One of the most heralded radio advances during World War One was the development of compact and rugged vacuum-tube transmitters, which allowed airplane pilots to communicate with persons on the ground. During the war, the Glenn L. Martin Aviation Company of Cleveland, Ohio was one of the companies which produced radio transmitters for military aviation.

At the time of the broadcast reported in this article, the ban on commercial and civilian operation of radio transmitters was still in effect, so the Martin company presumably operated its station under a wartime military authorization. (The ban on civilian radio listening had ended just two days before this broadcast took place.) The station's 375 meter wavelength corresponds to a transmitting frequency of 800 kilohertz. Although the weekly entertainment broadcasts soon ended, the April, 1920 issue of QST would later note that two Glenn L. Martin stations in Cleveland, still using their "air station" calls of UM and GMC, were on the air with radiophone transmissions. (A letter in the July, 1920 issue of this magazine listed the callsign of the Glenn Martin Aircraft station as GMA).

 
Cleveland Plain Dealer, April 17, 1919, page 1:

HEAR  CARUSO  SING  BY  WIRELESS  THURSDAY!
 


Amateurs  With  Radio  Instruments  May  Listen  to  "Concert"  Given  Through  New  Invention.

    Cleveland--or that portion equipped with wireless receiving instruments--will hear its first wireless concert tonight between 8 and 10 o'clock. And it's to become a permanent institution.
    Every Thursday night between these hours the radio amateur, or professional, who "tunes" his instruments to receive wave lengths of 375 meters will be able to listen to Caruso's voice and strains from Sousa's band, interspersed with baseball scores and the latest news bulletins. It's the first application in Cleveland of a wonderful war invention, the wireless telephone.
    The concert will emanate from the plant of the Glenn L. Martin Co., St. Clair avenue and London road N. E., and will be audible to wireless operators within 200 miles of Cleveland. It is estimated that fully 400 radio amateurs will "listen in" on the first concert night and that within a few months 2,000 amateurs within thirty miles of Cleveland will compose the audience for the concerts.
    This is because the government ban on the use of radio apparatus by amateurs has been lifted.
    Hundreds of amateurs are fitting up stations here now, J. S. Newman of the Newman-Stern Co., E. 6th street said yesterday. It will require no expensive apparatus to hear the wireless concerts, Mr. Newman said.
    The wireless transmission of music is accomplished by use of the wireless telephone, as developed by F. S. McCullough, wireless expert for the Martin Company. The same apparatus as is used in airplanes will be employed and the music will be produced by a phonograph.
    The transmitting apparatus of the telephone will replace the phonograph horn. The baseball news and bulletins will be announced by voice, so that no knowledge of "codes" will be required to receive them.