AM Broadcast

AM BROADCASTING

Home...Home..Take Me Home...

AM Antennas

In AM broadcasting, for a vertically polarized antenna, the tower itself is the antenna. Normally the antenna is operated above ground - that is, it sits on top of a base insulator. Under every AM broadcast tower there are 120 ground radials that extend out approximately the height of the broadcast tower. Their purpose is to reduce the ground losses and thus improve the overall efficiency of the vertical antenna.

The first AM broadcast is credited to KDKA, Pittsburg, Pennsylvania. The Harding-Cox election returns were broadcast on November 2, 1920 from a small wooden shack on top of the "K" Building at the East Pittsburg Works.

Common Terms

ND - Non directional Antenna

DA - Directional Antenna

DA1 - Operates with One DA Pattern

DA2 - Operates with Two DA Patterns

DAN - Operates with DA at Night

DAD - Operates with DA at Day

Null - Minimum signal in a given direction

Main Lobe - Maximum Signal in a direction

Monitor Points - FCC designated points used to check DA for proper operation.

In the beginning, AM broadcast antennas were simple. They were non-directional antennas usually consisting of a horizontal wire strung between two towers, with a vertical feed wire that dropped down. This formed a type of "T" or inverted "L" antennas.

The problem with those antennas was that the surface wave (ground wave) attenuation was very high. AM broadcasting relies heavily on ground wave propagation characteristics.
Vertical polarized antennas are now commonly used because of their superior ground wave characteristics, and their simpler design capabilities.

Vertical Scroll:

Typical Pattern for 3 Tower DA

The first directional antenna was introduced by Ray Wimotte. It was filed with the FCC on 5/27/32 and granted on 10/15/35. The antenna was filed for WFLA-WSUN in Clearwater, Florida to protect WTMJ, Milwaukee, Wisconsin.

Tower Height

The height of an AM tower is usually described in terms of electrical degrees, or wavelengths, rather than a physical height. This is based on the velocity of an electromagnetic wave, which is 3 X 10⁸in free space. One wavelength, or 360 electrical degrees, can be determined from the formula 300/f (MHz). It should be noted that the physical height of the tower is inversely proportional to the frequency. The higher the AM frequency, the physically shorter the tower is. Thus, a 360-degree tower at 540 kilohertz would be 555.56 meters in height, while at 1700 kilohertz, the same 360 degree tower would only be 176.47 meters. (To check the height of a tower in electrical degrees, go to Degree Calculator)

AM broadcast towers will not be one wavelength in height, or 360 electrical degrees. The ground wave signal begins to decrease above 5/8ths wavelength, or 225 electrical degrees. In fact, the ground wave signal will drop to zero for a one wavelength tower. A height of 190 degrees, or 0.53 wavelengths, is generally accepted as the maximum height for an AM broadcast antenna. If the AM tower is used to support another antenna (such as an FM antenna), then procedures are done to alter the electrical and physical heights.

When speaking of the tower height, AM broadcast engineers will typically speak in terms of the electrical degrees of their towers, such as 90-degrees, or 110-degrees. A 90-degree tower is a very common broadcast tower, remembering that a 90-degree tower at 540 kilohertz will be 138.89 meters, while a 90-degree tower at 1700 kilohertz will be 44.12 meters. A 90-degree tower is also called a quarter wavelength tower, a 180-degree tower is called a one half wavelength tower, and a 225-degree tower is called a five eighths wavelength tower.

A 90-degree tower, regardless of the frequency of the AM broadcast station, is considered an efficient tower (or efficient radiator), as long as the required ground system is installed below the tower as required to minimize ground losses. However, as will be shown, a 90-degree tower at another location from the transmitter site can also be an efficient re-radiator as well. This is one of the primary factors that causes Wireless Towers to distort the broadcast antenna pattern.

Why do AM broadcast stations change power at night, or switch antenna patterns??


As of March 31, 2003, there were 4,804 AM broadcast stations....

Directional and Non-Directional Stations

Many of the licensed AM broadcast stations operate with a single tower in what is referred to as non-directional. In theory, a non-directional station broadcasts with equal signal strength (which is measured as field strength, in mV/m) in all directions. In reality, the ground conductivity, terrain and other factors may alter that, but for purposes of discussion, the non-directional (ND) station will be considered a circle, with equal signal strength in all directions.

If another tower is placed in close proximity to the first tower, the non-directional antenna pattern will be changed. By varying the amount of power in each tower, and phase relationships of the towers, and the spacing between the towers in electrical degrees, a directional antenna pattern (DA) can be formed. By controlling the three factors, more signal can be placed in one direction than another. The main purpose though for the directional antenna system is to protect other AM broadcast stations that operate on the same frequency or an adjacent frequency from interference

If you consider that each tower is radiating a signal based on the power, phase and spacing, at a distance removed from the transmitter, the signals will add or subtract from each other. The system is designed so that a minimum signal (null) can be placed in directions to protect other broadcast stations from each other. This allows more AM broadcast stations to exist without interference.

Take as an example the directional antenna pattern on the right.

This pattern has nulls at approximately 45-degrees and 225-degrees.
In reality though, the nulls will not go completely to zero, but will
go to a minimum. Note also that at approximately 350-degrees, the
pattern does have a main lobe, although not as great as the 180-
degrees lobe. This main lobe can put a greater signal into an area
that does not require quite as much protection, but probably still is
protecting another station.

Two items should be noted here.

1. The area of the radiation pattern hasn't changed, just its shape.
     Field strength removed from one area is placed in another area.

2. Nulls are symmetrical - that is, a null at 45-degrees will also
     have a null at 225-degrees, which is 180-degrees
     different.

Directional Broadcast stations can have as many as twelve towers, and as few as two. It is also easy to spot a directional broadcast transmitter site as the towers will be grouped together.

`


Excuse me - I have a question?

Partial Proof of Performance

The basics of a Partial Proof of Performance are as follows:

1. 8 Field strength Measurements on each Radial that contains a Monitor Point, with a minimum of   4 Radials.

2. Measurements should be taken from 3 to 15 kilometers.



Transmitter Output Power

The transmitter output power of an AM broadcast station during the daytime hours varies from a low of 0.250 kilowatts (250 watts), to a high of 50 kilowatts (50,000 watts). In other countries, especially in Europe, 1,000,000 is not uncommon. The FCC limits the output power in this country so as to not cause interference to Canada and Mexico. The FCC has divided the broadcast band in Clear, Regional and Local Channels (a channel being an individual frequency, such as, 1230 kilohertz). It also is divided in four Classes: A, B, C and D. More information on this can be found at www.fcc.gov/mb/audio/amclasses/html.

A AM broadcast station will not measure their power in watts, but with a current meter that is inserted at the base of the tower for a non-directional broadcast station, or at the Antenna Current and Phasing unit (Phasor), usually located inside the transmitter building, for a directional broadcast station. The basic Ohms Law is used, p=I²XR, with R being the impedance at the measurement location, to determine the output power. The stations broadcast license will typically specify a current that the station is to operate at.

The amount of re-radiation that can be caused by the construction of another tower is somewhat determined by the power output of the AM broadcast station. Low power broadcast stations are less likely to be affected than high power stations. A further explanation will be provided in the Tutorial section.

Why AM Radio Stations Must Reduce Power, Change Operations, or Cease Operations at Night

Most AM radio stations are required by the FCC's rules to reduce their power or cease operating at night in order to avoid interference to other AM stations. Our rules governing the daytime and nighttime operation of AM radio stations are a consequence of the laws of physics. Because of the way in which the relatively long wavelengths (see Footnote 1) of AM radio signals interact with the ionized layers of the ionosphere several miles above the earth's surface, the propagation of AM radio waves changes drastically from daytime to nighttime. This change in AM radio propagation occurs at sunset due to radical shifts in the ionospheric layers, which persist throughout the night. During daytime hours when ionospheric reflection does not occur to any great degree, AM signals travel principally by conduction over the surface of the earth. This is known as "groundwave" propagation. Useful daytime AM service is generally limited to a radius of no more than about 100 miles (162 km), even for the most powerful stations. However, during nighttime hours the AM signals can travel over hundreds of miles by reflection from the ionosphere, a phenomenon called "skywave" propagation. (Shortwave stations, which operate using AM modulation on several bands between 2.3 MHz and 26.1 MHz, also use this phenomenon to broadcast still greater distances, up to thousands of miles). Because of this change in signal propagation from daytime to nighttime, if every AM station kept its daytime operating power at night, massive interference would result. (See also Hours of Daytime-Only AM Broadcast Stations, First Report and Order, BC Docket 82-538, 95 FCC 2d 1032 (1983) for a similar description.)

The FCC was established by the Communications Act of 1934, and was given the responsibility of making a "fair, efficient and equitable distribution of radio service" to the various communities of the United States. In order to meet this responsibility, and in recognition of the physical laws which govern AM radio propagation, the FCC established three basic classes of AM stations (Clear Channel, Regional, and Local), each of which operates on specified frequencies, or channels There have been some changes since then, but the main features of the plan have remained in place. Here is a quick summary of the three types of AM stations:

Footnote 1: The wavelengths of AM radio signals vary from a maximum of 555 meters (1821 feet) at 540 kilohertz (kHz), the lowest AM carrier frequency used in the United States for broadcast stations, to a minimum of 176 meters (579 feet) at 1700 kHz, the highest carrier frequency at the upper end of the AM expanded band. In contrast, the wavelengths of FM radio signals are much shorter; i.e., approximately 3 meters (10 feet). Because of their much shorter wavelengths, FM signals are not significantly reflected by the ionosphere.

Footnote 2: This is somewhat of an oversimplification, in that there are actually two classes of clear channels. On some of the clear channels, there are two dominant stations, which are separated by great distances and which employ directional antennas during nighttime hours in order to protect each other's skywave service areas.

Questions on this subject may be referred to Dale Bickel (dbickel@fcc.gov) at (202)-418-2700.

This page is located at http://www.fcc.gov/mb/audio/bickel/daytime.html.

                        Courtesy FCC.Gov
AM broadcast stations must follow rules and regulations extablished by the Federal Communications Commission...Some of these rules and regulations required AM stations with directional antennas to maintain and monitor certain parameters to ensure their pattern is within its licensed limits...stations are required to monitor the ratio of currents and phases from a reference to other towers in its array....They are also required to take field strength measurements at certain locations which helps establish that the pattern is working as designed...These points are called the stations Monitor Points...