This is the Broadcast History section of
The Broadcast Archive

Maintained by:
Barry Mishkind - The Eclectic Engineer
Last update: 6/11/17

20) What does the "AM" in AM Stations mean?

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)..

21) What is the highest power used by an AM station?

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.

  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. A Directional Array (multiple towers) "directs" power in a desired direction, increasing the "effective" power level.
  3. 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.

               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.



21a) What station has the most towers? 

21b) What station has the tallest tower? 

22) Why do some stations reduce power at night?

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.

22a) Where was the First Directional Antenna installed?

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 - KWFM - South Tucson, AZ. - Daytime: 2 kW, Nighttime: 5 kW, DA.
1160 kHz - WOBM - Lakehurst, NJ - Daytime 5 kW, Nighttime 8.9 kw, DA

There have been many more in recent years.

23) What use of subcarriers has there been on AM?

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.

24) The Growth of the Standard Broadcast Band

25) The sad AM Stereo Saga

26) Digital Audio on AM, including IBOC

27) Some interesting AM station sites