Page:Popular Science Monthly Volume 91.djvu/169

 Popular Science Monthly

��153

��How Germany's Secret Service Wireless Stations Are Being Weeded Out

THAT there are secret service wireless stations of Germany in and about our large cities and important harbors, there is not the slightest doubt. At the time the merchant submarine "Bremen" was to arrive from Germany, it will be remem- bered, German agents prepared moorings for her at New London, Connecticut. Neither the Government wireless stations nor our commercial stations received any communications from this submarine. Evi- dently, secret stations, whose messages we could not hear, must have kept in touch with her.

These stations, we can rest assured, have not dismantled, but are working now. They await only the opportunity to report the sailing of a Europe-bound ship to a sub- marine waiting offshore. Unquestionably, such a menace must be weeded out. Our Government has not been idle. According to reliable information, it has already located several of these stations. The problem is not an easy one, and is one entirely of wireless engineering.

William Dubilier, one of our most prominent experts, believes that the Ger- mans manage to keep their antennas con- cealed by stringing them inside high non- metallic structures; as, for instance, a hollow wooden flagpole. Though but one wire could be strung inside a flag- pole, what would be lost in antenna efficiency could be partly compensated for by the increased power of the station. Of course, these spies would not be compelled to use an aerial at all. They

���could use a close circuit system having two grounds. But this is not likely. The other method is more efficient and lends itself to better selectivity.

From the antenna concealed in the flag- pole, the spies could run the aerial lead directly through the roof and into the garret of the building, without exposing it. These instruments, we can take it for granted, are the best that German money can buy. The operators obtain their unusual selectivity probably by the use of a double heterodyne or ultra-ultra system. That is, they superimpose a number of oscillation circuits one upon the other between the exciting transformer and the aerial and ground connections. The various ways in which each of these many circuits can be tuned are almost unlimited. The waves such a system would send out could be efficiently received only by a receiving system of similarly complex configuration. The usual station which has not these superimposed circuits could not receive the signals distinctly.

These facts give some idea of the task

our wireless engineers are up against.

First, they must obtain a circuit which will

receive these secret signals — a most difficult

task which would involve the finding of

the exact number of the superimposed

circuits and the exact configurations!

Second, they must locate the stations with

some sort of direction-finder using this

type of circuit. The methods used with

these finders would then be similar to

that explained in a previous article on

the direction-finder, on page 232 of the

February 191 7 issue of the Popular

Science Monthly.

�� �.. '■ ■

� �1

�mk.

�'4a

�-

�. ■

��The Germans conceal their aerial in the hollow of the flag pole. Their complex waves can be received only by submarines and other stations fitted out likt themselves

�� �