Features of designing radio security systems.
Radio channel OPS systems are gaining more and more popularity. Read about the main stages of wireless equipment design, as well as about calculating the range of radio devices indoors in this article
The process of drawing up design and estimate documentation can be divided into the following stages:
Approval of the technical specifications by the customer
Drawing up a commercial proposal or a stage «P» project (for large objects)
Approval of the stage «R» project (working documentation)
Coordination of estimate documentation
Drawing up executive documentation (working documentation with amendments)
Technical specifications(TZ) is compiled according to RD 25.952-90 «Automatic fire extinguishing, fire, security and fire and security alarm systems. Procedure for developing a design assignment.» For small objects, a commercial proposal is then formed, and for large ones — a stage «P» project (feasibility study according to SNiP 11-01-95 «……….»???). It should be noted that when using wired equipment, the total costs of consumables, design, installation and commissioning of the system can reach twice the cost of the equipment itself, i.e. the final amount can be 3 times higher than the cost of the devices. Obviously, this fact, as well as a higher speed of project implementation, the ability to install equipment without decommissioning the facility, minimal intervention in the interior of the premises largely explain the increased popularity of radio channel fire alarm and notification systems.
The stage «P» project is the basis for determining the system operation tactics. For wireless equipment, the preparation of working documentation includes:
placement of security and fire alarms and annunciators on the facility plans taking into account reliable detection of signs of intrusion or fire;
placement of radio expanders on the plans taking into account the range of the radio channel;
determination of system parameters (sections, signal and actuator devices, control devices and their interrelation).
At the moment, the most incomprehensible (but not so complicated) process is the calculation of the range of radio channel devices. These calculations must be made for the optimal placement of radio expanders on the site plans — devices that control their daughter detectors, collect and retransmit messages from them to the security post.
Calculating the range of radio channel devices
So, each pair of radio devices (for example, «detector — radio expander») is characterized by an energy potential, which is determined by the power of the transmitting devices, the sensitivity of the receiving paths and the parameters of the antenna-feeder paths. This energy reserve on the radio line characterizes the signal/noise ratio in the receiving path of the radio channel devices and is expressed in decibels (dB). The recommended energy reserve, ensuring stable radio communication is at least 20 dB. Let's consider the algorithm for calculating the value of this reserve inside and outside buildings.
As a starting value, it is necessary to take the value of the energy potential between the components of the radio system in open visibility — this information must be provided by the equipment manufacturer. Then, from this value, we subtract the signal attenuation values (see below) depending on the distance of the devices and the obstacles located between them. The result obtained is the calculated energy reserve between the radio devices.
Signal attenuation (excluding fading) on radio links in free space depends on the distance between radio devices and is characterized by a decrease in the solid angle with increasing distance from the signal source. Fig. 1 shows (graphically) the dependence of attenuation on the distance between radio devices. Signal fading can be divided into fast and slow. Outdoors, fast fading is caused by radio wave interference, while slow fading is mainly determined by daytime and seasonal attenuation of the radio signal and changes in the dielectric constant of air. The depth of these fadings can range from 10 to 20 dB.
Inside a building with brick or concrete structures, radio signal attenuation is associated with its multiple reflections (from 10 to 40 dB). Rapid fading is caused by the presence and movement of people in the premises (from 15 to 25 dB).
In turn, the weakening of the radio signal due to passage through building structures depends on the material and thickness of these walls and partitions. Their maximum thickness is for:
concrete — 0.47 m (433 MHz) and 0.24 m (868 MHz);
brick — 4.3 m (433 MHz) and 2.18 m (868 MHz).
Thus, it can be assumed that the attenuation of the radio signal as a result of passing through one wall at an angle of incidence of a plane wave on a flat surface of 90° will be:
for wood and foam concrete — 3-4 dB;
for brick — 6 dB;
for concrete — 10 dB;
for reinforced concrete — 18-20 dB (with volumetric reinforcement it can reach up to 30 dB).
If a plane wave hits a surface at an angle other than 90°, the maximum wall thickness will be slightly less. Fig. 2 shows graphs of radio signal attenuation depending on the angle at which it hits them and the materials they are made of.
An example of calculating the range of radio communication at an object
As an example, let's take eight rooms. The first of them has a radio expander, the last one has a radio detector (Fig. 3).
The distance between them is 48 m.
Between them there are seven 15 cm thick walls made of foam concrete. The angle of incidence of the wave is -20°. Vpr.= 7 × 215; 5 dB = 35 dB.
Attenuation due to diffraction Vd=35 dB
Total signal attenuation due to obstacles Vd+pr= 32 dB
Attenuation in free space Vо = 58 dB.
Total signal attenuation V∑= 32 + 58= 90 dB.
The example considers the equipment of the radio system «Strelets», in which, when working with standard antennas, the energy potential between the radio expander and its daughter detector is 114 dB. As a result, the reserve for fading Pс = 114 – 90 = 24 dB, which in most cases is quite sufficient for organizing normal radio communication.
If the calculated reserve is insufficient to compensate for fast and slow fading (i.e. the result is less than 20 dB), it is recommended to either change the location of the radio devices or use additional measures to increase the energy potential (remote or directional antennas, antenna amplifiers, etc.).
It is also advisable to use additional methods for increasing the radio communication range in cases where the installation of intermediate links of the radio system (repeaters) is impossible or unjustified. In particular, such a situation is quite likely when designing a fire alarm system in summer cottage villages, gardening communities, garage cooperatives. The specificity of constructing a radio system in these cases is that it will probably be necessary to provide radio communication with a remote object (for example, a detached cottage or other building). In this case, installing an intermediate radio expander may be associated with certain difficulties. Firstly, the intermediate radio expander must be installed in an inaccessible place (protection from theft and vandalism), and it must be supplied with power. Secondly, if the only purpose of installing a radio expander is to retransmit messages, then installing an intermediate radio expander is economically inexpedient. Finally, the installation of additional links somewhat reduces the reliability of the radio system (power supply problems may arise, the possibility of sabotage cannot be ruled out, etc.).
The range is increased by using remote antennas (the influence of interference sources is reduced, no additional power sources are required) or amplifiers (easy installation).
Radio system parameters and recommendations for equipment installation
The final stage in developing the working design documentation is determining the parameters of the radio system: frequency channels, sections, signaling and executive devices and control devices with an indication of their interrelationships.
At this stage, the parameters of the radio expanders as receiving and control devices of the radio system are also specified:
general: operating parameters of the radio expander;
sections: local section – the main functional unit for control and indication of the system status;
relay: external response to events in sections;
subsidiary devices: signaling, executive and control devices included in the section, and their operating parameters;
users: who and with what code is allowed to manage the local section.
In conclusion, we will provide the main recommendations for the installation of radio system equipment:
radio expanders and daughter devices should be mounted as far as possible from metal objects, metal doors, metallized window openings, communications, etc. (Fig. 4);
Fig. 4. Radio devices should be mounted as far as possible from metal objects
Avoid installing radio devices near various electronic devices, computer equipment, current-carrying cables, wires in order to eliminate the influence of interference from operating voltage converters, microprocessors, etc. on the quality of radio reception. The recommended distance between radio expanders and electronic devices is at least 1–1.5 m (Fig. 5);
Fig. 5. The recommended distance between radio expanders and electronic devices is at least 1.5 m
It is recommended to install radio expanders so that the main antenna is in a vertical position. The recommended installation height of radio expanders is not less than 2–2.5 m (Fig. 5).
Fig. 5. The recommended installation height of radio expanders is not less than 2 m