Medium waves or hectometric waves are electromagnetic waves with frequencies in the range of between 300 to 3000 kHz.  The broadcasting band in MW, called the AM band, contains the frequencies allocated to radio broadcast.  These are from 535 kHz to 1705 kHz.

IBB MW stations receive broadcast programs from production studios via satellites and retransmit these programs to designated areas via frequency carriers in the AM band.  RF power for the carriers is provided by transmitters.

Medium wave transmitters in the IBB network typically deliver 500 or 1000 kilowatts of power and operate on a single carrier frequency in the AM band.   Highly efficient all solid-state 1000kW transmitters that are being phased into the network will improve coverage while reducing operating cost.  The solid-state transmitters are ideally suited for use in an automated or remote controlled environment that is now commonly found in our broadcast facilities.  

The RF power carrying the broadcast program delivered by the transmitter is then fed to a device called antenna to be transformed into radio waves.  The simplest antenna can be a vertical radiating tower which delivers RF energy equally in all azimuths.  This type of antennas is called omnidirectional.   Most frequently, in our IBB network, directional arrays consisting of many vertical radiating towers are used.  This type of antennas allows RF energy to be concentrated in certain directions while reducing potential interference to other broadcast.

Click on the left to see a map showing locations of IBB MW Stations.

Medium waves propagate in space by ground waves and sky waves.

1. Ground waves  consist of the direct wave, the reflected wave from  ground, and the surface wave which follows the curvature of the earth by diffraction.  Figures (a) and (b)  illustrate these ground waves.  Ground waves are stable, therefore reliable, and form the basis for domestic coverage.

2. Sky waves are the waves reflected from ionospheric layers.   (Figure c).

The ionosphere is a region of the upper atmosphere in which the constituent gases are ionized by extreme ultra-violet and x-rays radiations emitted by the sun.   Sky waves can bounce off between the surface of the earth and ionospheric layers to reach distant locations.

In the medium frequencies range, sky waves are absorbed during day time by an ionospheric layer in the ionosphere.   At night, this layer disappears and sky waves can propagate very far.  Sky waves are generally considered as a source of interference to domestic broadcast.   International MW broadcast on the other hand, uses sky waves to reach distant service areas at night.