Radio Amateur News, September, 1919, pages 104, 141:

Guarding  the  Ether  During  the  War
FEW persons realize that from April 3, 1917, until November 11, 1918, Naval radio operators, especially trained for this work were constantly on watch at listening-in stations erected in various parts of the United States for the purpose of carefully listening to all radio activity, and all signals which could not be readily identified by Naval communication experts as the official transmission of Allied radio stations were immediately investigated and traced to their source of origin. In fact a regular department was established which worked in cooperation with all United States Naval radio stations in the country. Operators at the listening-in stations soon became experienced in recognizing allied from enemy radio transmission. In order to effectively censor all signals, these listening-in stations covered a receiving range of as much as 6,000 miles and a tuning range of from 50 to 50,000 meters, several operators being required to cover various sections of these wave lengths simultaneously. Fig. 1
    That new and valuable aid to navigation--the Radio Compass shown in figure 1--played an important part in this work of investigating illegal radio activity; in fact, it was owing to the timely necessity of having a reliable means of ascertaining the direction of strange signals that the Navy Department developed the radio compass to its present stage of efficiency. The manner of operation was simple. As soon as a listening-in operator became suspicious of a certain signal or spark, he immediately communicated the fact to the radio compass operators stationed at various points of the district in such locations as to form a triangle, and they in turn secured the approximate location of the suspicious signal by plotting a point at the line of intersection resulting from the three different directions secured by each of the three radio compass operators forming the triangle.
    After having ascertained the approximated position of a strange or suspicious signal, Naval investigators would immediately reach the spot in fast automobiles. It must be understood, however, that very few radio compass directions are ever secured so accurate as to give the exact spot from which signals emanate, but it does give the location within a mile. It is then a comparatively easy matter to search the particular neighborhood given by the direction finder and investigate any suspicious-looking overhead wires which might be employed for transmission.
    It is surprising to what lengths young men of a playful turn of mind went in using substitute antenna; some of these were unused telephone wires, clothes-lines where the rope had been substituted with flexible stranded wire, insulated iron fence, chicken netting, etc. In this connection it may be said that after the executive order had been sent out by the authorities instructing all amateurs and private owners to dismantle their radio apparatus and store them away, many, being of a perverse turn of mind, attempted to see for themselves just how far the Government officials were prepared to locate those who had failed to comply with instructions by persisting in sending to each other, using prearranged call letters not known to anyone else, or, as was detected in one case, simply sending out unintelligible signals. Much to their surprise, however, they were soon located by the investigators, who, in the case of New York State, would be on the spot in a very short time, and having discovered them would confiscate their complete installations as well as give them a good scare, inferring what might befall them and their friends were they ever heard from again by radio until after the cessation of hostilities. While some of these boys did not exactly act as all law-abiding citizens should, their number was very small indeed as compared to the large body of amateurs in the country.
    Under these circumstances, and by giving wide publicity in the press to the cases detected, it was comparatively a short time before all radio stations not under the direct supervision of the Army or Navy had been effectively dismantled and closed for the duration of the war. Running down receiving stations, however, was not so easily accomplished since there was little external evidence of anyone doing this, the only possible clues being in the accidental discovery of secret antenna.
    For a considerable period of time trained observers, many of them former amateurs who were familiar with the various tricks anyone might resort to in order to keep their receiving station open and thereby copy all important cipher and plain language communications between allied stations, were constantly traveling about the country on the lookout for any indication of secret receiving or sending apparatus. In this way, many innocent-looking telephone and telegraph wires were often found to lead to elaborate and very business-like receiving outfits.
    One interesting case in particular was that of two young electrical engineers living in an apartment house in the Bronx section of New York City, and who had cleverly installed an elaborate system of receiving antenna. These enterprising young men had erected a series of very fine enamel wires which circled two high apartment houses in such a manner as to be practically invisible. This was accomplished by installing the wire on small insulated tacks driven into the brick wall at the top of the roof and under the terra cotta cornices, making it impossible for anyone to detect it either outside or inside the roof. Fig. 2
    Eventually the wire was accidentally discovered by an electrician who was repairing the electric bell system of the house. In leaning over the terra cotta railing of the roof, his hand unconsciously wandered under the railing where he had felt the wire, and upon investigating he discovered that it led to a window on the third floor. It happened that this electrician had a slight knowledge of radio telegraphy, and suspecting his find to be a secret radio installation he immediately reported the fact to the authorities. When the investigators walked into the apartment of the two young men they found a complete long-distance undamped receiving set employing three steps of application installed on a table and along with it an accurate log of the signals transmitted by European stations, including Eiffel Tower, France; Nauen, Germany, and Rome, Italy. Of course, these young men had no intention of making unlawful use of the information they were securing in this fashion, and merely were using the set for experimental purposes, at the same time probably exulting over the fact that they were "putting one over" on the authorities supervising radio activity. Had they not been able to furnish indisputable evidence of their citizenship, loyalty and character, things might have gone different with them, and they might have reflected over their lack of foresight while safely interned in a Southern internment camp.
    It must be said in fairness to the amateurs in general, that incidents of willful and deliberate attempts to engage in illegal radio activities were small compared to that large body of men. As a matter of fact, the greater part of these young men immediately enlisted at the entry of the United States in the war, in either the Army or the Navy, to serve their country in the best way known to them, making use of the practice and experience gained while amateur radio operators.
    The radio activities of German submarines which happened to be in the vicinity of the Atlantic Coast did not escape the attention of the authorities, and much valuable information was secured by actually intercepting and often discovering the system of communication the enemy submarines had devised in order to communicate with one another without detection.
    One outstanding trick of the German submarine radio operators was to use low wave lengths alternating between 60 and 75 meters. Until this was discovered there was, of course, little chance of nearby vessels intercepting their messages, since the lowest wave lengths used by commercial vessels is about 500 meters. The submarines were usually comparatively near to one another, and therefore could use such low waves with a very small amount of power; in fact, they often used high frequency buzzers similar to those used for testing detectors. German high-power stations, such as Nauen (POZ) and Berlin (LP), knowing quite well that Allied Radio listening-in stations were constantly on watch copying all of their transmission, would camouflage their important transmission by first sending out press items of the propaganda type on their regular and known wave lengths and simultaneously send their cipher dispatches on the same waves from a nearby antenna, their receiving operators depending on a system of critical weeding out in order that the press items cause no interference. Another method was for the nearby station to drop down or step up to various prearranged waves and there send dispatches, probably inferring that allied operators in their desire to copy all items of the interesting (?) press would fail to cover other waves. Whatever their motives for these tactics, and if their intent was to decoy listening-in stations, these were crude methods indeed.