LFR and other systems that would follow were (a) specifically developed as large-scale programs with numerous sites, (b) were widely adopted by civilian and military aviation and (c) utilized for decades. All were used internationally. Here are the dates they were developed and implemented:
- LFR (Low Frequency Radio Range): developed in 1926, implemented 1928, last used in the US in 1974, by the mid-1980’s elsewhere. 4-course aural system.
- Lorenz Beam: developed in 1932, commercially available 1934, last used in 1960. Single beam aural system.
- Elektra-Sonnen & Consol: developed and implemented 1940, last used in 1991. Rotating multi-beam aural system.
- Gee: developed and implemented in 1942, last used in 1970. First hyperbolic system.
- LORAN (Long Range Navigation): developed in 1941 implemented 1942, discontinued by 2015. The most widespread hyperbolic navigation system used. May be revived as a backup to GPS.
- VAR (Visual Aural Radio Range): developed in 1937, implemented 1944, last used 1960’s US and the 1980’s elsewhere. 4-course system.
- Decca: developed in 1942, implemented 1944, last used 2001. An additional hyperbolic system.
- VOR (Very High Frequency Omnidirectional Radio Range): developed 1937, implemented 1946. Visual system, still in widespread use.
- Starting in the 1980’s when GPS first became available for civilian use, Satellite Navigation Systems (SATNAV) would rise to dominance by the start of the 21st century.
By this account, it appears that LFR had a six-year head start ahead of the Lorenz Beam which took off in the 30’s and over a decade on the other systems that would propagate in the 1940’s. Of these only Lorenz, VAR and VOR also provided immediate “real time” guidance along a specific airway or course with minimal work for the pilot. The rest only established a bearing or fix at one point in time, which the pilot would then need to analyze further to determine any necessary course correction.
LFR also already had at least 7 operational stations in the US by the time England's Orfordness Beacon went on-air July 1929. It used a similar concept to the Telefunken system described above, as well as an experimental 1920 maritime beacon developed by Marconi, where a rotating loop antenna was used instead of a fixed array to smoothly sweep the directional signal after the omnidirectional timing pulse. Only one sister station was built at Farnborough before it was surpassed by other efforts.
The basic principles of VAR and VOR, the successors to LFR, are covered on the Its Fate Page. A quick overview of the other systems follows below:
Germany’s Lorenz Beam (named after the company) was, in essence, a narrow single aural “beam” that used two slightly overlapping 33.33Mhz signals, one of dots and the other of dashes, that merged to create an on-course "equisignal" tone. The two signals were actually a single signal transmitted on a center dipole antenna. A cam switch activated two adjacent dipole reflectors on either side that would rapidly deflect the signal pattern back and forth across the centerline to form the dot and dash lobes. It was used successfully by the 1930’s as an early Instrument Landing System and for long range navigation. Australia preferred this system over LFR and rapidly implemented it after the 1938 crash of the DC-2 Kyeema in poor weather. During the “Battle of the Beams” the “Knickebein” bombing aid intersected two powerful versions of such beams over targets in England. The rise of the VOR, VAR and modern Instrument Landing Systems would all make the Lorenz Beam obsolete by 1960.
The Elektra-Sonnen system (simply “Sonne,” German for “sun") was implemented by Germany at the start of World War II in occupied Europe and neutral Spain. The British also surreptitiously used it and named it Consol (Latin for “by the sun”). It combined the Lorenz Beam and Orfordness concepts by radiating 18 rotating equisignals that, after the omnidirectional timing pulse and station ID was sent, would each then sweep across a fixed pie-shaped sector roughly 15° wide over a 30 second period. These sectors were plotted on charts and arranged in two equal opposed “fans” about 135° wide on each side of the station. They were formed by a continuous 250-350khz signal from a center antenna, flanked by two more spaced 3 wavelengths away (about 1.8 miles). The signal alternated between these outer antennae at 1-second intervals to create two heart-shaped “cardioid” dot and dash patterns. Additionally, their signal phase was adjusted to create deep “ripples” (e.g. lobes formed by wave interference) that counterrotated around the edges of both patterns. These interacted to form the many moving equisignals. Technically, these were condensed hyperbolic waveforms between the 3 antennae but from miles away these patterns appeared to radiate from a single point like the rays of the Sun, hence the system’s name.
After using a radio compass, last known position or other means to determine the sector the aircraft was likely in, the number of dots heard after the timing pulse would be counted until the equisignal emerged which then faded into dashes. A chart or table would convert this count into a bearing accurate to within ½° to ¼°. It took a few minutes to assess the 2 to 3 counts needed to most accurately derive a bearing, but like LFR only a simple AM receiver was needed. Its simplicity (e.g. just count the dots) and 1,000-mile range kept it in service in for 50 years, the last station going off-air in 1991.
Hyperbolic Navigation also emerged during the war. It used the time between an initial signal pulse sent by a “master” station that was responded to by another pulse from two or more or “slave” stations located hundreds of miles apart along a defined “chain,” after a short time delay, to determine location. Mathematically, a line that has a constant amount of signal delay between two points forms a hyperbola, hence the origin of the term. Dozens of such “lines of position” or LOP’s were plotted on aerial and marine navigation charts for each chain, typically in 100 microsecond (ms) increments. By reading the time difference on a dial or cathode ray tube between the master and at least two slave stations (or a pair of stations on another chain) the user could determine which two lines of position they were on and the intersection marked their location. LOP’s could also be checked against depth soundings, bearings to landmarks or other beacons, etc. to establish location.
The British developed Gee as the first hyperbolic system for its bombing operations which operated at 20-90 Mhz and had a range of a few hundred miles. This inspired the US to shortly follow with LORAN to extend radio navigation for its Atlantic convoys. It operated at a lower frequency of 1.75 to 1.95 Mhz that used skywave propagation to boost its daytime 700-mile range up to 1,400 miles at night. It was fairly easy to use and was accurate to within a few miles. Decca was a similar hyperbolic system (also developed in the US but first implemented in Britain) but used the phase comparison of low frequency waves between 70 and 129Khz to determine distance within a 200-to-400-mile range. In late 1950’s, after “LORAN B” was abandoned during development, “LORAN C” emerged that used both timing and signal phase, like Decca, to provide accuracy to a few hundred yards. It initially used very expensive 100lb receivers; however, the now cheaper, down-market original “LORAN A” receivers became common globally and by the 1970’s microelectronics made LORAN C units small and affordable. Decca further evolved into Omega, a US military only global network of 8 stations that served from 1971 to 1997.
Satellite Navigation would spell the end of hyperbolic navigation. After the successful 1960 deployment of the US Navy Transit satellite system that allowed military users to determine their location from the observed doppler shift of a radio signal from a satellite, the US began to launch the first part of the Global Positioning System in 1978. The accidental 1983 downing of Korean Air Lines Flight 007, which inadvertently strayed into Soviet airspace due to a navigation error, prompted the US to make GPS available to civilian users (initially with a less accurate “selective availability” constraint that was removed in 2000). As compact, cheap and highly accurate GPS receivers became commonplace by the start of the 21st century, hyperbolic navigation systems would be fully decommissioned by 2015, the exception being the Russian CHAYKA and Alpha systems, its parallel versions of LORAN-C and Omega respectively, which are still in use as of 2021. There are now several satellite navigation systems in use; however, as nations realize the vulnerability of satellite constellations to everything from solar flares to unfriendly neighbors, there has been work to create a new “eLORAN” system as a potential terrestrial backup.
QUESTION 2:
Did the Ford Motor Company really invent the Low Frequency Radio Range? How was it developed?
Yes, the Ford Motor Company is indeed the official inventor as is made abundantly clear by US Patent #1,937,876 filed in Ford’s name in 1928. The Ford Museum still retains an original 1927 transmitter building and other components in its collection. However, like many inventions in history, there was a long road of others who developed and refined its key concepts and prototypes, and in the end, Ford and the US National Bureau of Standards would have different accounts of its creation.
In 1907, just four years after the Wright Brother’s took flight, German engineer Otto Scheller (who would go on to develop 70+ patents for the German radio giant Lorenz AG) conceived a radio beacon that used 4 vertical antennae to create two overlapping figure-eight signal patterns. One signal was a dot (the Morse letter “E”) and the other a dash (the Morse “T”) that interlocked to form an on-course signal along 4 beam lines. This was the core principle of the Low Frequency Radio Range. A patent for the system was filed in 1907 with a diagram that resembled a later Adcock range. In 1916 he had the foresight to update the patent to include the recently invented radio goniometer to adjust the beams. A demonstration station was set up in 1917 using the “A” and “N” letters but as with the Kompass Sender above there simply wasn’t yet a fleet of radio equipped planes to take advantage of it. In the economic devastation and severe sanctions that followed Germany’s defeat in the First World War there was little interest in pursuing it further. The idea was simply ahead of its time.
Some sources credit the United States Bureau of Standards for the invention of the LFR, and they certainly had a hand in its development. In 1919 the US Army Air Service established its Engineering Division at McCook Field in Dayton, OH (the Army outgrew it by 1927 and replaced it with Wright Field, now Wright/Patterson Air Force Base). In 1920, the Army further established an “Instrument Section” to develop the fundamental instruments needed for all weather flight. Their research led to the turn and bank indicator and gyrocompass that gave a pilot a sense of “up” and direction in the clouds, but a solution was needed for navigation. That year, they asked the Bureau, the government’s main R&D arm at the time, to develop a suitable radio beacon.
In response, between 1920 and 1923 Bureau scientists Percival Lowell, Francis Dunmore and Francis Engel developed a “directive type” radio beacon that used two compact loop antennae to create a four-course system using the letters “A” and “T”. Sources vary as to the extent that they were aware of Scheller’s design – but vertical antennae were not used and the system lacked a goniometer. The antennae crossed at 45° to accentuate two of the beams along a main course that was assessed. A station was successfully tested in Washington DC by land and sea in May 1921, and in the air by the US Army Air Service and Signal Corps at Dayton in 1923. However, its rudimentary spark gap transmitter required that a plane drag a weighted 200’ antenna behind the aircraft to receive the signals. It was a promising experiment but it was not quite ready for practical commercial use. For reasons unknown, the Bureau’s funding for further development was cut off in 1923.
Although the Bureau was temporarily out of the picture, the US Army Air Service and Signal Corps continued their experiments and were apparently aware of Scheller’s patent. An improved beacon was set up in Dayton by 1924. By 1925 at Monmouth, IL, General Superintendent of the US Airmail Service Carl Egge and Edward Warner of M.I.T. gained permission to set up an “Equi-Signal Radio Beacon.” The Transcontinental Airway had been in service for a year, and it was probably becoming clear by then that the tower beacons were still no match for clouds. It was built under supervision of McCook’s Radio Laboratory which was involved in the 1923 test. It used the latest vacuum tube radios, interlocking “A” and “N” signals (chosen for their short, equal durations) and first employed a radio goniometer tested by the Signal Corps to adjust the beams – a key ability of future stations. A later addition was a three-light indicator to show if the pilot was on, right or left of course.
A September 12, 1925 Air Service News Letter describes these developments and stated “perfected, the Radio Beacon is bound to be of inestimable value.” Pilots stated it was a “very simple matter to remain on course.” It was another step closer, but still required the use of trailing wiring antennae and its cost of $6,000 per plane, or $89,000 in 2020 dollars, was prohibitive. Surprisingly, Egge faced considerable internal headwinds. At the outset, Egge’s superiors told him to avoid “experiments that do not lead to anywhere,” and a later internal review found “little of importance was being done.” Egge was accused of misappropriation of funds and had to resign. The project was terminated but not before a paper was published - as such, many in the industry were aware.
The stage was set in 1926 when Congress passed the Air Commerce Act which established the US Government’s role in regulating but also furthering development of aviation by its creation of the Department of Commerce's Bureau of Air Commerce. Both public and private sector aviation R&D boomed. At this stage there were published accounts and papers on the Monmouth Station, the Army Air Corps tests, the Bureau’s initial “directive” beacon as well as Scheller’s original patent – it would be difficult for anyone to now claim that they had a fully original ideal for LFR. Ford’s first range station would be finished later that year and in July 1926, the Bureau of Standards was given an expansive budget and a mission to find a suitable radio navigation system for the Department of Commerce. Events transpired quickly over the next few years:
- In March 1926, Ford hired Eugene S. Donovan as the chief radio engineer for its Dearborn airport. He was a 37-year-old Army veteran and civilian radio engineer that was part of the Army Dayton team that continued experimental work at McCook after the Bureau withdrew in 1923 and later oversaw the Monmouth station. The Army would also be present for later Ford tests. When he finished and licensed the Dearborn station by October 1926 it, unsurprisingly, contained nearly identical improvements, e.g. vacuum tube transmitters, the goniometer, etc. It also added marker beacons, a more robust automatic key and the ability to handle Morse communication with the pilot by temporarily suspending the signals - all later features of LFR. The subsequent patent stressed its objective was to create an “inexpensive” setup: Ford engineers “borrowed” receivers from the quickly evolving radio technology market and found the simplest, least costly units and antennae that could easily fit on board regular aircraft – the final challenges to be solved. The stations vertical masts were spread 90° apart to create 4 equal courses, similar to Scheller’s original concept. It was recognizable as a range station.
- The first test flights per Ford airport’s flight log commenced on November 8, 1926 with “radio man” Donovan on board. By February 1927, when Ford started to use its beacon to ferry auto parts through snowstorms, LFR ceased being an experiment and first became a commercially practicable system that was proven to work under adverse conditions. Ford would add a second beacon at Lansing, MI by 1928 and the system would prove “quite successful in improving the bad weather reliability of the cargo flights.”
- A February 27, 1927 New York Times article reported on the experimental beacons at Ford Airport that were “undertaken with cooperation with the Department of Commerce and Army Air Corps and several radio companies from which receiving sets were borrowed.” Department officials were “encouraged” by “splendid reception” along a 142-mile test course that pilots flew “quite easily.” It appears then that there quite a bit collaboration across the field between government and private industry, a likely exchange of ideas, and certainly wide awareness of Ford’s progress. The April Ford News stated “credit for much early development work on radio beacons of the type described is due to the United States Bureau of Standards, Air Service and Signal Corps, who have been of the greatest assistance to the company by the cooperation.”
- A month later on March 25, 1927 the Baltimore Evening Sun reported that Bureau staff checked out Ford’s progress on a test flight between its Dearborn Beacon and McCook field, Donovan’s old post. In what became a clear trend in later Bureau press releases, any reference to Ford was omitted, even though its beacon and a Trimotor were clearly used, and partial credit was given to the Bureau which had actually been absent from any testing since 1923. That said, the article credited the Army with a “large share in the development of the radio beacon.”
- After a 3-year absence, the Bureau was fully back in the game on July 1926 as described above. Construction began immediately on its College Park, MD station which started field experiments in December 1926, about three months after Ford had constructed its Dearborn station. College Park was officially “inaugurated” in May 1927. In December 1927 (10 months after the New York Times article above) the Bureau began the first demonstration runs at Hadley Field NJ, and Cleveland, OH under the direction of Section Chief John Howard Dellinger (incidentally, some using Ford Trimotors). The first three stations entered service in 1928. These new stations, the first in the US LFR network, strongly resembled Ford’s 1926 system and patent and were not similar to the Bureau’s 1923 prototype as seen above. The vertical loop antennae, vacuum tube transmitters, and other improvements they utilized were all first used by Ford and the Army. Of additional note is that Ford decided to file its patent application in May 1928.
- This is where the stories truly diverge: In reviewing the Bureau's later accounts and Air Commerce Bulletins, the Bureau credited itself with the creation of LFR . The Bureau’s 1929 treatise “Development of The Visual Airway Beacons," co-authored by Dellinger, which was intended to give a “general” history of the system, stated that it was the “result” of the Bureau’s research from 1926 to 1929 and that it was the “outgrowth of a development undertaken by the Bureau of Standards in 1920.” Remarkably, neither Ford’s beacon or patent application, or the Monmouth station, were mentioned here or in later Bureau and Department of Commerce publications. A passing reference is made to a “German” patent and the Army's work with the goniometer. The Bureau also made it clear that the system’s future would be built on two key improvements developed by Harry Diamond and Francis Dunmore: the visual reed indicator (discussed on the It’s Use page) and an experimental 12-Course range station made possible by adding a third pair of antennae.
- So, if LFR was indeed the “result” of the Bureau’s work, one would think that it would have had a solid case to appeals Ford’s patent: however, as Ford’s patent held, it appears that any challenges to it were unsuccessful. Ford’s patent was granted in 1933 – interestingly after a 5-year delay. Ultimately, Diamond would file patent No 2,172,365 for the improved Adcock type range on behalf of the Bureau, but that wouldn’t be until 1933 (granted 1939).
- In 1931, the Federal Communication Commission (FCC) denied the radio license renewals for Ford’s two LFR stations plus a third Dearborn airport station. Ford executive William Gassett appealed the decision in subsequent hearings in 1934. The FCC’s position was that the licenses were issued for experimental purposes, not for ongoing commercial use. Further, the FCC made it clear that navigation beacons, like lighthouses, should be the exclusive province of government and the Department of Commerce. The appeal was ultimately rejected and Ford’s LFR stations were decommissioned. Gassett, dejected, suggested that the Ford charge royalties on its patent – this demonstrates that Ford believed that the Department of Commerce’s range stations were using technology covered by its patent. Indeed, a 1946 Ford biography noted “the present ‘radio range’ is merely a refinement of the system originally designed by Ford Engineers.” Ford declined to demand royalties, stating “aviation is bigger in possibilities that anything else in the world. Too big to be a one-man patent concern. Many of our patents have been picked up and used by others. When a thing is useful it is bound to come into use. Patents cannot stop it.” Perhaps this was gesture to help mend fences with the US Government. In any event, its patent stood.
- A final coda: as the fiscal reality of the Depression settled across the US, the incoming Roosevelt administration terminated the Bureau’s research budget on March 4, 1933 (Inauguration Day). Further development on the 12-course version and visual indicators died with this decision, quite possibly to aviation's immediate loss: LFR would remain a 4 course, aural system. One source indicated “inter-departmental bickering” was another factor in the demise of these innovations.
Dated photographs and radio license in Ford Museum, and the excellent chronologies in Bonfires to Beacons and Beyond the Model T (see Resources) helped round out much of the timeline here and at top of the of the What it Was page.
The bottom line: Ford, through Mr. Donovan, finished the Army Air Service and Monmouth efforts to make LFR practical, which appear to have been more directly rooted in Scheller’s patent. Donovan’s hire and the Army’s presence at subsequent Ford tests would seem to imply a close working relationship between the two. Their crucial developments came after the Bureau’s nascent 1923 station that wasn’t ready for real-world service, and before its mature 1928 stations which incorporated The Army’s and Ford’s subsequent enhancements that made LFR truly viable. Certainly, there was wide collaboration across the field and new and old ideas circulated between groups. Arguably, many of the more logical improvements (e.g. vacuum tube radios) could have been intuited separately by Ford and the Bureau. But for whatever commonalties these factors led to in their final designs, Ford was the first to build a practical physical station that ordinary aircraft could use, the first to use it for a commercial purpose and, perhaps most importantly, it was first to the Patent Office. Afterwards, Ford clearly believed the Bureau and the Department of Commerce was using its innovations, but declined to enforce its rights.
It is also apparent that the US Army Air Service was the key driver behind LFR’s creation. They naturally turned to the Bureau for help in 1920 and kept its development going after the Bureau lost funding. When Ford and it resources presented itself as another avenue to achieve this goal the Army took the opportunity.
It’s not clear why the Bureau and later Department of Commerce accounts couldn't concede this. Even many modern accounts omit this history, likely as they simply relied on the Bureau’s records as gospel. It appears that the Bureau even started a PR campaign in mid-1927 in various newspapers to promote their position that they were “first.” Ford rebutted with its own articles and advertisements, culminating with its 1934 announcement that its patent was confirmed. The Bureau’s papers were instead focused on its successes and improvements possibly done to work around the patent, e.g. the 12-course and visual indicators that never came to be. Was this just a private versus public sector rivalry which led to the FCC’s ultimate push to get Ford out of the beacon business? Did the Bureau’s academic scientists, stymied by earlier funding cuts, have difficulty giving Ford’s well-resourced “commercial” engineers (and others) any credit for apparently making “their” earlier concept practical? On the other hand, the story of large corporations using the patent process to “appropriate” the honest work of others isn’t entirely unheard of. This is notwithstanding the fact that both parties heavily relied on prior innovations. A century later, it may be impossible to exactly ascertain what transpired, but it’s clear that Ford and the Bureau didn’t see events eye-to-eye. If anyone has any additional info that can clarify this history, please reach out to me!
Regardless, in the end, it was the US Government that awarded the patent to Ford and the US LFR network came to be only after Ford’s successful 1926/27 stations. Making the Low Frequency Range a practical reality was likely Ford’s most important contribution to aviation, outliving its famed Trimotor and the entire aviation division which shut down by 1933 due to the Depression. But, as with many inventions, Ford’s patent was rooted on prior efforts: every step that led to it heavily borrowed from previous ones. These included not only the Army’s 1923-1925 improvements and the Bureau’s early work, but Scheller’s original patent and the important addition of the Italian radio goniometers.
Ford’s achievement should also not diminish the Bureau’s many other accomplishments. Diamond would invent the first Instrument Landing System and made key improvements to the proximity fuse, one of the technologies vital for winning World War II. Dellinger had earlier helped popularize the word “radio” (vs. "wireless") in his efforts to standardize international conventions, would discover how solar flares impacted radio communications and would eventually have a lunar crater named after him. The College Park station was the birthplace of radio voice communication in aviation, it standardized LFR station design and the Bureau would continue to make important refinements to the system (including adding the British Adcock antenna). Renamed the National Institute of Standards and Technology (NIST) in 1988, the Bureau had gone on to develop the atomic clock fundamental to GPS, improved nearly everything from computers to missile systems and developed numerous innovations that are part of daily life from automobile standards to closed captioning systems on TV.
After nearly 30 years, the 1954 Civil Aeronautics Administration “Pilot’s Radio Handbook” would clearly acknowledge Ford’s role and patent.
QUESTION 5:
Who was Eugene S. Donovan?
Mr. Donovan did not appear to leave a legacy of scientific papers, articles and Wikipedia entries like some of his contemporaries referenced on this site; however, given his key role in the history of LFR we thought we should attempt to develop a biography for him. Thanks to sources such as Ancestry.com we can distill some details of his life, family and career:
- Social security records show that Eugene Smith Donovan was born in the unincorporated town of Redden, Sussex County, Delaware on January 20, 1891 to Enoch W. Donovan and Arian Smith.
- The 1900 Census shows that 9-year-old Eugene lived with his father, 4 other brothers and a sister. The mother was not present in the household at of the time of this census.
- At the time of the 1910 Census,19-year-old Eugene lived with his brother in Delaware and listed his occupation as a wage earning “farmer.”
- An Army Service card showed that Mr. Donovan served in the US Army from 1917 to 1918 (during World War1), achieved the rank of Sergeant and attended the Army Air Service School for Radio Mechanics. Per later census records, at some point he completed a 4-year college degree.
- Marriage records show that Eugene married Matilda De Hart in New Jersey in 1920. They had at least two sons Donald, born 1922 and Thurman born 1925.
- The 1925 “Official Register of the United States”, a listing of all government personnel, showed Eugene working as an “Assistant Radio Engineer” for the US Army Signal Service in Dayton with a $3,000 salary (about $57,000 in 2020 dollars). His legal residence was in New York.
- A March 30, 1926 Dayton Herald article stated that Eugene had been hired as the “chief radio engineer” for the Ford Airport in Dearborn, MI. In 1928 at age 37 he would file the patent for its range beacon.
- The 1930 Census shows that Mr. Donovan was still working for an “Auto Factory” and lived at 1314 Berkley in Dearborn. The house appears to no longer stand or has been renumbered.
- In 1934 the Army Service Card listed Philadelphia as his residence.
- A 4/25/1937 Indianapolis Sunday Star article shows a picture of Mr. Donovan, now a Civil Aeronautics Bureau (CAB) Radio Technician, attending an initial test of the Lorenz Beam Instrument Landing System at that city’s airport.
- By the 1940 Census he and his family had returned to Dearborn, Michigan.
- The 1948 Official Register, shows Eugene was a “Chief Radio Engineer, Landing Area Communication Unit” for the CAB with a $7,102 salary (about $75,000 in 2020 dollars). A city directory listed him in Silver Springs, MD. The 1949 Register showed that he became the “Acting Chief, Special Projects Section” with a slight bump in pay. At 58, Mr. Donovan would have been close to retirement.
- Per Social Security records he died in June 1981, aged 90 years.
Perhaps low key, but it appears he was an earnest family and career man blessed with a long life. It would be great to find out more of this man’s story beyond these data points. If anyone can provide any more details on Mr. Donovan please reach out to me.
QUESTION 6:
Any further details on the Fan Marker Stations? Why aren’t they shown on your maps?
Fan markers stations (type “FM”) were fairly simple structures, each consisting of small shed (perhaps 10’x 10’) containing a 100-watt 75 Mhz transmitter with an adjacent row of four short horizontal half-wave length antennae elements, each about 6.5’ long, over a wire screen reflector the same distance above ground. The entire footprint could easily fit within a 20’ x 80’ area. In urban areas they could be part of other structures. Like a Z marker this antenna projected its signal upward, but with greater horizontal spread, to either form a lens shape, or a “dog bone” shape that was more pinched in the middle at the airway to provide a more precise marking of station passage. The nominal size marked on charts was 3 x 12 miles at 3,000 feet; however, the signal spread to over 6 x 20 miles at higher altitudes.
Their locations are very difficult to find for two reasons: (a) the locations on the charts were shown by the broad 3 by 12-mile shaded regions with unmarked center points and (b) the fan markers’ smaller size make them difficult to distinguish from other small structures. It was sometimes hard enough finding something as large as a range station on early aerials. Only the clearest aerials would likely show a fan marker station and their antennae, assuming that you somehow found the exact location and picked it out from other surrounding buildings and ground clutter. If researchers need to locate a fan marker, we’d recommend digitally overlaying the sectional chart over an historic aerial to identify potential candidates. .