Maximum radio communication range

Many radio device manufacturers specify a parameter referred to as the “maximum transmission range (in the open space)”. The term “in the open space” refers to a theoretical and ideal condition, in which the radio waves are propagated in a vacuum, i.e. in conditions not available to the standard wireless system users, e.g. intruder alarm systems.

The article discusses actual conditions of radio wave propagation in reference to ISM Industrial, Scientific, Medical equipment. The bands are mostly used by Class I devices (i.e. devices that may be operated without a radio licence) in industrial, scientific, medical and domestic applications. Currently, ISM bands are classified into several groups (Table 1).

Table 1. ISM band groups

Remember, the table shows frequency range allocated in the EU. Radio transmission in the ISM bands is limited, and selected channels may be assigned a specific use, e.g. monitoring. Other limitations include output power, channel width or band usage period. Detailed information are available in the ITU Radio Regulations, ITU recommendations and the National Frequency Allocation Table.

Currently, the most utilized ISM band is UHF 433 MHz used by many radio devices, and it is difficult to find a channel free from interferences in this band, in particular in large urban areas. The frequency range 433.05–434.79 MHz is intended for amateur applications. There is a risk that the intruder alarm devices operating in this band may be interfered by radio amateur stations operating at significantly higher powers, approx. 100 W; for comparison, the intruder alarm device power in this range does not exceed 10 mW. Most of the currently manufactured intruder alarm systems operate at higher frequencies, e.g. 868 MHz, 2.4 GHz or 5.8 GHz. At 868 MHz, the frequency occupancy factor is relatively low, in particular at 868–869.7 MHz.

Maximum transmission range: in actual conditions may be several times smaller than the maximum transmission range in the open areas. Wave attenuation can be affected by many factors.

The terrain can be separated into three basic types.

Fig. 1. Three basic terrain types: (1) space with obstacles, (2) semi-open space, (3) open space

The illustration shows three types of spaces, where communication can be planned. If the transmitting and receiving antenna are not optically "visible" to each other (for bands over 300 MHz), communication is not possible (Fig. 1.1). In Fig. 1.2 (semi-open space), the communication may be unsatisfactory, since the signal power is low.

The radio wave at ISM frequency may be attenuated by different objects, e.g.:

– internal building wall – attenuates signals by 10–15 dB,

– external building wall – attenuates signals by 2–38 dB,

– floor – depending on material used - attenuates signals by 12–27 dB,

– window – attenuates signals by 2–30 dB depending on material used and the glazing cavity gas composition. Older windows filled with air do not attenuate the signal too much, however the windows filled with noble gas attenuate the signal to a greater degree. For comparison - the signal attenuated by 30 dB is a thousand times weaker than the initial signal (before attenuation).

The best solution is to install the receiving and transmitting antenna without any obstacles between them (Fig. 1.3), and if the transmitting devices are located outside, the receiving antenna should also be installed outside.

Built-in antennas may reduce the signal power due to their design.

It is an elliptical zone, with the straight line between the transmitter and the receiver being its axis (Fig. 2). In practice, the first Fresnel zone directly affects the transmission range.

Fig. 2. First Fresnel zone

When designing the communication system, remember to avoid any obstacles, either natural or artificial in the first Fresnel zone (the wave will be attenuated and the communication may be interrupted). The zone is an area in which most of the signal energy is transmitted.

A phenomenon describing the wave (e.g. radio) propagation in the medium. In case of wireless sensors, alarms and monitoring systems, propagation defines the radio wave propagation in the air.

Any obstacles with smooth edges will scatter (attenuate) the radio waves to a significantly higher degree than any sharp-edged obstacles. Wave propagation may also be affected by the weather: strong winds, rain or storm may result in electrostatic charges being accumulated on the antenna and may cause interferences. The more heavy the rain, the worse the conditions of wave propagation, especially at higher frequencies.

The maximum transmission range and the wave propagation mode directly depend on its frequency. The higher the frequency, the lower the signal susceptibility to interferences and the lower the range.

The radiated power (ERP or EIRP) has a significant effect on the maximum transmission range. ERP is determined by the transmitter power, transmitting antenna gain, transmitter-transmitting antenna path attenuation (mainly affected by the quality of cables and connectors used). ERP is a combination of those three parameters. A radiation power is the actual power at the transmitting antenna output. The maximum transmission range may also be affected by the height of the electric centre of the antenna (meters above sea level) and altitude of the antenna mast base (meters above sea level).

The receiver can be defined by its receiving antenna gain, sensitivity and selectivity.

Receiving antenna gain affects the received signal quality and increases the maximum transmission range. Sensitivity of the receiver is a measure of its ability to receive weak signals. Selectivity is a measure of its ability to separate the usable signal from many other signals (e.g. interferences and noise).

Similar to the hardware factor, the maximum transmission range may also be affected by: height of the electric centre of the antenna (meters above sea level), altitude of the antenna mast base (meters above sea level) and receiver-receiving antenna path attenuation for the receivers.