, it is not possible at this time to furnish data regarding the distances covered but it is probably sufficient to say that the tests have shown that the auto-radio-phone is entirely practical and the near future should bring extensive developments along these lines and we may soon hear an "SOS": "Send an emergency service car to car No. 999-999 three miles east of "Suburbanville."