Aircraft Radio
January 1950 Radio & Television News Article

January 1950 Radio & Television News

[Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio & Television News, published 1919-1959. All copyrights hereby acknowledged.

Having never been a sports aficionado, I have not spent much money or time at baseball, football, or soccer fields, hockey rinks, bowling alleys, curling sheets, or basketball courts. When an air show comes to town, however, I'm there. I'll stand in line for 45 minutes to tour the inside of a DC-3, B-25, B-17, PBY-5, or just about anything that will admit me. What is particularly enjoyable is being able to inspect the radio equipment racks and bays. The sight and smell (I consider it an aroma) of the old UHF and VHF sets, recording equipment, power supplies, generators, synchros, and the associated wiring and connectors is something I never tire of experiencing. I always imagine the men who operated and maintained everything doing their assigned duties to keep those wonderful machines flying. Maybe you know of what I write. This article provides a nice overview of the state of the art for airborne electronics in the post WWII era. A couple photos are included here. Dig the Zenith console radio that I found lashed into in a C-54!

Funny anecdote: A couple years ago Melanie and I were standing in line at the Erie Airshow waiting to crawl (literally) through a B-25. The fellow in front of us was rather portly and I wondered whether he would be able to squeeze through the area in the mid fuselage that separated the front and rear portions of the airplane. Up the ladder he climbed and got a few photos of the cockpit area. Then, he turned to proceed to the rear and I could see his visage fall as he was confronted with the narrow passageway behind him. The poor guy had to back down the ladder and give up his hopes of seeing the rear gunner position and some of the bomb bay mechanisms. Fortunately for him, there are plenty of pictures online of anything he missed.

One of American Airlines' postwar fleet of DC-6's.

By James Holahan*

A brief review of the field of aircraft radio, covering the nature of the work, the necessary qualifications for entering this field, and the available opportunities. The various types of radio equipment found in modern commercial planes are also discussed.

In the few brief years that comprise the history of aviation the expansion of air travel has been tremendous. The blockade of Berlin has proven to the world that life in a modern metropolis can, in an emergency, be sustained through the medium of air transport, in spite of the hazards of weather.

Throughout the United States hundreds of airfields, built by the military for training, are now serving in a commercial capacity as municipal airports. Few populated areas in the country are farther than a short drive from a local landing field. Foreign travel, too, has expanded. Giant wings and powerful engines are shrinking distances, formerly measured in weeks, into hours.

With this expansion of air travel new opportunities are opening up for the radio technician and electronic specialist.

Practically every aircraft in use today, both privately and commercially, carries some form of radio equipment ranging from a single receiver to complete racks of electronic gear.

Modern day flying has placed radio in the "essential" class. Improperly functioning radios are not only useless but hazardous because such great dependence is placed upon them.

This was pointed out as far back as 1929 in an article by an unnamed manager of the National Air Transport Company, when he wrote:

"There is no question as to the assistance that radio can give the air transport pilot. The problem is to insure against failure of the system. A catastrophe may easily be caused if the pilot relies on radio and it fails him for one cause or another. It would be far better if he had no radio at all."

These were the days when flying was in its infancy and many airmen used radio with caution and distrust. Today our entire system of instrument flying is built around radio. Complete radio failure in an airplane could result in disaster not only to the occupants of the affected aircraft but to other aircraft in the vicinity.

The use of radio in aircraft has transformed the airplane from a contraption reserved for daredevils to an instrument of commercial utility. It permits the pilot to fly through the worst weather without reference to the ground. It gives him a course to follow and enables him to keep in contact with ground stations and other aircraft.

Aircraft radio may be divided into two categories, communications and navigation. The radio gear carried on any aircraft is used for one or both of these purposes.

The communications category includes voice transmission and reception of pertinent messages, such as takeoff and landing instructions, flight plans, weather information, emergency calls, etc. On long trips away from ground stations, such as on transoceanic flights, c.w. is used.

The aircraft receives these voice and c.w. messages on the aircraft band (200-400 kc.) or on v.h.f. (100-150 mc. voice only) and transmits on a variety of medium high frequencies between 3 and 9 mc. with 3105 kc. and 6210 kc. being most frequently used in the United States. Communications from air to ground are also transmitted on any of the allotted channels between 100 and 150 mc.

Air navigation by radio requires more of a discussion. An extensive system of radio aids to navigation, which form the civil airways of the United States, is maintained by the Civil Aeronautics Authority, an agency of the Department of Commerce. The principal component of this system is the radio range station.

A radio range station consists of a transmitter emitting a carrier on an assigned frequency in the aircraft band. It has two independent r.f. channels differing in frequency by 1020 cycles, controlled by matched crystal oscillators. The antenna system is comprised of two crossed Adcock antennas that are 90 degrees in space with respect to each other, together with a center vertical antenna. The central tower is constantly fed by the output of one r.f. channel while the output of the other is switched from one Adcock antenna to the other. First the code letter "A" is fed to one Adcock then an "N" is fed to the other. A receiver monitoring these signals will pick up the 1020 cycle beat note which corresponds to the difference between the r.f. channels. A steady tone is heard whenever the energy from the two antenna systems is received with equal intensity. This is called the "on course" or beam. Each range station produces four beams, the direction of which is controlled by the directional antenna systems. These beams give the pilot a definite course to follow toward or away from the station. In the sectors between the courses either an "A" or "N" is received depending upon what quadrant the plane receiving the signal may be in.

Actually the civil airways are aerial routes whose courses are determined by these beams. While flying on the airways the pilot is aware of his position with respect to the range station, which he can locate on his map, from the type and intensity of the received signal. The point directly over the range station is identified by the lack of signal because the vertical antenna system used will radiate negligible energy directly upward. This "no signal" area widens with height and for that reason is called the "cone of silence." On most ranges the cone is further identified by a "Z" marker, operating on a frequency of 75 mc., which constantly keys the letter "Z". A special receiver is necessary to pick up this signal.

An important piece of equipment for radio navigation is the loop antenna.

We all know the directional qualities of a loop. For instance, a portable radio with a loop antenna will receive the maximum signal when the plane of the loop is in line with the station and minimum signal when the plane of the loop is at right angles to the station.

This same principle is used in navigation. In its simplest form, the loop is used with a receiver as an extra antenna which may be switched into the circuit for direction finding.

More elaborate receivers employ amplifiers and balancing networks which greatly increase the loop's sensitivity.

The loop is the heart of the radio compass or, as it is also called, the automatic direction finder. The operation of an ADF is simple - just tune in the desired station, identify it, flip a switch and a needle, mounted on a 360 degree azimuth scale, points to the angular direction of the station relative to the aircraft.

Formerly the radio compass, because of its weight, was confined to large aircraft only. Now a manufacturer has come out with an ADF weighing 24 lbs. including the power supply.

About the latest thing in radio navigation is the omnidirectional range which will soon be in operation all over the country. These ranges operate in the v.h.f. spectrum between 90 and 110 mc. In contrast to the low frequency ranges which present four beams or pathways to the station, the omnirange allows the aircraft to come in "on the beam" in any direction. The aircraft having this equipment installed will have a "To-From" indicator which tells the pilot whether he is going toward or away from the station he is working. On the low frequency ranges the pilot determines this from the build or fading of signal strength which can often be difficult in times of poor reception due to precipitation static and the like.

For navigating great distances by radio, such as in transoceanic flying, a system called "loran" is used. The name is a coined word derived from the words LOng RAnge Navigation. Developed during the war, this system is composed of a receiver operating on a band just above the broadcast frequencies. There is no audio. Pips appearing on the face of the cathode-ray tube furnish visual information, which when applied to special loran charts gives very accurate fixes. The only installation required in the aircraft is the loran receiver.

Ground installations consist of several pairs of transmitting stations operating on the same frequency. In each pair one is termed the "master" the other the "slave" station. The system is based upon the microsecond interval between the reception of the signals emitted by the master and the slave.

A navigator, specially trained in the loran method of navigation, is required to operate this system. It is possible to obtain accurate fixes from stations up to 1500 miles distant by means of loran.

The instrument landing system may also be included in the navigation category. Two receivers and an instrument panel indicator comprise the main components of the ILS equipment as it is installed in the aircraft.

One receiver, the localizer, operates on any of six crystal-controlled frequencies located between 108 mc. and 110 mc. This receiver tells the pilot, through movements of a vertical needle of a specially designed micro ammeter, whether he is to the right or left of the runway. Actually the needle is differentiating between an area of 150 cycle modulation and 90 cycle modulation.

The path of descent can be seen in the movement of a horizontal needle of another micro ammeter utilizing the same meter face as the vertical needle. A glide path receiver controls the movement of the horizontal needle. It operates on three crystal-controlled channels between 332 mc. and 335 mc. The glide path signal when modulated by 150 cycles will indicate position above the correct path while 90 cycle modulation is used to show position below the correct path.

Also used for blind landings is the much publicized GCA-ground controlled approach. All the equipment that is needed in the aircraft in order to use this system is voice communication between the pilot and ground. The path of the aircraft is accurately plotted and the pilot is literally talked into a landing by ground operators.

Non-directional radio beacons and marker beacons are two more of the facilities offered by the Federal airways system. The former is simply a station transmitting a continuous carrier in the aircraft band interrupted by regular station identifications. It is used only with direction finders as a homing aid.

Marker beacons are vertically directed signals on 75 mc. These are located along the "on course" of low frequency radio ranges. They enable the pilot to definitely establish his position while flying the ranges.

Thus it may be seen that the present day aircraft is well equipped with radio gear. In flying, radio is essential. Just as essential is the personnel which maintains this equipment.

Commercial operators (airlines, flight schools, charter companies, etc.) and most private owners who engage in regular and frequent flights away from their home airport have their radios checked periodically.

Scheduled airlines probably have as complete a radio maintenance system as can be found in the aviation industry. Here all radio equipment is given periodic checks according to the number of hours which the ship has flown. There are checks approximately every 25 hours. Each succeeding check involves more operations until, at the end of a set period of time, say 2000 hours, all the radio gear is removed for overhaul and bench checks.

Aviation companies have found that it doesn't pay to wait until a unit breaks down before servicing it - preventive maintenance is the watchword. Those used to servicing home sets would find aircraft radio quite different in this respect - less than half the time is spent repairing defective units; more time is consumed looking for troubles before they cause the delay of a flight.

The more technical aspects of an aircraft radioman's job involve making new installations and modifying certain types (especially surplus) radio equipment, either to improve performance or to conform with government regulations. In large companies very little of the installation or conversion work is left to the initiative of the technician. In small outfits the success of the installations and conversions is largely dependent upon the skill of the technician.

The qualifications necessary for an aircraft radio technician can be summed up in five basic requirements:

1. Technical knowledge. He must possess a high degree of widely diversified theoretical and practical knowledge of radio.

2. Mechanical ability. He must be able to work well with tools and be fairly skillful in designing and building things of a mechanical nature.

33. Familiarity with aircraft. Since most of the equipment is installed in aircraft, familiarity with the location of the power sources, switches, controls, etc., are of vital importance.

4. Physical fitness. This work involves a good deal of climbing in and out of tight places, much walking, and lugging of heavy equipment.

5. FCC license. A 2nd class phone is the minimum license requirement. If a man has the proper technical knowledge all the preparation needed to pass this exam is a study of the FCC regulations governing the holders of commercial licenses. However, a review of a "question and answer" book (sold at most technical book stores) for this exam, might prove helpful to those who might be rusty on theory.

Now, providing a man has the proper qualifications, the next step would be applying for a position. Let us consider the main sources of employment. There are five.

1.     Scheduled airlines

2.     Non-scheduled airlines

3.     Maintenance and overhaul shops

4.     Civil service

5.     Own business

The scheduled airlines hire the bulk of the personnel, each major airline having in the neighborhood of 50 to 175 men over-all, doing radio work. The starting pay is fair - about $1.52 per hour with periodic increases governed by the length of service. The work is shift work. Working conditions and equipment are excellent. There is a strict union shop with its attendant seniority regulations.

Non-scheduled airlines pay on a par with the scheduled lines but do not offer the same security since the "non-scheds" have not as yet attained any form of inherent stability.

The working conditions are just fair while ofttimes the equipment is worse. Here more skill and versatility is required. Hours are frequently long and irregular. However, experience is plentiful and the opportunities of high gains that go along with new enterprise are ever present.

Maintenance and overhaul shops usually pay scheduled airline rates to the regular technicians but the rates of the leadmen and foremen are somewhat higher. Here the hours and working conditions depend upon many factors, such as the size and policy of the particular shop.

Civil service positions are sometimes available in the aircraft radio field as civilian technicians with the armed services and with the Civil Aeronautics Authority.

With the latter the work is for the most part installing and maintaining the ground stations of the radio aids to aerial navigation that span the entire country. Such positions for technicians pay between three and four thousand per year, depending upon the classification of the work. Announcements of examinations for these positions appear in the Civil Service periodicals.

Operating one's own business will offer opportunity only to those experienced in aircraft radio as well as business techniques.

Not unlike similar ventures in the home sales-service field it offers advantages, such as being your own boss, on the one side and disadvantages, like a seven day-ninety hour week, on the other.

For the benefit of those considering an entry into this phase of electronics let me outline briefly the future of aviation radio.

Aircraft radio's future is tied up with aviation. If aviation rises or falls so will its electronic offspring.

Do not be deceived into thinking that aviation is the industry open to a chosen few booted and goggled supermen where everything is booming and everybody carries off a bucket of gold for his daily labors.

It is a new industry; in its short life it has had its share of depressing times when many employees were furloughed. In the years since the war most of the companies have lost money. Those that survived are beginning to get into the black and show signs of an upward trend.

Showing the positioning of the radio compass loop in relation to the fuselage.

What can be definitely stated is that aviation is a young industry and that aviation is here to stay.

There is a tremendous amount of wealth, both government and private, invested in aviation.

In spite of all its fine achievements aircraft radio has its shortcomings; it still has vast room for improvement.

Much of aviation's future is dependent upon the advancement of electronics.

How a radio signal is used to "home" a plane during adverse weather conditions.

Loop position characteristics.

The administrator of Civil Aeronautics, D. W. Rentzel, recently made the following statement:

"The most urgent need of aviation today - civil and military - is a reliable, all-weather navigation and landing system... The United States already has approximately six billion dollars invested in civil airports. Because of weather these airports are closed fifteen percent of the time ... The gravity of this situation, which is a bottleneck to commercial aviation and a weak link in our national defense, has been recognized in every group which has studied air transportation problems. Both the President's Air Policy Commission report and the report of the Congressional Aviation Policy Board emphasize the need for a safe, efficient, all-weather navigation system, estimated at one hundred million dollars to implement and requiring fifteen years until 1963 - to be placed in operation. This represents about five percent of our present investment in aviation in the United States ... The system must accomplish a task of almost fantastic complexity. Before the war, such a system could not have been established. But new developments, such as radar and other electronic devices using extremely high frequencies, have given us the tools which make such a system possible."

*The author is a radioman who became a pilot in the USAAF during the war. At the end of the war he was placed in charge of radio and radar maintenance at Rapid City Army Air Base. Since separation from service he has been in the aircraft radio field and is employed by Air Associates Inc.

Posted September 20,2021
(updated from original post on 9/3/2015)