# Propagation

When an alternating current is passed along a conductor, the magnetic field surrounding the conductor will decrease and build up the opposite direction repeatedly. A magnetic field cannot collapse instantaneously, so at higher frequencies insufficient time is available, and the energy is propagated in the form of electromagnetic waves. The magnetic field, as seen, is caused by current variation. An electrical field, inseparable from the magnetic field, is associated with the corresponding voltage changes. These two fields are at 90° to each other, and both at 90° to the direction of propagation. The direction of the electric field determines the direction of polarisation.

The frequency spectrum is conveniently divided into bands, and each particular frequency will have a corresponding wavelength X, where

The major modes of propagation are:

ground wave | (up to about 2MHz)
ionospheric wave | (about 1- 30MHz)
tropospheric wave | (above about 30MHz)

The earth is surrounded by the troposphere and then the ionosphere. The ionosphere is divided into layers (D, E, F1 and F2) at heights of about 80km, 120km, 200km and 300-400km respectively. These are only approximate and vary both daily and seasonally. Skip distance and skip zone are the distance and area respectively where a receiver is between the limits of a ground wave and a returning ionospheric wave. Critical and maximum usable frequencies are of concern as a guide to propagation expectations.

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