Synchronous AM
Broadcasting –
A Long and Tortuous History
By John F. Schneider, © 2019
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Synchronous AM
Broadcasting – By John F. Schneider, © 2019 |
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www.theradiohistorian.org Copyright 2019 - John F. Schneider & Associates, LLC
(Click on photos to enlarge) This graphic from the April 1931 issue of Popular Science explains the concept. 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.
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AN UNREQUITED
TECHNOLOGY: 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. CLEAR CHANNEL
TESTS: 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 POLITICS
OF BOOSTERS: 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. WARTIME
TECHNOLOGIES RENEW INTEREST 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. RECENT
HISTORY: 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 REFERENCES: Broadcasting Magazine
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.
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