Early in April, 1927, we decided to make a small practical radio receiver for use in an airplane. The idea was to make tests on short wave lengths to see just how badly the ignition would interfere without shielded ignition leads from the magneto to the spark plugs of the motor. We realized that such a receiver must be one of minimum weight, compact, and yet show a performance that would compare favorably with the average good table model receiver.
This receiver was finished May 23. It weighed less than nine pounds. The entire receiver was mounted in a shielding case of aluminum, which measured eight inches wide, seven and three-quarters of an inch high and four and three-quarters of an inch in depth. Filament batteries consisted of three number 2370 Burgess “C” (four and one half volts) batteries connected in parallel to heat the filaments of the CX-299 tubes, the current consumption of which is sixty milliamperes per tube, or one hundred and eighty milliamperes for the three of them, there being a detector and two audio stages of amplification. Plate supply batteries consisted of two number 4156 Burgess “B” (twenty-two and one-half volts) batteries connected in series. Batteries also were mounted in the same case, the receiver, proper, in the upper compartment and all batteries in the lower one.
Four interchangeable coils were made to cover the wave length range from about fifteen meters to ninety-two meters. These coils were wound on the bases of discarded CX-301A tubes, the four prongs being used for terminal contacts which made connections to the two secondary and regeneration control terminals brought up to the socket. The circuit is the standard regeneration control which has been the amateur’s stand-by since 1915. Any circuit that can stand up for thirteen years and still be the favorite of ninety-nine per cent of the radio amateurs must be a pretty good circuit. This is all the more impressive when consideration is given to the fact that the untiring efforts of some fifteen or twenty thousand radio amateurs for years and years have been given to develop something better. When Paul Godley gave the amateur world the idea of regeneration on wave lengths around 200 meters, little did he or anybody else realize that it would stand this most acid test of time and effort.
Of course, the airplane receiver was tested to see that it would function properly before it was installed in a plane. Mounted on a desk in the office of the radio laboratory and with the receiver cord of the line telephone for an antenna pick-up, signals were heard. Our log shows that we heard PQW, a station in Germany, transmitting to SPU on a wave length of about sixteen meters. That was the greatest distance over which signals were heard and this happened at twelve minutes to two o’clock C.S.T. on the afternoon of May 26. When further tests were made, dozens and dozens of short wave signals were heard.
Finally, June 13, we installed the receiver in the cockpit of the Waco airplane at Pennco Field in Madison, Wisconsin. The plane had a Curtis OX-5 motor. We had hopes of hearing signals from our own 500 watt short wave transmitter at 9EK, providing we did not fly more than a mile or two away from the transmitter. The plane was in the air fifteen minutes and even before the plane came to a stop after landing we knew that Phil Zurian had heard signals— the broad smile was the telltale. Signals were of maximum strength and in spite of our knowledge that the greatest distance the plane flew from the transmitter was not more than two miles, our enthusiasm jumped up several points. Station 9EK transmitted on a wave length of 38.4 meters.
The same receiver was again tested June 26, in a flight from Washington, D.C., to Pittsburgh, Pennsylvania, in an airplane of the Navy Department. Mr. C. W. Horne, of KDKA, Westinghouse Electric and Manufacturing Company, kindly cooperated by transmitting both voice and code on a wave length of about sixty-three meters during the flight. Not a peep was heard nor on the return flight next day, although attempts were made to hear signals on the forty and eight meter bands of wave lengths. This was rather discouraging in some measure, yet there was no interference from ignition, one of the biggest bugaboos in reception in a plane. The Army and Navy have done extensive experimental work in connection with shielding to prevent ignition interference.
In the course of time the little transmitter, weighing less than five pounds, exclusive of dry batteries, was completed and ground tested before it was taken into a plane. September 21 we installed the transmitter and receiver in the Air King, at Madison airport, and using a trailing wire antenna about seventy feet long established two-way communication with 9EK for the first time on seventy-nine meters. The airplane used our experimental call, 9XH. Communication was perfect for a distance of seventeen miles.
Don Mix, MacMillan’s first radio operator to accompany him to Etah, North Greenland, was next in line for an airplane test which took place September 30. This, too, was in the Air King with the Curtis OX-5 motor. After reaching a distance of about forty-five miles, during which time communication was maintained with 9EK, the Air King was running low on gas, necessitating an immediate return from Wales, Wisconsin, against a head wind. Signals from the plane were remarkably steady in frequency during the entire flight although there was some slight fading at times. The altitude during these flights was between one and two thousand feet.
October 4, we made further tests, especially to learn if higher altitudes would show different results. Accordingly, the plane took off and flew some thirty miles south, climbing all the time until an altitude of 6,400 feet was reached. Way up above the clouds where we could not hear the motor nor see the plane, we knew Phil Zurian was there because we maintained communication with him. It was on this day we first tasted greater distance when we heard two amateurs calling the plane, 9XH. One was 9CLW, Danville, Illinois, and the other was 9DGW, Goldfield, Iowa, distances of about 250 miles. Station 9CLW reported signals r-4, while 9DGW reported r-7, both strong signal reports. Such reports were extremely encouraging, considering the power of the airplane transmitter which is about four watts, one fifteenth of the power used in the average electric light bulb.
Then came slight changes on the transmitter when some means of adding voice were carried out. Ever realizing that additional weight in the radio equipment would mean that much less in pay load, we did add a voice transmitter with practically no additional weight. Not the most efficient, but something to start on for preliminary tests. Although we used dry batteries for plate and filament supply, the best supply that can be obtained, we hardly thought it possible that voice signals would come through with any clarity because of the vibration of the transmitter caused by the airplane. We knew this, because when using code, the signals had considerable modulations due to the vibration and we were somewhat skeptical about results.
The only way to find out was to try it, which we did on October 21. When Zurian said he was going to change over and use voice, we held our breath only to heave a sigh of relief when Zurian started in with, “Hello, 9EK; hello, Hoff, this is the Burgess Flying Radio Laboratory flying over Madison, Wisconsin.” And for fifteen miles we kept up a running conversation, 9EK using code in reply to Zurian’s voice. Encouraging, to say the least.
The big test came October 22 when we were going after real distance. We had enlisted the cooperation of radio amateurs of the American Radio Relay League and they were keen and on edge for a test of this kind. Only unforeseen things could prevent new results. The Air King was filled to capacity with gas and oil and we knew it would be good for several hours steady flying. Of course we picked Saturday afternoon when we knew there would be many amateurs on the air. While the airplane apparatus was being installed in the plane, another receiver was connected up at the hangar. This was used for comparison of signal strength with those received in the plane. The Air King left the ground at twenty-eight minutes after three o’clock with Zurian as operator. After the antenna was reeled out, there was a short preliminary test with 9EK to see that all was well. No sooner had Zurian signed off with 9EK than he picked up 9DTK, Milwaukee, Wisconsin, for the first outside contact. Station 9DTK reported signals r-7 and also handed in a very complete record of everything Zurian sent from the airplane 9XH. At twenty-two minutes to four o’clock, 9JA, Madison, Wisconsin, was communicating with 9XH. Then followed 9BBT, Cocato, Minnesota, at 3 -.44 p.m. Station 9EHO, Arlington, Minnesota, was next at 3:45 p.m. This operator said he could have copied every letter sent by 9XH. At 3:51 p.m., 9DCE, Milwaukee, Wisconsin, reported r-6 as did 9DBW, at North- field, Minnesota, at 3:55 p.m. R-3 was the best he could get from 8CVQ, at Kalamazoo, Michigan. Station 9EMD, Manchester, Wisconsin, didn’t give a report although communication was established with him at 4:06 p.m. We, on the ground, were a bit excited when 8CEO, Oakmont, Pennsylvania, called 9XH at 4:11 p.m. Would Zurian hear 8CEO we wondered, and hadn’t finished when 9XH called 8CEO and communicated with him and received report of r-4 to r-6. Five hundred miles from and to an airplane on seventy-nine meters! And with such a small outfit! The whole outfit weighed but sixty-five pounds and was powered entirely from dry batteries.
In a report from 8CEO, Mr. McAuly, operator said:
I consider this to be the most important test of the year in amateur radio circles. Many planes have taken off for long and dangerous trips with either no radio set at all or with a range which would render it of little value in emergencies. With a set like that used by 9XH today, arranged to use the forty and twenty meter bands as well as the eighty meter band, and working with amateurs, it would be possible, by changing from one wave to the other as distance made it necessary, to work continuously no matter where the plane might go. With such a set, a good operator, such as manned 9XH today, and the policy of working with amateur stations, airplanes need not be without communication at any time.
Next came 9QT, Minneapolis, Minnesota, with a report of r-5 at 4:15 p.m. Then 9BKV, Akron, Iowa, who reported perfect voice modulation of r-7 at 4:28 p.m. The other transmissions were key and code. At 4:33 p.m. we heard Lieutenant H. P. Roberts, U.S.A., of Wright Field, Dayton, Ohio, calling 9XH and Zurian communicated with him. Lieutenant Roberts, AW5, used his voice transmitter which was received perfectly by Zurian. Station 8AMU, Brentwood Boro, Pennsylvania, reported r-4, followed by 8EJ, Warren, Ohio, at 4:54 p.m. with r-6. The last station was 8ARC, also at Oakmont, Pennsylvania, with r-3 at 5 :oo p.m. The plane landed a few minutes after five, having communicated with fifteen amateurs in six different states.
Zurian heard two amateurs in metropolitan New York and later we received reports from the following amateur stations: 9CHC, Lafayette, Indiana; 9CE, Ishpeming, Michigan; Aero Products Company, Chicago, Illinois; 9CVJ, Ashland, Wisconsin ; 9CVI, South Milwaukee, Wisconsin; 9AIR, Sleepy Eye, Minnesota; 9AMG, Council Bluffs, Iowa; 8ARK, Detroit, Michigan; 9DLD, Jefferson, Wisconsin; 8BCM, Deposit, New York, 725 miles; 9ACL, Davenport, Iowa; J. H. Brager, Mount Horeb, Wisconsin; 9DSV, Davenport, Iowa. From all of the above stations came reports of reception of signals from the airplane, all being regarded as very good signal strength reports.
Two weeks later, Monday evening, November 11, a demonstration which meant night flying was put on for a technical club. A receiver and transmitter were installed at the club meeting place and although the interference was quite bothersome, the demonstration was pretty good. While Zurian was talking to the members at the meeting, an amateur in Dayton, Ohio, listened in and heard the voice transmission from the airplane. So much for the seventy-nine meter tests.
Immediately after concluding these tests, work was started on a forty meter transmitter and receiver for airplane use. Also a low power transmitter was installed at the radio laboratory for voice and code communication with the plane. This was operated from dry battery plate supply, using a seven and a half watt vacuum tube in each socket for modulation, oscillation, and amplification. This was tested for range at no regular time of the day but just as we happened to find a few minutes here and there.
Voice communication was established with 9DYT, Ladoga, Indiana; 9EJO, Geneva, Illinois; 9DOE, St. Louis, Missouri; 9BTX, Wilmette, Illinois; 9EFZ, Franceville, Indiana, and 9AAW, Chicago, Illinois. In each case the report was very good on signal strength and reproduction or modulation.
The first forty meter airplane test was made December 6. A “Travel Air” with a Curtis OXX-6 motor was used. Zurian established communication with 8DCE who reported r-7. The voice received at 9EK was considerably better at close range than on seventy-nine meters although the test was of short duration. It was not as good at a greater distance. Another test was made December 23, in a “Travel Air” equipped with a Wright J5C whirlwind motor. It was done in haste with unsatisfactory results. Mix said the antenna fouled on a brake wire which caused the difficulty and there wasn’t time to correct it before darkness came on.
To close the year, one final forty meter test was made December 29, when Zurian, in the “Travel Air” J5C took off from Madison at 1156 p.m. headed for Great Lakes Naval Training Station. We sent a dispatch to Great Lakes advising that the airplane would fly over the station, weather permitting, during the afternoon. Knowing that the radio station at Great Lakes would be busy with schedules, Zurian took a chance and called NAJ. We heard him call several times and finally telephoned Great Lakes and asked them to listen for 9XH because the pilot wanted to land there in order to make a test from the ground. Because this apparatus will operate on the ground, since it is powered by dry batteries, we did want to get a ground test. The pilot was unable to find the landing field at Great Lakes and although he circled around for more than a half hour, he had to go back to Milwaukee where they put in for the night.
Station 9EK maintained communication with 9XH all the way to Great Lakes and back to Milwaukee without the slightest difficulty. Zurian had to shift from one wave length to another in order to keep an ear out for 9EK and NAJ. He heard NAJ, but NAJ did not hear 9XH. The plane left Milwaukee next morning and communication was maintained with 9EK until the plane landed at Madison Airport shortly after ten o’clock. Immediately after landing, the antenna was reeled out above the ground about three feet when two-way communication was established with 9CPH, Peoria, Illinois, who reported signals r-6, pure d.c.
The results of these tests should in no way be regarded as conclusive. They do point the way and may be used as a basis for those who are interested in airplane radio.
Fiction, fad, or fancy—there is belief in some quarters that some day airplanes and airports will be equipped with short wave radio transmitters and receivers. The time seems close at hand, yet time alone will tell. Whether it be passenger, mail, or express plane, communication may be regarded as highly desirable if not as important and necessary as communication with ships at sea. The factor of safety is considerably increased when weather reports, flying conditions, and landing field conditions are available to the pilot of an airplane. Surely, the additional weight of a reliable, compact, and easily operated radio set is justified by the insurance of the safety of the airplane, not to mention the life of the pilot and perhaps the passengers.
Such radio apparatus, to be considered at all, becomes the wrist watch of the radio family, yet it must do what is expected of the huge and cumbersome grandfather’s clock. It may be the means of preventing serious, if not fatal disaster.