The Hazards of the Higher Frequencies

Radio Record, Volume V, Issue 11, 25 September 1931, Page 31

The Hazards of the Higher Frequencies

GEVERAL months.ago the explosion of a gun turret on one of the U.S. Navy’s warships was stated by experts to have been "caused by heat generated by high-frequency waves from the ship's short-wave transmitter." On the very short waves these induced currents take on a much more serious aspect, so serious, in fact, as to amount to a real menace to. the safety of equipment and even of the ship itself and its crew. That, at any rate, is the view held by Admiral Hughes of the U.S. Navy, who has recently issued a preliminary summary of what he officially terms. the "hazards of high frequency," and promulgated some very definite orders restricting the use of high frequency (shortwave) wireless. According to these orders : Minimum of 75 Metres.

"No radio transmitter on board sbip shall be operated on frequencies above 4000 ke. (75 metres) when the following conditions hold: "1. During target’ practice’ when powder is out of the magazines, "2 When fuelling ship with fuel oi) or gasoline. "3. When wing-tip flares are installed on planes and the flares are near objects which might be seriously damaged or cause serious damage as a result of the flares igniting. Wing-tip flares should only be installed on planes when neces: sary. In addition, the order ° forbids the operation of transmitters on frequencies wbove 4000 ke. while aireraft or boats are being fuelled on deck. In his report on the subject the Ad wiral says: "Investigations: have been conducted in the fleets and, under the direction of the Bureau of Engineering and Ordnance, at the Naval Radio Laboratory, to determine what. if any, extra fire or explosive hazards may be introduced by the use on shipboard of high frequency Tadic transmitters, "Tt has been known for years that eurrents of considerable magnitude are induced in conductors in the vicinity of an energised antenna whenever their electrical length is such as to approach resonance with the antenna, and that potentials are induced in large metallic bodies, insulated from the ground. when in.a strong field under an antenna.

"An example or the first kind is the current in standing rigging not broken up by insulators; an, example of the second is the spark which can be drawn from an automobile parked under a large energised antenna. . A gun, if loaded, might be prematurely by either (1) heat generated by eddy currents, or (2) currents induced i fire-control wires. Tt should perhaps explained here that modern naval guns are fired by remote electrical contro], an electric spark touching-off the firing charge.

Powder exposed in open containers might be fired by a chance spark between two metal objects, one unearthed and in resonance with the transmitting aerial, and the other earthed. Possibility of Sparks. Similarly, resonance between the transmitter and an armoured petrol-filling, hose might result in a spark occurring between the metal nozzle and the filling cap of the petrol tank; and, again due to’ resonance, currents might be induced in the wiring to wing-tip flares which might be of sufficient magnitude to ignite the flares. perhaps causing a serious fire or explosion on the deck of a battleship or aireraft carrier. Old-time wireless men, and especially those who have had sea experience, will by this time be thinking: "Why don’t they earth everything properly and stop making a song about it?’ Very excellent-on the longer waves. But this example will serve to illustrate the difficulty on short-waves. During the course of investigations which were made on the U.S. battleship Texas, it was found that on turret No. 3 the muzzles of the guns became quite hot when a nearby aerial radiated only 1 kw. at 16,000 ke. (19 metres). The guns were one full wavelength in length, and the distance from the trunnions to the muzzles was three-quarters of a wave-length. It is conceivable that at considerably higher powers, or higher frequencies, the breeches themselves might get hot enough to prematurely fire the guns. :

Easily Measurable Voltages. At any other frequency the trunnions would have effectively earthed the guns. When the muzzles of the guns are exposed to high-frequency radiations, as they must be, the entire gun commenees to oscillate electrically. with the result that low though easily measurable voltages are produced at the breech, even though this end is shielded from the electric field by the turret. Actual experiments showed that the energy, under certain conditions. was sufficient to light an 18 to 24 volt lamp connected between the breech and the turret. explode a primer in the breech of the turret gun by using the energy fed into the turret by a special tuned circuit. light aeroplane running lights when the plane was exposed to the field of the aerial. and set off the primers of wingtip flares. Evidently. therefore, seme very real hazards exist, and restrictions are obviously uecessary.