Installation guide

Technical Notes

Propagation/Distance

For any radio link, the distance that can be achieved between sites depends on a

number of factors, the main ones being; the output power, the antenna height (above

ground), the type of terrain and ‘line-of-sight’. Line-of-sight means how clear the path

is between the two antennas, whether there are any obstructions and how dense

these obstructions are. The major effect on distance however, is the height of the

antennas above ground. The basic relationship between signal strength, antenna

height and distance is given in the following formula:-

Signal strength : (TX antenna height)² x (RX antenna height)²

(Distance)

From this formula it can be seen that if the distance doubles, the signal strength

decreases to one sixteenth. In other words, to increase the distance from 1 Km to 2

Km, requires an increase in signal strength of sixteen times. It can be seen therefore

that increasing output power has a limited effect on distance.

Increasing the antenna height however, has more effect. If the antenna height (above

ground) is doubled, the effective signal strength increases four times. This is true for

either the transmitting or receiving antenna so by increasing the height of both a

significantly greater distance can be achieved.

Clear line-of-sight

When we refer to a clear line-of-sight for radio signals, this is different to a visual line-

of-sight. You may be able to see the transmitting antenna from the receiver site, but

this does not necessarily mean a good radio path.

A radio signal transmitted between two antennas will spread out forming an elliptical

shape that is widest at the mid distance between the two antennas. The area within

this ellipse is known as the Fresnel Zone and any obstructions within this zone will

cause an obstruction and interference to the signal path.

TX Fresnel Zone Radius RX

Note that this Fresnel Zone is three dimensional and has both height and width. The

maximum radius of the beam, at the centre, increases with the distance between the

two antennas. As a guide, at 2.4GHz the radius for a particular signal path is:

1Km signal path 4 - 5Mtr radius

2Km signal path 5 - 6Mtr radius

5Km signal path 8 - 9Mtr radius

This means for example, that if the link is 2Km long, the antennas must be at least 5 to

6 metres above the ground and above any building or obstruction. There must also be

the same 5 to 6 metres clearance to either side.

It should also be remembered that radio signals at 5.8GHz will not normally pass

through buildings and they will be severely attenuated by vegetation, such as trees.