RDT RM96xx Guía de instalación descargar pdf (Pagina 7)

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 transmit or receive 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 1.4GHz the radius for a particular signal path is:

1Km signal path 5 - 6Mtr radius

2Km signal path 8 - 9Mtr radius

5Km signal path 12 - 14Mtr radius

This means for example, that if the link is 2Km long, the antennas must be at least 8 to 9

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

same 8 to 9 metres clearance to either side.

It should also be remembered that radio signals at 1.4GHz will not normally pass through

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

Even with a clear line-of-sight (as defined above) the ground, or nearby buildings, can

cause reflections. These reflected signals will also arrive at the receiver and can interfere

with the direct signal causing a reduction in signal strength or lost data.

Reflections

The best possible performance and therefore distance, will be achieved if there is perfect

unobstructed line-of-sight between the two sites ‘A’ and ‘B’. Even in this situation however

there is almost certain to be some degree of reflections from the ground or nearby

structures.

These reflections will have an effect on the signal strength and the final picture quality

obtained. It is useful therefore to understand the basic principals of a radio link and the

effect of reflections. The most common reflection is from the ground although other

structures, buildings etc. will have a similar effect. The reflections from these other

structures are less predictable because they depend on the shape of the structure,

construction material etc.

Consider a radio path between site ‘A’ and ‘B’ with each antenna 10 metres from the

ground. Assuming good line of sight, there will obviously be a path directly between the two

antennas, drawn as a straight line on the diagram. There may also be one or more paths

that are reflected from the ground and reach site ‘B’ indirectly via a point on the ground, ‘X’.

Obviously the distance A-X-B is longer than the direct route A-B. The receiver at site B will

receive a direct signal and a reflected signal via point X.

The effect of this reflected signal depends on its strength and the difference in length

between A-B and A-X-B. If the difference is a multiple of the wavelength ( approximately

21.4cm for 1394MHz ) the reflection is 180º out of phase and will reduce the received signal

strength. In fact if both signals were the same strength they would cancel and no signal

would be received.

Because at these frequencies the wavelength is quite short, even a small adjustment in

antenna height can affect the path length of the reflected signal and so change the received

signal strength. By raising the antenna gradually it will cycle through a regular pattern of in

phase and out of phase signals giving a corresponding pattern of reduced and increased

signal strength.

The reflected signal will show up as ghosting on the monitor. Occasionally this may be

seen in adverse weather conditions when a signal may be reflected from rain, snow, dense

cloud or fog.

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VideoWave 1.4GHz 1

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RDT RM96xx Guía de instalación Pagina 7

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