PACIFIC RADIO NEWS
Volume 1, Page 291
August, 1920
NEW STATIONS OF THE AERIAL MAIL SERVICE
By H. L. Rodman
| East Tower and Station Building at Elko, Nevada |
Operating Station at Elko, Nevada |
(Above) An Overland System of Radio Using the Standard Federal 2 K.W. Arc Transmitter and the Navy Type S. E. 1420 Vacuum Tube Receiver
What is believed to be one of the most efficient radio circuits in the world, in terms of power-inputs and distance covered is in daily operation between two of the untied States Aerial Mail Service radio stations, one at North Platte, Nebraska, and the other at Elko, Nevada.
The distance between these two stations is approximately one thousand miles, over one of the most broken portions of the United States.
Four schedules a day are worked constantly, two in the forenoon and two in the afternoon, and the traffic handled is coped direct on typewriters.
Although there are three other stations between these points at intervals averaging 250 miles, North Platte and Elko, work direct for the purpose of clearing through traffic and saving the time that would otherwise be required to relay from point to point.
The most remarkable part of this circuit is that the stations are equipped with arcs of only 2 K. W. nominal power.
The receiving apparatus consists essentially of navy standard 1420 receivers and two-step amplifiers, utilizing both Moorhead and Western Electric tubes.
The amplifiers were added simply to facilitate the use of the typewriting in copying, as there is quite a good basic signal.
These two stations are links in a chain of six arc stations stretching across the western part of the United States which were recently built by the Air Mail Service, coming under the jurisdiction of the Second Assistant Postmaster General at Washington, D. C.
North Platte is the easternmost arc station and communicates with an Air Mail spark station at Omaha, Nebraska, constantly, and with the U. S. Naval Radio Station at Chicago at frequent intervals.
North Platte is the easternmost arc station and communicates with an Air Mail spark station at Omaha, Nebraska, constantly, and with the U. S. Naval Radio Station at Chicago at frequent intervals.
Proceeding westward we find a 2 K. W. arc radio station at or near each landing field, as follows: Cheyenne, Wyoming; Rock Springs, Wyoming; Salt Lake City, Utah; Elko, Nevada; Reno, Nevada.
Reno is the westernmost station and has regular schedules with the San Francisco Navy Radio.
It was stated that the stations are “at or near” the landing fields. At Cheyenne which was the first of the arc stations
to be installed, the towers were placed on the field. This was found to be an error, as radio towers on airplane landing fields have proved to be more of an obstruction to aerial navigation than a guide. Several instances are on record where pilots have crashed into radio towers on landing fields.
For this reason the radio towers are now placed at from one to five miles from the landing fields, communication with the field being made by telephone.
The two towers are 110 feet in height, built up to 100 feet on a modified Howe Truss plan, with 10 foot topmasts above the trusswork, the base centers being separated by 300 feet.
At the base the towers are four feet square; at the top about two and one half feet, giving the towers a slight taper which is very pleasing in appearance.
To guard against high winds, four sets of guys are employed.
The antenna at Elko is of the “T” type. Five wires each 280 feet in length, spaced 3 feet 6 inches apart on 15 foot spreaders comprises the flat-top portion, and five wires, cabled together about ten fee from the top, are used for the down-lead.
The down-lead is anchored by a string of eight goose-egg insulators, to relieve the entrance insulator of any excessive wind strain.
The large grounding switches for protecting the installation against lighting is located on the outside of the building near the entrance insulator, and a heavy lead is taken to ground from this switch so as to provide the best possible protection to the set during a severe electrical storm.
The arc was modified slightly at Elko, as was done at the other air mail arc stations, so as to obtain certain desired results.
These two kilowatt arcs are designed to operate on 600 meters as well as on 2,400 meters. A very strong magnetic field is necessary for the arc to function on 600 meters due to the high corresponding frequency of 500,000 cycles. On a wave-length of 3000 meters with its corresponding frequency of 100,000 cycles, the same strength of magnetic field is neither necessary nor desirable. The action of the arc is naturally slower at this lower frequency and the strength of field employed at 500,000 cycles (600 meters) for maximum radiation is much too strong for obtaining maximum radiation at 100,000 cycles (3,000 meters) due mainly to the fact that the arc is blown out too soon during each cycle, not allowing the full amount of current to flow during the discharge portion of the cycle.
As the wave-lengths selected for these stations range from 2800 meters to 3600 meters, and no work was contemplated on shorter wave-lengths, it was at once apparent that a weaker field than that created by the four field coils in series was desirable.
Leads from each filed coil were therefore brought out and the series-multiple scheme of connections utilized. This served to weaken the field strength to allow a greater current to be used through the arc, and at the same time reduce the heating of the field coils when using this greater current.
With the series-multiple scheme of connections, a current of 15 amperes is used in the arc at 200 volts, making the actual power input 3 kilowatts.
It will be noted that although a heavy current is being used for so small an arc, the power consumed in kilowatts is not proportionately great due to the low voltage used, this latter being made possible by the low antenna and ground resistance at the wave lengths employed.
The method of signaling used at Elko and other air mail arc stations is the coupled compensating method, using two turns of radio-frequency cable loosely coupled to the antenna loading inductor. The signaling or lower wave is separated from the upper or compensating wave by approximately 50 meters.
Although these particular arc sets are equipped with the ignition key signaling apparatus, the coupled compensating method was found best adapted to these stations, and in addition to being best adapted, it is the preferred by the operators.
A surprisingly large amount of traffic is being handled by the air mail stations, their principal use being the reporting of arrivals and departures of mail planes and handling the telegraph business of the air mail service between Washington D. C., and the officials in the field.
The circuit is complete between San Francisco and Washington, connection being made east of Chicago through other air mail radio stations employing spark systems.
There is however, no communication with the mail planes, as the planes are not as yet quipped with radio. This will doubtlessly come with the growth of the air mail service in the near future, as will the direction finding apparatus and other radio aids to aerial navigation.
Pacific Radio News, original source.
Volumes 2 & 3