Synchronous AM Broadcasting –
A Long and Tortuous History

By John F. Schneider, © 2019


Copyright 2019 - John F. Schneider & Associates, LLC

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(Click on photos to enlarge)

  Synchronous AM diagram

This graphic from the April 1931 issue of Popular Science explains the concept.

WBZ Synchronous System

  WBZ in Boston synchronized its frequency with booster station WBZA in Springfield, Mass., beginning in 1926. The dual station operation — first on 900 kHz, then 990 and finally on 1030 — lasted until 1962, when Westinghouse was forced to shut down WBZA in order to purchase WINS in New York.

WBBM & KFAB Synchronous coverage map

 WBBM in Chicago and KFAB in Lincoln, Neb. (later Omaha) synchronized operations on the 770 kHz clear channel frequency from 1934 to 1941. This map showed their respective groundwave coverage areas. At nighttime, both stations broadcast the same CBS network programs, generating what was essentially a single received skywave signal.

Western Electric ad

This 1934 ad promoted Western Electric's synchronous AM transmission system, as imiplemented by WBBM and KFAB.


A 1934 trade magazine advertisement promoting the combined stations WBZ in Boston and WBZA in Springfield.


This 1933 advertisement promoted the synchronous operation of WHO in Des Moines and WOC in Davenport.  WHO had recently increased power to 50,000 watts, and its synchronous operation with WOC would end shortly afterward.

WLLH magazine cover

Cover of a 1940s promotional booklet for WLLH in Lowell, Mass. The station began operating a synchronous booster in neighboring Lawrence in 1937. The booster received a license from the FCC in 1941, the only known case of a booster receiving anything more than experimental authorization. WLLH continues to operate with two 1 kW transmitters on 1400 kHz today.

WLLH two transmitter sites

This is a view of the WLLH two-transmitter operation in the 1940s. At left was the station’s main antenna in Lowell. At right was the 150-foot tubular steel tower on the roof of the Cregg Building in Lawrence.

WOL synchronization
This 1954 article from Broadcasting Magazine described the synchronous booster operation at WOL in Washington, D.C.




For the past few years, as AM improvement has been on the radar of the FCC and radio broadcasters, there is renewed talk about the subject of AM “boosters” – the frequency synchronization of multiple transmitters.  In 2017, the FCC opened a comment period on the subject of licensing AM boosters, but this is not the first time the FCC has explored – and failed to act – on this topic.  In fact, AM boosters have been considered and tested numerous times since the Commission’s predecessor, the Federal Radio Commission, first took up the topic in 1930.  But even though the technology has repeatedly been proven effective, and has been widely utilized in Europe, American regulators have consistently declined to allow the licensing of AM boosters, for a variety of reasons.  Let’s take a moment to look back at the history of this beleaguered technology.

In 1930, crystal control of transmitter frequencies was still an emerging technology, and the allowable frequency tolerance of a broadcast transmitter was +500 Hz.  Two stations operating on the same channel, even if widely separated, could generate a heterodyne note of up to 1 kHz – a disconcerting annoyance to listeners.  Accordingly, only a few stations were allowed to operate nationwide on a single channel at the same time.  Also, there were forty clear channel stations, meaning that each one had exclusive national use of the frequency.  As most of these clear channel stations were network affiliates, it meant that many of these clear channels were wastefully duplicating the same programs.  In 1929, the respected radio engineer Frederick Terman proposed that, if stations could synchronize their carrier frequencies (within + 0.1 Hz) to eliminate the heterodyne beat notes, they could coexist on a single channel, freeing up spectrum.  Terman proposed that all NBC and CBS stations broadcast on two just frequencies, freeing up dozens of new channels.

Synchronization had already been proved successful by the Westinghouse station WBZ in Springfield, Massachusetts.  Operating from the roof of the Westinghouse factory, WBZ did not cover Boston well, and so WBZA was opened in Boston and the two stations synchronized on 900 kHz beginning in 1926. (The two stations later swapped places, with WBZ becoming the primary station in Boston. The frequency was later changed to 990, and then to 1030 kHz.) 

The system developed by Westinghouse used a tuning fork as a frequency reference. Synchronization improved the coverage in some areas, but it also created interference zones with distortion and fading in areas where the two signals were roughly equal in strength.  It took a year of experimentation before a successful technology was found:   a master crystal oscillator in Springfield set the frequency for both transmitters.  Its frequency was divided down into the audio range and sent by phone line to Boston, where it was multiplied upward again to drive the WBZ transmitter.


The pressure on the networks to test synchronization was building, and in October, 1930, the Radio Commission oversaw an overnight test on a single clear channel frequency, 660 kHz.  Three NBC 50 kW stations – WEAF, KDKA and WGY – all broadcast the same program on the channel.  It worked remarkably well - long-distance skywave reception was greatly improved, and fading was virtually eliminated because as one station’s signal faded, the others would continue to provide clear reception. 

Although NBC couldn’t deny that the technology had worked well, it still resisted.  The network’s true reasons were not technical, but economic.  It had become common practice to feed different network programs to different parts of the country to serve advertisers who only wanted regional coverage.  With synchronization, such “split network” operation would no longer be possible.  Also, NBC feared that giving up its clear channels would lead to the creation of competitive networks using the same system.

Nonetheless, NBC supported synchronization in the cases of several affiliate stations who were sharing time on clear channel frequencies, thus allowing these stations to operate full time.  Starting in 1931, it synchronized its New York flagship station, WEAF, with WTIC in Hartford, but the stations were too close to each other and the experiment ended due to interference.  But its attempt at synchronizing WJZ in New York with WBAL in Baltimore was more successful, and this operation continued until 1937.   In both cases, synchronization was accomplished by sending a 5,000 Hz audio tone to each station by phone line from a central location.  For its part, CBS also experimented with the synchronization of WABC and WCAU.

Then in 1931, CBS applied to the Radio Commission to build a 250 watt synchronous booster in Washington, DC, to repeat the programs of its key station, WABC in New York (now WCBS).  But the commission rejected the application because of the objections of other Washington stations, and because it would put the region “over quota” based on the Davis Amendment to the Radio Act, which apportioned the number of stations equally around the country.  (CBS subsequently purchased WJSV in Washington.)

Another successful synchronization experiment was with WHO in Des Moines and WOC in Davenport, Iowa, starting in 1930.  Both stations were owned by B. J. Palmer, operating the two stations together on 1000 kHz under the dual call sign “WHO-WOC”.  Synchronization was accomplished using a new system developed by Bell Labs.  An operator at a monitoring station halfway between the two stations monitored the beat note between the stations and remotely adjusted WOC’s crystal oscillator every ten minutes.  The system was successful, but the operation ended in 1933 when WHO increased its power to 50 kW.

Another technology developed by Western Electric was used to synchronize WBBM in Chicago with KFAB in Lincoln, Nebraska.  The two CBS stations had been time-sharing the 770 kHz frequency since 1928.  They synchronized their carriers in January, 1934, broadcasting separate programs during the daytime and the same CBS program at night.  A highly accurate 4 kHz signal was delivered to both stations by phone line, where it was multiplied up to the carrier frequency and compared with the transmitter.  A motor-driven variable capacitor adjusted the transmitter’s crystal oscillator.  A unique part of this system was an audio delay line that retarded the programs of WBBM by about 20 milliseconds to compensate for the phone line delay from Chicago to Lincoln.  The combined nighttime coverage of these two stations was excellent with almost no fading, and it continued in operation until KFAB moved to 1110 kHz in 1944.

Synchronization experiments were not limited only to the high-powered stations.  In 1936, WLLH in Lowell, Massachusetts, received permission to operate a booster nine miles away in the city of Lawrence.  This was the first time that synchronization had been authorized on a local channel, using 250 watts in Lowell and 100 watts in Lawrence.  The booster in Lawrence was said to add 250,000 people to WLLH’s service area.  The special temporary authority became permanent when a license was issued for the booster in 1941.  It continues in operation today, the only booster ever licensed by the FCC.  All other booster operations in the United States have been permitted as temporary experimental authorizations. 


The FCC’s attitude towards boosters seems to have change towards the end of the 1930s, apparently for political, rather than technical, reasons.  In 1939, during an FCC oversight hearing, Congressman William Connery of Massachusetts declared his opposition to booster stations.  He stated that they created new stations in new communities, but provide no employment in that community and succeeded in diverting advertising from local newspapers.  He named several boosters operating in his own district.  It’s likely that complaints about WLLH from a Lawrence newspaper had reached his ears.  The result was that the FCC started to oppose boosters that added coverage in a new community instead of filling holes in a station’s existing coverage.  About a dozen booster requests were denied for this reason.

In 1941, It made an exception to this policy out of necessity for WBT in Charlotte, North Carolina.  When the NARBA Treaty frequency reallocations caused KFAB to move onto WBT’s 1110 kHz frequency, WBT was forced to operate with a nighttime directional antenna to protect KFAB.  To mitigate its loss of coverage, the Commission allowed WBT to build a 1,000 watt booster in Shelby, NC, operating only at night.


The FCC’s interest in booster technologies seemed to revive again during and after World War II, although its treatment of the subject was inconsistent and seemed to be based on political and strategic interests.  Powel Crosley’s WSAI in Cincinnati was allowed to feed a 100 watt downtown booster from 1942 to 1945, but only during the daytime.   In 1944, a new booster was granted to WRBL in Columbus, Georgia in order to cover Fort Benning.

In 1944, the FCC approved two boosters to operate in its own city of Washington, DC.  WWDC on 1450 kHz was allowed to install a 50-watt booster in Silver Spring, Maryland, and WINX was granted a 250-watt booster in Arlington, Virginia. 

After World War II, manufacturers who had developed new military radio technologies sought to find peacetime uses for them, and one of their targets was broadcast synchronization.  In 1945, the Radio Manufacturer’s Association established a “Subcommittee on AM Satellite Transmitters” to set standards for an improved synchronizing system.  It proposed setting aside nine channels in the 1 gHz region for one watt microwave links.  They proposed transmitting the entire modulated AM carrier from the master station to its booster as a microwave subcarrier.  In 1948, WINX added a second booster, presumably using this system.  Its 250 watt boosters in Arlington and Bethesda increased nighttime coverage threefold.  Although the microwave booster system was never implemented commercially, the proposal foreshadowed the later development of microwave studio-transmitter link (STL).

But soon afterwards, the FCC seemed to have set an internal goal of eliminating AM band boosters, even while promoting the concept for TV and FM stations, and it increasing made decisions that resulted in the elimination of existing AM boosters.  This, plus the changing economics of AM radio in the face of competition from television, caused the shutdown of most of the boosters still in existence.

In 1954, the FCC ruled that Class IV (local) stations could not use boosters to extend their coverage areas because it was against the purpose of a local station license.  In 1954, the renewal of the booster permit for WOL, who had taken over WWDC’s facility, was set for hearing.  Under apparent pressure from the FCC, it decided to withdraw its application and terminate the operation.  For its part, WINX (now known as WOOK) moved its main transmitter to its downtown studios in 1951 and discontinued its two boosters.  It instead filed for a new booster in Rock Creek Park, but the application denied in 1955 “since Commission rules do not provide for such operation”.

Another factor in the elimination of boosters was the FCC’s “7-7-7” rule, adopted in 1953.  It prohibited any station group from owning more than seven AM, FM or TV stations in the country, and the Commission counted booster transmitters as one of the seven stations.  In 1953, CBS was forced to shut down its booster at WBT Charlotte in 1953 to comply with the FCC’s new ownership limits.  And in 1962, when Westinghouse wanted to purchase WINS in New York, it was forced to shut down WBZA in Springfield, even though it had operated successfully for nearly forty years.


The FCC’s Jekyll-and-Hyde attitude towards AM boosters has not changed in modern times.  In 1987, the FCC showed renewed interest in synchronous booster technology, and it opened up a period of comments under Docket 87-6.   A station that was allowed to experiment with a booster that year was KLSQ in Laughlin, Nevada, a 10 kW directional station on 840 kHz.  The station was allowed to build an experimental three-tower 5 kW booster site in Henderson, on the outskirts of Las Vegas.  Tests of the combined operation showed excellent coverage in both communities, and the booster operated for at least seven years before all operations were consolidated at the Henderson site.

Another system installed about that time was at KKOB in Albuquerque, New Mexico, whose new 50 kW nighttime array placed a major null squarely over the state capitol in Santa Fe.   As KKOB was the primary emergency station serving the capitol, the FCC allowed construction of a 230 watt nighttime-only booster in Santa Fe, which continues to operate to this day.

But once again, after two years of comments and deliberation, the Commission chose not to decide.  It closed its Docket 87-6 proceedings in 1989, stating that it would continue its policy of authorizing boosters on a case-by-case basis using experimental authorizations. 

Beginning in about 2000, WISO in Ponce, Puerto Rico, was allowed to operate synchronized boosters in Aguaduilla and Mayaguez under experimental authority.  But in 2011, the FCC denied WISO’s request for an additional booster station in Guayama, and in 2017 it abruptly cancelled all of WISO’s experimental permits, without a clearly-stated reason and despite letters of protest from government authorities in Puerto Rico.

Most recently, the FCC once again opened a rulemaking proceeding to consider permitting AM boosters, part of its AM Improvement initiative, under Docket RM 11779.  Supportive comments have been received from the AFCCE, Kintronic Laboratories, and dozens of other interested parties.  They noted that carrier synchronization, a complex technology in the 1930’s, is now easily and accurately accomplished thanks to GPS satellites.  But yet again, the FCC has so far failed to take any action.  Will this be the time that the Commission finally takes positive action on AM boosters, or will Lucy once again pull the football away from Charlie Brown in the final seconds?

This article originally appeared in the January 27, 2020, issue of Radio World Magazine


Broadcasting Magazine

  • 10-15-31; 11-1-31; 12-1-31, CBS Washington, DC, booster application

  • 11-1-31, “Action Awaited on CBS Request”

  • 1-15-32, “Synchronization status considered at hearing”

  • 5-1-36, “WLLH Granted Right to Test Booster Station”

  • 5-15-36, “WBZA Shift to 550 kc. Sought by Westinghouse”

  • 5-1-37, “WLLH Operating Booster Station”

  • 11-1-36, “Successful Duplication of WTIC and KRLD”

  • 1-1-39, “Satellite Station Plan of WFBR Recommended”

  • 5-1-39, “Sentiment for FCC Legislation”

  • 1-1-41, “More Boosters are Granted”

  • 2-15-44, “New Grants under FCC”

  • 10-1-45, “Boosters for All AM Stations Possible”

  • 1-7-46, “Seven League Boosts”

  • 3-3-47, “FCC to Study Booster Pleas”

  • 7-28-47, “WBT Extends Coverage With Booster Station”

  • 5-17-48, “WINX Operates Two Boosters”

  • 8-6-51, “WINX Transmitter Move Granted”

  • 7-12-54, “FCC Rules Against Booster Bids”

  • 5-7-62, “Westinghouse Buys WINS”

Federal Radio Commission Annual Report, 1930:  “Synchronization of Broadcast Stations”

“Popular Science Monthly”, April 1931, “Chain Broadcasts On One Wave”

Wikipeda, WBZ History.

Radio World, 1-14-2017, “Rackley on Synchronous AM Boosters”

“A Wavelength for Every Network” by Michael J. Socolow, University of Maine, 2007.

Proceedings, Institute of Radio Engineers, March 1936:  “Present Practice in the Synchronous Operation of Broadcast Stations, as Exemplified by WBBM and KFAB”, by L. McC. Young, Columbia Broadcasting System.

Federal Communications Commission, MM Docket 87-6, 3-3-87, “Amendment of Part 73 to Authorize the Use of Multiple Synchronous Transmitters by AM Broadcast Stations”,

“KOB and Decades of Conflict”, by Mark Durenberger, 2017.