This is the Broadcast History section of
The Broadcast Archive
Maintained by:
Barry Mishkind - The Eclectic Engineer
Last update: 3/12/08
Contrary to popular belief, AM does *not* mean "Ancient
Modulation." It refers to the method of modulating the amplitude, or
strength of a fixed frequency carrier to allow detection of the program matter.
The Standard Broadcast Band (using AM modulation) in the USA runs from 540 kHz
to 1700 kHz in 10 kHz steps. In other regions of the world, there are different
spacings (often 9 kHz)..
In the US, the highest nominal authorized carrier power is 50,000 watts
(although directional stations are permitted 52,500 watts TPO to compensate (5%)
for any losses in the phasing system).
There are several factors that affect the actual radiated field.
- The Tower Height: the transfer of power from an AM transmitter to
the tower is affected by the tower matching network and tower height, as a
fraction of the station's wavelength.
- A Directional Array (multiple towers) "directs" power
in a desired direction, increasing the "effective" power level.
- The Ground Conductivity can have a major impact on the station's
coverage. Salt water is considered the "best" with a ground
conductivity of "5000" ... typical desert land will be 4 or 2
(or less), farmland may run at about 15 or 30, for example.
The highest power ever authorized for an AM station in the US was 500,000
watts, authorized to WLW, Cincinnatti in the 1934. The "Nation's
Station" transmitted at super high power until 1939, when other stations
complained. WLW was limited to 50,000 watts except for some "special"
broadcasts to Germany during the War.
In 2000, WHO, Des Moines, IA, with a 300 degree tall, sectionalized
radiator has the highest reported efficiency of 471.54 mV/m/kW at 1 km. With
it's 50 kW input, the radiated field is equivalent to 85 kW input to a
"conforming" radiator.
A directional station, WWL, New Orleans generates a reported 3934.902
mV/m at 1 km.
INTERNATIONALLY - Continental Electronics has delivered AM
transmitters at the 2 Megawatt level to the broadcasting organizations of
several countries. It is reported that several fought a tendency for this level
of RF power to melt and fuse the insulators and sand around the tower.
Moving off the AM Band, Shortwave and Longwave stations can develop
tremendous power levels, both from transmitters and directional arrays.
- Shortwave - In the USA, the minimum power for a shortwave station is
50,000 watts (transmitter power out).
Most shortwave stations are directional, using various forms of antennas.
1. The Tower Height: the transfer of power from an AM transmitter to
the tower is affected by the tower matching network and tower height, as a
fraction of the station's wavelength.
2. As an example, the Greenville transmitter for the VOA runs 500 kW, but
with the antenna, the ERP can exceed 100 Megawatts.
3. The VOA Bethany site was able to focus their 200 kW transmitters on a
nine degree arc. Peak ERP was enormous.
- Longwave - Radio Luxembourg is said to run a 2 Megawatt transmitter on
this band.
- Dave Hersherger adds: "Apparently, the highest ERP is produced by the
interplanetary radar transmitter at Arecibo, Puerto Rico. Continental built
this transmitter too - it's a 1 megawatt (CW, not pulsed) S band (2300 MHz)
source. It's two klystrons, at 500 kilowatts apiece, combined for a megawatt
CW.
"The dish at Arecibo (seen in the movie "Contact") has a gain
of 74 dB (!) at S-band. And with the interplanetary radar running, the ERP
is 23.6 - not megawatts - not gigawatts - but TERAWATTS (i.e. 23.6E+12 watts
or 23,600,000,000,000 watts). I have been told that this is equivalent to
the energy flux of a small star.
"When the Arecibo transmitter was being built, it was called the "E.T.
Phone Home" transmitter. When that thing was fired up it glitched the
power line so bad that every PC that wasn't on a UPS rebooted, and the
phones all started ringing."
- There are now many stations with 9, 10, 11 and even 12 towers.
- KFXR, 1190 in Dallas has 16 active towers - four daytime, twelve at night.
- KHTN, 1070 in Houston has the most steel in the air. Their eleven towers
total 4960 feet - yes, if they were laid end-to-end, it would stretch almost
a mile!
- WRDT, Monroe, MI (Detroit), runs 14 Watts from their nighttime tower - 302.47
meters/995 feet/203.4 electrical degrees - develops the equivalent of
394.75 mV/m
- WNAX, Yankton, SD, has a 277.59 meter/913.27 foot/190 electrical
degree tower. It develops RMS of 395.9 mV/m per kW.
After dark, there are changes in the upper atmosphere (ionosphere) that
actually reflect radio signals in the AM band back toward the earth. A station
can be heard hundreds, or thousands, of miles away, even with relatively low
power levels. In order to reduce the interference suffered by the majority of
stations, some are ordered to go off the air at sundown. Others are ordered to
reduce power, and some of them must utilize directional antennae, to minimize
radiation toward the primary stations on each frequency, as well as the adjacent
channels.
- The first DA in the world was designed by Raymond Wilmotte in 1932 for WFLA-WSUN,
Clearwater, FL on 620 kHz. Its purpose was to protect a Milwaukee station, WTMJ.
Wilmotte's directional used lengths of transmission line for its phasing
elements.
- Instead of protecting another station, another reason for employing
directional antenna designs was to reduce "wasted" power sent out
over the Atlantic, and instead redirect it over land.
- The whole idea of directionality must have caught on quickly. According to
Dr. Brown's book, of 646 AMs on the air in 1937, 39 were using directional
antennas.
- In March 1935, WLW, Cincinnati added a directional antenna to
protect a station in another country. It had to restrict its power output to
"only" 50 kW toward Niagara.
- According to a number of anecdotal accounts, WOR, as well as WEAF
and WADO were experimenting with Bell Labs on this approach. Although
the first date that appears in WOR's FCC file is January 1939, the
notation could be interpreted to indicate something was in operation
earlier. Dates as early as 1933 have been suggested, although no
contemporary citation has, as yet, been discovered.
- WDAE in Tampa also had an early directional, designed by Dr. George
H. Brown of RCA, using lumped constants in a cabinet for phasing elements.
As with WOR and WEAF, the FCC files show 1939 as the official time, but
unconfirmed anecdotes place it in the mid 1930s, allegedly as the second one
built.
22b) Are there any stations that raise power at night?
There are not a huge number of these, but there are more and more have been
designed and installed in recent years, since the FCC began permitting "non
standard" power levels on any AM channel. The idea is nighttime protections
are different on some channels, so more power is permitted than when, for
example, a daytime allocation exists locally to the station. Another reason is
that most stations prefer a Non-Directional day signal of lower power than a
directional array that might actually reduce coverage in some directions.
Some examples:
620 kHz - WSNR - Newark, NJ - Daytime: 3 kW, Nighttime: 7.6 kW
950 kHz - Chicago, IL - Daytime: 1 kW, Nighttime: 5 kW.
1330 kHz - KJLL - South Tucson, AZ. - Daytime: 2 kW, Nighttime: 5 kW, DA.
WIOD-AM in Miami for years relayed the U.S. Navy's "Fox Schedule" via +/-
10 Hertz FSK of its carrier, presumably for the benefit of the U.S. Naval Base
at Guantanamo Bay, Cuba.
Later uses included pilots for stereo operation, "remote control"
of transmitters, and "load transfer" signals for utility companies. (A
PSK system somewhat like the WIOD FSK system was being used [in 1996] by the BBC
to transmit control signals for the electric power industry in Britain. This
uses the three synchronized long wave transmitters of Radio 4 [one 500 kW, the
other two 50 kW each] which reach all of the UK day and night..
A particularly interesting feature common in the 40's and 50's was facsimile
transmission, primarily from stations owned by newspapers as they tried new ways
to deliver the news.
- The Standard Broadcast Band began in 1921 with just one frequency (833
kHz).
- In December 1921, 619 kHz was added for Weather and Market reports.
- In September 1922, 750 kHz was added as a new
"Entertainment" frequency.
- On May 15, 1923, the Standard Broadcast Band was expanded to 550-1360
kHz using 10 kHz steps.
- In April 1925, this was further expanded to 550-1500 kHz.
- November 11, 1928 saw the first big "realignment" as the FRC
shifted many stations, and deleted others, in an attempt to reduce
interference.
- On March 29, 1941, the band was extended to 550-1600 kHz. On this
date, the NARBA frequency shift changed many stations spot on the dial.
- In the early 1950s, the band was extended to 540-1600 kHz. The
first 540s appeared in 1954 or 1955.
- In mid-1961, most Class IV stations were permitted to increase power from
250 Watts to 1/.25 kW.
- In 1983, most Class IV stations were permitted to increase power from
1/.25 kW to 1 kW.
- In 1995, the FCC again extended the Standard Broadcast Band to 540-1700
kHz.
WJDM, 1660, Elizabeth NJ, was the
first U. S. regular broadcast station to operate in
the expanded band, using 10 kW
day, 1 kW night.
25) The sad AM Stereo Saga
- 1963 - WHAZ runs a stereo program on eight AM stations, four on each
channel.
- 1960 - AM Stereo first demonstrated on XETRA, Tijuana, MX, using the Kahn
ISB system.
- 1984 - The FCC begins AM Stereo testing with five systems. Eventually
selects the Magnavox system.
- November 23, 1993 - FCC makes Motorola's C-QUAM the AM stereo
standard
- Lawsuits and other issues result in the FCC permitting "the
marketplace" to decide.
- AM Stereo dies a slow, lingering death, as more and more stations adopt
talk or foreign language programming, and kill the stereo subcarrier.
- Digital broadcasting offers some hope, but the existing AM stereo systems
lose more support.
26) Digital Audio on AM, including IBOC
- June 16, 2003 - DRM debuts around the world.
- November 2003 - A new codec is released, but the nighttime
interference problem persists.
- 2004 - As stations install more and more equipment, the hope is that
receivers will appear.
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