Radio signals, like all electromagnetic radiation, usually travel in straight lines. However, at low frequencies (below approximately 2 MHz or so) diffraction effects allow them to partially follow the Earth's curvature, thus allowing AM radio signals in low-noise environments to be heard well after the transmitting antenna has dropped below the horizon. Additionally, frequencies between approximately 1 and 30 MHz, can be reflected by the ionosphere, thus giving radio transmissions in this range a potentially global reach (see shortwave radio).

However, at higher frequencies, neither of these effects apply, and so any obstruction between the transmitting and receiving and antenna will block the signal, just like the light that the eye senses. Therefore, as the ability to visually sight a transmitting antenna (with regards to the limitations of the eye's resolution) roughly corresponds with the ability to receive a signal from it, the propagation characteristic of high-frequency radio is called "line-of-sight".

In practice, the propagation characteristics of these radio waves vary substantially depending on the exact frequency and the strength of the transmitted signal (a function of both the transmitter and the antenna characterics). Commercial FM radio, at comparatively low freqencies of around 100 MHz using immensely-powerful transmitters, easily propagates through buildings and forests.

Impairments to line-of-sight propagation

Low-powered microwave transmitters can be foiled by a few tree branches, or even heavy rain or snow.

If a direct visual fix cannot be taken, it is important to take into account the curvature of the Earth when calculating line-of-sight from maps.

The presence of objects not in the direct visual line of sight can interfere with radio transmission. This is caused by diffraction effects: for the best propagation, a volume known as the first Fresnel zone should be kept free of obstructions.

Reflected radiation from the ground plane also acts to cancel out the direct signal. This effect, combined with the free-space r^-2 propagation loss to a r^-4 propagation loss. This effect can be reduced by raising either or both antennae further from the ground: the reduction in loss achieved is known as height gain.

How do cellphones work, then?

The frequencies used by cellphones are in the line-of-sight range. So how do they work in cities? The answer is a combination of the following effects:

• r^-4 propagation over the rooftop landscape
• diffraction into the 'street canyon' below
• multi-path reflection along the street
• diffraction through windows, and attenuated passage through walls, into the building
• reflection, diffraction, and attenuated passage through internal walls, floors and ceilings within the building

The combination of all these effects makes the cellphone propagation environment highly complex, with multipath effects and extensive Rayleigh fading. These problems are tackled using:

• use of many base stations (a phone can typically see six at any given time)
• rapid handoff between base stations
• extensive use of error correction and detection in the radio link

External links:

http://www.tapr.org/tapr/html/ve3jf.dcc97/ve3jf.dcc97.html