Designing systems

Factors affecting the quality and distance of the radio connection include:

  • Power of the radio transmitter

  • Sensitivity of the radio receiver

  • Tolerance of spurious radiations of the radio modulating signal

  • Amplification of transmitting and receiving antennas

  • Antenna cable attenuation

  • Antenna height

  • Natural obstacles

  • Interference caused by other electrical equipment

The transmitter power of the radios using 410 to 473 MHz frequency range is 1 W (maximum) and the sensitivity of the receiver better than -115 dBm. Thus in a flat area and in free space with a 1/4 wave antenna (antenna amplification 1dBi) and antenna height of 1 m communication, distances of 3 to 4 km can be achieved. Distances may be considerably shorter in situations where there are metallic walls or other material inhibiting the propagation of radio waves.

Over long distances, increasing the height of the antennas can often solve problems caused by natural obstacles. A ten-fold increase in distance can be achieved with the use of amplifying antennas. Frequent topographical variations over long distances may require that at least one of the antennas be raised to a height of 10 to 20 m.

Adding a repeater station can also solve problematic radio connections. In systems with many base stations, the RSSI-signal can be used to help choose the base station with the best signal. A communications network can also be built with a combination of cables and radio data modems.

The TDL510 radio tolerates normal levels of interference that occur. However, exceptionally high levels of interference can break through the safeguards and thus cause errors in data transfer.

To obtain a sufficient safety margin, test the communication path using an extra 6 dB attenuation at the antenna connection and with slightly less effective antennas than those to be used in the final system.

Radio field strength

Radio signal strength must be good enough for successful data transfer. Where field strength is above a certain level, the operational results are very good. Below this level, a few dB marginal areas occur in which errors begin to be generated by noise and interference that eventually lead to loss of connection.

The field strength is at its optimum level in open space, although increasing distance will still reduce it. Note that one open space has different environmental and external factors to another, and you need to take into account the effects on transmission quality when planning the system.

Ground, ground contours, and buildings cause attenuation (loss of energy through absorption) and reflection of radio waves. Buildings reflect radio waves and therefore the effects of attenuation are not as acute when transmission is over a short distance.

However, the reflected waves will often be a bit delayed, and when they combine with the direct radio waves they interact in either a weakening or a strengthening way. This causes the fading effect in mobile systems. In reality, very sharp signal drops spaced about 35 cm apart may occur. The attenuation may even reach 40 dB, normally less.