GENESIS OF THE LOW FREQUENCY RADIO RANGE

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“Blind flying” or “Instrument Flight,” (being able to pilot a plane through the densest weather without any reference to the horizon or ground) was recognized as fundamental to reliable air travel.  Gyroscopic instruments, namely the artificial horizon, would solve the first problem: giving pilots a sense of “up” in the spectacularly disorienting conditions within the clouds (this hard lesson was and is still learned the fatal way by untrained pilots). However, this technology could not guide pilots to their destination - more was needed.

Aviation and radio started their lives in parallel at the beginning of the 20th century.  Early on it was realized that the latter technology could penetrate the weather and night sky to help pilots find their way. Experimental radio compasses appeared on aircraft as early as 1920 and as the name implied, they could point their way to beacons or even a commercial radio station. However, in the era of vacuum-tube radios the technology was complex, bulky, temperamental and expensive – only larger planes could carry the weight. Additionally, until World War II, most systems required that a pilot manually turn an antenna or turn the entire plane to find a signal – and even then it couldn’t immediately tell the pilot if he was moving toward or away on that bearing line (the “180° ambiguity” problem).  If there was a continual crosswind, a pilot could actually fly a curved course to the beacon per the diagram below. “Tracking” at a slight angle to compensate could help, but as winds could fluctuate along a course, it was not precise. The ideal system would allow pilots to use cheap, light equipment to follow a fixed course, a highway in the air, which made it easier to fly a straight line with a crosswind, avoid hazards such as high terrain and allowed for better separation of two-way air traffic along a course.

The answer that became the Low Frequency Radio Range system was first made workable by Ford Motor Company Engineers. A patent was filed in 1928 by Eugene S. Donovan. Earlier concepts for the system  were developed in Germany in 1906 which were later experimented with by the US Bureau of Standards and Army Signal Corps. Henry Ford personally saw the future of aviation and wanted to part of it, stating in 1924 “it is now or never to get a hold of commercial flying and make a success of it.” He placed his considerable financial resources behind his ambitions, buying the airplane manufacturer that would create the famous Ford Tri-motor and building the new state-of-the-art Ford Airport in Dearborn, MI just outside of Detroit. 

In 1926, as part of this effort, Ford engineers looked to perfect a reliable radio beacon to help them ferry auto parts between their Chicago and Dearborn plants using Tri-Motors during the brutal Midwest winter. The first station at Ford Airport was built and licensed by October 1926 and by February 1927 the system proved to very successful. Citing the transformative impact it would have on aviation and the public welfare, as well as the probable growth in sales for its aviation division, Ford declined to collect royalties from US Government for using the invention. 

Progress was very quick once the 1926 Air Commerce Act empowered the US Department of Commerce's new Aeronautics Branch to establish a system of radio navigation aids. They, in turn, enlisted the US Bureau of Standards to find suitable technology. Experimental work commenced at their College Park, MD station in December 1926, and test runs began between additional demonstration stations at Hadley Field NJ, and Cleveland, OH from December 1927 through February 1928. Right after that point, the Bellefonte, PA station went on-air as the first commercially available range station. It was joined by the two earlier demonstration stations to establish the first “Radio Airway” between New York and Cleveland in November 1928. This grew to 9 stations by 1929.  On September 24th of that year, James Doolittle, who would go on lead the famous raid on Tokyo in 1941, successfully took off, circled and landed a plane with a tarp-covered cockpit using a radio range and another specialized radio beam for landing, proving once and for all that flight solely on instruments was possible. Coast-to-coast service from New York to San Francisco was achieved in 1931. By 1935 the US Mainland was crossed by a network of 117 stations, and by the end of World War II over 400 were in place with a couple hundred more across the globe. 

The LFR system was considered an "indispensable" godsend in its era – for the first time, pilots could reliably navigate long distances without seeing the ground.  

The crux of the Low Frequency Radio Range system was simple.  The airways were established by the hundreds of stations, typically near a significant intersection or airfield. A pilot would listen to their signals by tuning into a station over a simple AM radio set in the cockpit with headphones, about the same size and cost of a small tabletop radio from the era. 

Each station had two diagonally opposed sets of vertical antennae that transmitted a figure 8 directional pattern of radio waves at a 90° angle to each other per the diagram above creating four sectors or quadrants. Two antennae emitted a Morse code dash-dot tone, the “N” signal, into the two yellow quadrants and the other pair emitted the dot-dash tone, the “A” signal, into the two opposite blue quadrants. Note that each signal was an exact negative of each other, the noiseless gaps in one signal were synced with the dot/dash sounds in the other. When the pilot was equally placed between the towers on the line that separated the quadrants, whether it be one mile away or fifty, both signals would precisely intermesh into a steady, continuous 1,020 hertz tone. These four lines were the “legs” or “beams,” the course a pilot would fly along. When “flying a beam,” if a pilot heard that simple monotone, they knew they were on-course, or “on the beam”, and that is how that term entered the American lexicon in the mid-20th century. If the pilot was closer to the A side or the N side, that tone would become dominant, and the pilot would make the necessary course corrections. These beams were roughly 3° wide and could extend 100+ miles from the station.

If a pilot was on the edge of the beam in the “twilight” zone, one signal would faintly emerge above the tone and the pilot knew to make a slight correction. When there was two-way air traffic along a beam, standard practice was for opposing aircraft to fly the right twilight edge of this beam to ensure proper separation. Farther away from the beam, both signals (one much louder) could be easily distinguished in the “bi-signal” zone. Deeper into a quadrant only one signal would be heard, the “pure quadrant” or “clear” area – especially at the “bisector,” a line in the middle of each quadrant equidistant from the beams at either side. These variations were subtle but could provide clues to help a pilot navigate to a beam. Another aid was that, in the US, true north from the station always lied in an N quadrant (in Western Canada this was shifted 45° eastward due to magnetic deviation). 

Every 30 seconds a two-letter Morse code station identifier (lasting 8 seconds) was repeated on the A side and then on N side in Morse code to allow the pilot to confirm that they were listening to the right station. For example Los Angeles was “LA”, Chicago “CG”, etc.  Starting in the 1940's, some station identifiers were changed to three letters (for example, “LA” became “LAX”) due to the increasing number of overall radio facilities. 

Recordings of an actual range station can be found on the next page

A pilot could tell that the plane was approaching a range station when the volume in the headphones steadily increased. When a pilot was over the range station, as the radio waves were projected outward much more than upward, the signal would suddenly die off in a “cone of silence” then as just quickly resume which became proof-positive that a pilot was directly over the station. Additionally, the A and N sides would reverse. Later on stations added a “Z” marker, a second upward pointed radio beam that would activate a beacon light on the instrument panel and a 3,000 Hz tone in the headset to more positively confirm the crossing. Flying between stations, the system could not tell you the distance from any station. As such, the ranges were complimented by additional similar “fan markers”, which as the name implied, had a wide 3 by 12-mile elliptical or bone-shaped beam spread perpendicular to an airway to mark particular waypoints. These beacon lights would flash 1 to 4 times per their compass direction from a reference station (1 for north, 2 for east, etc.) to make a positive ID. This basic system survives today in the form of the inner marker beacons used on instrument landing system approaches to airports.

LFR stations had two additional critical abilities. The first was the broadcast of regular weather reports to all aircraft on an entire part of an airway, typically scheduled every 30 to 60 minutes. The reports gave current and forecasted conditions for each station; finally, pilots could get a “look ahead” for their journey. For the planes equipped with a “radio-telephone” that could transmit and receive (normally only larger commercial and military planes in this era) the range station could provide a two-way ground link to the newly formed air traffic control and other services. The LFR network was thus a vital link for keeping in contact with aircraft on remote stretches. A station called up by a pilot in this manner was referred to as a “Radio”, e.g. “Oakland Radio”, “La Guardia Radio” etc. Nearly a century later, when pilots call a ground-based flight services station, this convention is still used.

The stations were fairly large as the longer wavelengths of low frequency radio required large antennae. First generation “loop” stations were square shaped and were roughly 300’ on each side (2-acre site area). They had 40’ wooden masts at each corner that supported an “A” and “N” loop antennae in an “X” configuration across the site to create the signal quadrants and beams. Later “Adcock” stations, described in more detail below, were larger at around 800’ on a side (15-acre site area). These more advanced stations had 135’ riveted steel antennae towers or “mast radiators” at the corners, spaced 600’ diagonally, that performed the same function. Fortunately for the US Government, land was cheap back then; especially at the rural sites most of these stations were set up at. Federal land agents would commonly appeal to the owner’s sense of patriotic duty to secure a lease or purchase agreement.

The system used Low-Frequency radio (190 kHz to 400 kHz, and up to 536 kHz for military ranges), just below the AM Radio broadcast range. As 300 kHz and above is considered “Medium-Frequency” some sources referred to the LF Range as the “Low-Frequency / Medium-Frequency” or LF/MF Range. These frequencies have long wavelengths that tend to travel further over the curvature of the earth and reflect off of the ionosphere, e.g. the “skywaves” that make global shortwave radio possible. Longer ranges were possible at night when the ionosphere is most reflective. For this reason, many stations used only a 150-watt transmitter (equivalent in power to a large incandescent household lightbulb) to be heard over100 miles away, although some stations had up to 1,500 watts of power. As discussed later on, this frequency was also susceptible to other negative effects but this low power requirement made the station apparatus economical and easy to set up, and only a cheap and lightweight AM receiver was needed on each aircraft, which greatly aided its adoption.

With the loop type antennae used at the first series of stations, transmission or reception is strongest in the direction of the loop and is weakest at the “null” point 90° to its plane. This principal works to transmit radio signals from a loop in a specific direction or, in the case of a radio compass, to align it with the source of a signal. As applied to these LFR stations, it caused the A and N loop to emit two opposed signal lobes which appeared from above as crossed figure-eight patterns forming the 4 quadrants. However, this arrangement required that horizontal wires be strung hundreds of feet between towers where they and their beams could be deformed by the wind. It also sent significant amounts of horizontally polarized radio energy upward, creating excessive skywaves that could interfere with the station’s own signals beyond a 30-mile radius, especially at night.  

In 1919 British Engineer Frank Adcock determined that two electrically connected but physically separate pairs of rigid vertical antennae acted as a more effective version of a loop antenna. In 1933, US Bureau of Standards scientist Henry Diamond realized this antenna form, lacking the troublesome horizontal lines, would eliminate the skywave problem by transmitting all of the radio energy outwards as vertically polarized waves. The Department of Commerce, not wanting to reference the name of a British inventor, initially called these new stations the “T-L” or Transmission Line” type. However, common sense would prevail and “Adcock Range” became the common term. These stations would become the dominant type by the late 1930’s. However, loop stations would still be used for many medium and lower power sites as they were simply more economical with their smaller size and wooden masts, especially during the steel shortage of World War II. 

Adcock ranges also solved another nuisance: with loop stations the range signal had to be shut down to allow the voice or weather broadcast. Pilots who had the ability to transmit sometimes had to ask the operator to suspend a report if they were in the middle of an approach or other critical phase. Adcock stations added a dedicated fifth center antenna and a “simultaneous transmitter” that could handle the voice communication separately to allow the range signals to run uninterrupted.  

A transmitter building, hut or “blockhouse” lied at the center of each station: a generally utilitarian structure approximately 500 sf in area that housed the transmitters (16’ x 32’ was a common size). Two dual-redundant transmitter units were provided to allow for backup and switchover for routine maintenance. An automatic telegraph key triggered by revolving, notched Bakelite disks (akin to a music box) continuously keyed the dots and dashes for the signals and station identification. The signals were then sent through a radio goniometer, a set of adjustable nested coils, that could rotate the entire beam pattern as needed to best align with the airways (this device was developed by Italian engineers Ettore Bellini and Alessandro Tosi in 1907 for the reverse application of finding the direction of a radio signal). If the power failed to the site, assuming that it wasn’t a more rural site that already generated its own power, an automatic standby generator was provided in the same structure or a nearby separate hut. 

Shielded coaxial cables, to minimize signal leakage and stray skywaves, ran from the blockhouse to each antenna either underground or via ground level supports. Antennae need to have a good electrical ground to efficiently function. Many sites, especially in desert regions, had dry and rocky soil that was not electrically conductive enough to provide this. In these situations, a “counterpoise”– a square or disk of wire mesh or cable at the base of antennae compensated for this.

Nearly all stations ran unmanned, one of the first “lights out” facilities of the 20th century, illuminated only by the faint warm glow of vacuum tubes within the transmitters accompanied by the sounds of whirring cooling fans and the never ending clicking of the automatic key. Range station operators were actually located at central offices many miles away (typically at airport terminals) that handled the weather broadcasts and radio communications for several stations. They could control each one remotely by a landline with a rotary telephone-like device that could send 16 preset commands, ranging from turning signals off to allow a voice broadcast, to turning on the lights on the radio towers at night. Generally, a daily inspection visit was all that was needed. A few remote stations, such as the Donner Pass range located on frequently snowbound 7,135’ ridge, did have a live-in station keeper but this was, by far, the exception. 

A few details for the more technically inclined: the central tower actually emitted a continuous omni-directional signal at the nominal station frequency (let say it was 200 kHz), and this carrier wave could be modulated for the voice and weather broadcasts. As it was continuous it could also be used as a homing beacon by a radio compass by those planes so equipped. The A and N towers actually broadcasted their signals at that frequency plus 1.020 kHz (so 201.02 kHz); the 1,020 hertz tone the pilot heard for the dots and dashes. When both signals were converted into sound waves by the radio’s electronics, like a piano chord, they blended and produced the audible, out of phase “beat” frequency of 1,020 Hz. Later radio receivers had a bandpass filter that allowed the pilot to switch the receiver to hear the tone only, voice only (as the human voice is lower between 300 Hz and 800 Hz) or both. Many aviation receivers from the era also had an “automatic volume control” (AVC) which made it easier to listen to conventional AM stations as they drifted in and out of range; however, handbooks earnestly warned pilots to always disable this feature when using the range so the pilot could properly hear changes in the signals. 

Especially given the fact vacuum tubes were prone to occasional burnout, there was a rigorous schedule of inspection, cleaning and preventative maintenance. Several ground monitoring stations, a fleet of CAB inspection aircraft, as well as the end users were always checking for deviations. Any departure from normal would be investigated immediately and a Notice to Airman (NOTAM) would be dispatched via teletype to all ground facilities to warn pilots. Later stations also incorporated an early form of automatic internal fault detection. The station would monitor its own radio output and if beam alignment drifted or an automatic key stuck, the station would transmit a series of three U’s (dot-dot-dash) between station identifiers telling the pilot to be especially wary using this station.

Stations were grouped into five classes based on type and power as follows. The lower power of medium range stations produced less interference due to reflected signals from terrain - an important consideration at mountainous sites:  

• RA – Adcock Range (new style), greater than 150 Watts – range  100 miles
• RL – Loop Antenna (old style) , greater than 150 Watts  – range  100 miles
• MRA – Medium powered Adcock Range, 50 to 150 Watts - range  50 miles
• MRL - Medium powered Loop Antenna, 50 to 150 Watts - range  50 miles
• ML – Low powered loop antenna, 50 watts. These had about 15 miles of range and provide localized homing to smaller fields. As these were never mapped and didn’t show in aerials, we’ve maintained a separate list of these stations. 

In terms of cost, the May 15, 1939 “Air Commerce Bulletin” gave the 1938 cost of a new simultaneous Adcock radio range station as $44,000 or $813,000 in 2020 Dollars.  A group of 3 medium and low power ranges also from that year cost $87,000 or roughly $29,000 per site - $536,000 in 2020 Dollars. These figures did not include the extensive infrastructure (power and telephone) needed to support the sites, as well as developmental and operational costs which a 1928 report indicated was roughly half the of station’s initial cost per annum. The overall program cost would certainly be in the billions in today’s dollars.  

These were all the major pieces, let's see how well it did (and sometimes didn't) work...

Next: Using the Range