Tel +44(0)870 235
1314
Fax +44(0)870 235 1366
sales@equinox-communications.com
|
Tel +44(0)870 235
1314 sales@equinox-communications.com
|
Line of sight
Radio signals require a clear path between antennas. It is necessary to know the requirements of a clear path (known as 'line of sight').Besides the
LOS there is the Fresnel zone. This zone is an elliptical area immediately
surrounding the visual path. It varies in thickness depending on the length of
the signal path and the frequency of the signal. The necessary clearance for the
fresnel zone can be calculated, and must be taken into account when determining
the height of the antennas.
Expected Range* of a Wireless Network.
| Effective Gain | Line of sight range Meters | Line of sight range Miles | ||
| 0dB | 100 | 0.06 | ||
| 3dB | 200 | 0.12 | ||
| 6dB | 400 | 0.24 | ||
| 9dB | 800 | 0.48 | ||
| 12dB | 1600 | 0.96 | ||
| 15dB | 3000 | 1.8 | ||
| 18dB | 6000 | 3.6 | ||
| 21dB | 12000 | 7.2 | ||
| 24dB | 24000 | 14.4 | ||
*These figures are for guidance only and vary depending on local conditions
A rule of thumb for the clearance needed above and below the signal path (H2) is that for every 1.5 km a clearance of 4 meters is necessary. A result of this rule is that large distances need great heights.
Think about the following things if you have to determine the line of site:
- Trees grow leaves
- Make sure you have enough height
Signal Boosters
Increase the coverage area and distance your wireless can be used with a signal booster.
Mechanical
installation of the antenna
A solid mechanical installation of the antenna is the base of a quality link. An
outdoor mounted antenna is exposed to extreme weather conditions. Metal brackets
oxidise, and vibrations will cause slack on connections. Therefore, the
mechanical connection between the antenna and its carrier needs to be solid. Use
locktite on screws. The connector between the cable and the antenna needs to be
sealed with threading tape. Corrosion on the connector will cause signal losses
due to changing impedance of the connection.
Outside mounted antennas are lightning-strike sensitive. Lightning strikes always are common at highest and smallest point, typically an antenna. The antenna cable will then conduct the enormous amount of energy towards the equipment. If no lightning arrestor is placed in-between the antenna cable the bridge unit will be damaged, or worse, set on fire. Therefore, always use a lightning arrestor.
When placing the antennas
the following points should be kept in mind:
· With an outdoor installation of the antenna always use a lightning arrestor
· Make sure that the mounting of the antenna can handle high forces. Wind can
cause misalignment or even permanent damage
· Keep the cable length between the antenna and the bridge limited to the
minimum. Every meter of cable causes signal loss
Step by Step installation
1. Determine the antenna
location
· Line of sight
· Make sure your antenna fits the range
· No large conducting objects in the area
· No other antennas close
· When mounting on a pole: Let the antenna rise above the pole
· Remember that trees grow!!!
2. Mount the antenna
· It can never be too solid
· Use Locktite with screws
· Seal connectors with threading tape
· Use a lightning arrestor
3. After installation
· Double check
Lighting protection
Lightning strikes can
cause major damage on equipment and buildings. An antenna placed on a roof is
often the place of strike. Dish antennas mounted against a wall are less risky.
The cable conducts the lightning into the building. Due to this behaviour an
antenna installation requires dual protection. The antenna needs to be grounded,
and the cable requires a surge protector.
Equinox-Communications sell a suitable lightning protector which will offer some protection however it is very important that the lightning protector has a proper earth to ground; preferably using a proper, high voltage earthing strap or braid.
Below are some general hints on lightning protection:
Never mount an antenna
though the roof, only on the roof
Keep the antenna cable as much as possible out doors
The cable shield needs to be grounded on the highest possible point, and at the
point of entering the building.
Place a surge protector on the antenna connection at the bridge, and also on the
power supply of the wireless device.
Multi path effects
Direct Sequence technology (802.11b) is more sensitive for multi-path effects
than the traditional Frequency Hopping. Multi-path effects can be explained as
coherent signals (signals from the same source) arriving at the antenna at
different times due to the difference in path length. Several signals from the
same source meeting at the receiving antenna can arrive in phase, out of phase
or in between.
When the level of each incoming signal is included, the effects can be any of
the following:
| Level | Phase | Result |
| Equal | In | Strong reception |
| Unequal | In | Good results |
| Equal | Out | Possible loss of reception |
| Random |
Random | Variable levels |
Vertically polarised signals are perpendicular to the reflecting surface, and will either be totally reflected from a conductor, or will propagate along the surface (depending on the angle of the arriving energy). Since radiated waves penetrate lossy materials, energy can be lost to heat generation.
Refraction
The velocity of RF energy is delayed with prolonged passage through dielectric
materials that are denser than dry air. This is refraction, or the change of
direction of propagation. The time spent in the denser material controls the
degree of refraction and the resulting direction of the emerging wave.
Diffraction
Perfect shadows that are cast by RF-opaque structures or objects are rare at
wireless system frequencies. Because of diffraction, energy scatters at the
edges of the obstruction. Diffraction is more pronounced in sharp or knife edged
corners. According to field theory, edges cause secondary radiation when
illuminated. This is generally independent of polarisation when the scattered is
much longer than the impinging signal's wavelength.
Diffraction forms a fuzzy signal source at the edges, which tends to fill in the
shadowed regions. The signal levels are low but often usable. These edges can be
corners of a building, window frames or large vehicles. Diffraction also figures
significantly in scattering at hilltops lying in the propagation path.
Locating the
antenna
Free space radiation patterns are the baseline performance criteria before
installation in the real world environment. Since sites are located on rooftops,
or on the side of buildings; many opportunities exist for interference from
chimneys, walls, masts, towers and other antennas. Although it is difficult to
predict level changes you can take steps to minimise pattern distortion.
Obstructions are either conducting or non-conducting. Conducting objects create the most severe disturbances. Those near the resonant length of the antennas, such as other antennas, behave as the parasitic elements of an uncontrolled array, producing random nulls and lobes. Larger conducting structures causes severe shadowing. The closer the spacing, the larger the shadow.
Free space
losses
Spreading is the principal contributor to signal loss for line of sight
propagation. As a signal radiates it spreads or expands into a spherical
surface. The available RF power is distributed over this surface and weakens
with increasing range. The signal is reduced by 6 dB for every doubling distance
from the source. We compute the loss path between the source radiators with
spherical patterns using the following equation:
Lp= 92.45 + 20Log10F + 20Log10d
Lp= Path loss in decibels
F= frequency in GHz
dB= decibels
d= Distance in kilometres
Example:
A distance of 6 kilometre provides a free space loss of -115,67 dB.
| Distance/Km | Gain Loss/dB |
| 0.5 | 94.0 |
| 1.0 | 100.0 |
| 1.5 | 103.5 |
| 2 | 106.0 |
| 3 | 109.5 |
| 4 | 112.0 |
| 5 | 114.0 |
| 10 | 120.0 |
| 15 | 123.5 |
| 20 | 126.0 |