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Product Warranty Information

 


Basics of Wave Propagation (Most Frequently Asked Questions)

With the exponential increase in voice and data communication traffic, the airwaves are often subject to
interference. Although more people depend on wireless links due to their cost-effectiveness, few of the wireless
devices available today can provide guaranteed performance unless prior planning is made. In order to use these devices to communicate over the airwaves without problems, we must recognize and follow some basic precautions.

Line of Sight: Both 900MHz and 2.4GHz frequency bands require line of sight (LOS) for outdoor applications or distances greater than 500m. Once a clean LOS is available, the two sites can be literally miles and miles away. In some cases links have been successfully established over distances as great as 45km.

Antenna and Cable Selection and Installation: Depending on each application, antenna requirements vary. For distances greater than 1.5km, a high gain Omni or Yagi antenna may be required on one or both ends. For cables less than 10ft (3m) in length, signal loss can be ignored as it is negligible. In most circumstances, however, cables will be far longer than 3m. Low loss cable should be used in such situations; and path loss must be calculated to determine the signal strength. For antenna installation, the rule of thumb is “the higher the better”: the higher it is placed, the better an antenna will perform. All antennas come with mounting hardware that may include pipe clamps or U bolts. These can easily be used to mount the antenna to an existing pipe or wood pole. With all outdoor antennas, lightning protection is absolutely essential. A lightning arrestor/protector is connected in series with coaxial cable and grounded to the building. The installation of pipe or wood poles must be done by professionals.

Extremely Important: Our tests have proven that the performance of any directional antenna depends on the physical orientation i.e. the perpendicular (vertical) or horizontal polarization, varying from one site to another. We recommend testing with both perpendicular and horizontal polarization setups. In the case of directional antennas, the higher the gain is, the narrower the beam will be. To find the exact direction of the antennas go through the following steps:

Set up the link with H-polarized antenna at both ends.

Record the BER for 5 minutes then turn the antenna slightly to one direction
and record the BER for 5 minutes. Do the same at four or five different angles.

Tighten the screws at the angle with minimum BER.

Repeat the process at the other end.

In applications that require a link over distances greater than a few kilometers, a path loss analysis must be done before the equipment is actually installed. The following parameters and formulas can be used:

Loss = 32.44 + 20 log f + 20 log d                  
where f = frequency in MHz
and d = distance in km

System Gain = Po (Tx power) + Gt (Tx antenna gain) + Gr (Rx antenna gain) - Mf (fade margin) - Ms (system margin) - Pr (minimum receive signal level)

Use 10 dB for Mf and 15dB for Ms (standard)
Example: Using a 500mW Smartamp (27dBm), 24dB antennas and -80 (dBm) as receiver sensitivity we get:
System gain = 27 + 24 + 24 -10 - 15 - (-80) = 130dB
using in formula (1) we get d = 95.5km

Assuming that the entire system gain covers the maximum path loss, we get:
Loss=130dB, 20logf=67.60, d=antilog 1.5 or 32km.

In the event that the connection between the radio and the antenna is an RF cable, the cable
loss will further reduce the system gain.

Coping with Undesired Interferences: Most of Teletronics’ modems and voice equipment offer channel selection options. If used correctly, most interference can be eliminated. In some instances, you might be better off using 2.4GHz equipment than 900MHz equipment. If possible, select an antenna with minimum beam width: smaller beam widths provide high gain while reducing your link’s vulnerability to interference.

Data Cables: RS232 cable is available almost everywhere. For V.35 interface consult with your sales person first. For category 5 cables, make sure whether you need straight through or crossover cable.

Power: If the available power is other than 110V AC, please ask for other alternatives.


 

 

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