I. Overview of how to set up a repeater station
In the wireless intercom system, the relay station plays an extremely important role in increasing the communication distance and expanding the coverage. It is an indispensable and important equipment for professional wireless communication systems.
Relay stations consist of receivers and transmitters. It usually works in transmitting and receiving the inter-frequency state, and can demodulate the received modulated radio frequency signal to transmit the audio signal to other devices. At the same time, the audio signals sent from other devices can be transmitted via radio frequency modulation. The other devices mentioned above have controllers for various systems, wireless connectors, etc., as well as other repeaters required for the interconnection. The signal received by the relay station is directly forwarded through its own transmitter. This is the most basic application of the relay station.
Therefore, the relay station must be capable of full-duplex operation, that is, transmitting and receiving work at the same time, and cannot affect the normal operation of the receiver when transmitting. Due to the basic characteristics of the relay station's work and the combination of multiple relay station combinations, the technical indicators of the relay station have higher and more special requirements than the mobile station.
In addition to a single-channel conventional terrestrial communication system consisting of a repeater station, relay stations can also use coaxial cables and power splitters to set up multiple distributed antennas to provide underground and ground coverage communications for buildings and hotels. In addition, multiple relay stations form a cluster system and a variety of band-shaped or star-shaped communication networks.
The indicators of relay station installation and installation directly affect the system communication distance and system network voice quality and function.
Second, how to set up a repeater station relay station communication distance engineering calculation
1. Radio wave transmission loss engineering practical formula
LM(dB)=88.1+20lgF-20lgh1h2+40lgd
Where: F—communication operating frequency (MHz)
h1—A-point antenna height of the communication target (m)
h2—B antenna height of the communication object (m)
Communication distance between d—A point and B point (m)
The above practical formula is limited to the VHF 150MHz and UHF 400-470MHz frequency bands, and the terrain undulation height is about 15m, and the communication distance is within 65km.
2. Calculation of the communication distance of the system wireless device (examples below)
(1) Assumed known conditions
a. System operating frequency:
TX 465MHz RX 455MHz
b. Relay station parameters and setup data:
Transmission power: 20W (43dBm)
Receiver sensitivity: -116dBm
Coaxial cable loss: 2dB (1/2′′ feed tube 40m long, 5dB/100m)
Omni antenna gain: 9.8dbi
Antenna erection height: 30m
c. Intercom parameters
Transmission power: 4W (36dBm)
Receiver sensitivity: -116dBm
Intercom antenna gain: 0dBi
Walkie-talkie height: 1.5m
(2) System gain of repeater and intercom
In this example, the so-called system gain is the maximum allowable attenuation of the receiver's transmitted signal to the repeater receiver, if cable loss and antenna gain are not considered:
System Gain (dB) = Transmitted Power (dBm) - Receiver Sensitivity (dBm)
Considering cable loss and antenna gain, the system gain in this example is:
SG(dB)=Pt+PA-(RA+CL+RR)
=36+0-(9.8-2-116)
= 144.2 (dB)
Where: Pt - Transmitter transmit power
PA——Interphone Antenna Gain
RA - repeater antenna gain
CL - Coaxial cable loss
RR - Receiver sensitivity
(3) If the system gain is equal to the loss of the radio wave transmission, it means that the radio wave energy of the communication distance has reached the limit. If the system gain is less than the transmission loss, the communication may not be established.
Substituting the system gain into the transmission loss engineering formula:
144.2=88.1+201g455-201g1.5×30+401gd
144.2=88.1+53.2-33+401gd
35.9=401gd
d=7.9km
(4) The above equation only calculates the conservative distance that the uplink signal (the intercom sends to the relay station) can communicate, and does not calculate the distance that the downlink signal (relay station sends to the walkie-talkie) can cover, usually because the relay station transmits power. Large, its downlink signal is often better than the communication distance of the uplink signal. Since the system call is two-way, the system's conservative call distance is often calculated based on the previous line signal.
Third, how to set up the interference in the communication system of the relay station
1. There are many interferences in the system, some are unavoidable in product manufacturing, and some are generated during system integration. We mainly discuss the latter. There are two types of interferences that we often encounter in daily communications: Intermodulation interference.
2. Co-frequency interference means that the operating frequency in one system is interfered by the same frequency of another system. Although the nominal frequencies of the two transmitters are the same, they are still somewhat different. When a receiver receives two transmitter signals at the same time, it will produce two beat noises with different transmit frequency differences. If the two transmit frequencies differ by 1 kHz The 1KHz beat sound can be heard in the receiver. In addition, although the transmission frequency is the same, the phase of its frequency is not necessarily the same, so it will produce low-frequency hum noise.
3. Intermodulation interference is generated by non-linear devices in the circuit. There are two main causes of intermodulation in our daily work:
(1) Interfering signals intrude into the transmitter's final stage, causing intermodulation with the wanted signal. This kind of interference is called transmitter intermodulation interference.
(2) Two or more radio signals in the intermodulation relationship are received by the receiver and produce intermodulation at the receiver high-level, called receiver intermodulation interference.
4. Intermodulation interference is mainly reflected in the multi-channel system or installed together with other systems. Multi-channel frequencies are generated by mixing with each other or other system frequencies and multi-channel frequencies. The main mechanism of the generation is two forms of expression. If we use A, B, C ... represents the frequency value, there are:
2A-B=C
A+DC=B or A+DB=C
For example: The transmission frequency of a system with 4 channels is:
1CH 160MHz 2CH 161MHz
3CH 162MHz 4CH 163MHz
If you do not use intermodulation suppression measures, it will produce: 2x160MHz-161MHz = 162MHz (false), that is, the second harmonic of 1CH enters the final stage of 2CH power amplifier. The difference frequency forms a false 162MHz, which will be received by 3CH. The device causes interference.
There will also be: 160MHz + 163MHz - 162MHz = 161MHz (FALSE), that is, the beat of 1CH and 4CH and the beat of 3CH produce a false 161MHz, which will interfere with the true 161MHz frequency received by 2CH.
5. In order to avoid intermodulation interference, there are usually two approaches: add a one-way isolator to the output port of the transmitter power amplifier according to the mechanism of intermodulation, attenuate the external incoming signal, and do not allow it to generate sum frequency and difference frequency. For receivers with large dynamic range devices, reduced nonlinearity is reduced. Another method is to artificially avoid the frequency of generating intermodulation combinations when using frequencies. The former needs to increase equipment investment, which is more economical but depends on the possibility of the frequency of approval.
6. Engineering calculation method without (third-order) intermodulation interference frequency:
(1) Sort the frequency allocated or used from low to high;
(2) Calculate the number of channels corresponding to each frequency according to the minimum channel interval;
(3) Calculate the difference in the number of adjacent channels;
(4) Calculate the sum of the differences (by summing as an example);
(5) Check that the difference does not have the same number as the sum.
For example: The existing set of frequencies 156.275MHz 156.150MHz 156.200MHz 156.125MHz calculation whether there is an intermodulation combination (Figure 1).
The same number 3 indicates that there is intermodulation in this group of frequencies, and only the frequency of 156.275 is shifted upwards to 156.300 or other frequencies to avoid the intermodulation combination. The figures in parentheses above are the result of the calculation after the exchange.
Fourth, how to set up relay station's key technical parameters and antenna system
1. Key technologies of relay stations
The technical parameters of the repeater station are different from the transmission power, frequency stability, modulation characteristics, receiver sensitivity, audio output power and distortion, modulation receiving bandwidth and other technical parameters. Work, that is to receive signals at the same time, but also to send signals, the key technical parameter is the duplex receiver working sensitivity, that is their own transceiver interference situation, a good relay station should not affect the receiver's sensitivity . If the relay station has received and sent interference, it means that the sensitivity of the receiver is reduced. When the system works, it directly affects the communication distance between the walkie-talkie and the vehicle-mounted station.
2, forming a relay station to send and receive interference factors
(1) The transmitter's high-power signal enters the front end of the receiver and causes the receiver to block, causing the receiver sensitivity to drop.
(2) The receiver frequency components in the transmitter sideband spectrum are too large to be similar to the same-frequency interference, which degrades the sensitivity of the receiver.
3, to solve the problem of relay station to send and receive interference
In addition to the higher technical indicators of the repeater's own transmitter and receiver, it is important that the transmitter antenna port and receiver antenna port have high isolation and attenuation. There are usually two methods for isolating the transmitter and receiver antenna ports: (1) Duplexer mode; (2) Transceiver antenna coaxial vertical spacing mode.
4, the advantages and disadvantages of using a duplexer
(1) Advantages of using a duplexer
a. send and receive sharing an antenna, a cable, easy to set up;
b. Usually used notch duplexer, small size, light weight, low insertion loss, good isolation performance, can guarantee good transceiver interference indicators.
(2) The disadvantages of using a duplexer
a. Requires a wide antenna bandwidth, often the antenna can not take into account the transceiver frequency;
b. The stopband of the notch duplexer is very narrow. When multichannel is used, the frequency used is limited and must be limited to the stopband width of the duplexer.
5, the advantages and disadvantages of using dual antennas
(1) Advantages of using dual antennas
a. Send and receive respectively using their own antenna, get better matching and gain;
b. The transceiver frequency used for the channel can be arbitrarily set within a wide frequency range.
(2) The disadvantage of using dual antennas
The dual antenna must achieve a certain degree of transmission and reception isolation. When it is erected, there are strict requirements, which are often not easily met, resulting in worsening of the transceiver interference index and seriously affecting the communication distance.
(3) Requirements for using dual antenna mounting
a. When the dual antenna is horizontally erected, the horizontal distance between the two antennas should be ≥ 390λ;
b. When the two antennas are set up coaxially vertically, the vertical distance between the two antennas is ≥9λ.
(4) Dual Antenna Setup Table 1)
6, after the installation of dual antenna relay receiver to send and receive interference test
(1) Using walkie-talkie test method
First, set the transmit power of the intercom to the lowest level, and the repeater station is placed in the non-transmission state: Take off the radio antenna and transmit it intermittently for a short time. Let the walkie-talkie and the repeater gradually pull apart until the repeater receives the intercom signal with noise. At this time, the repeater station is placed in the forwarding state or transmitted using the microphone of the repeater station. The noise of the repeater station should not be changed. If the noise is strengthened or the squelch circuit is turned off, no noise is heard, indicating that there is a transmission and reception interference.
(2) Using the instrument test method
Using RF RF signal source, the output port is connected to the instrument's own antenna or walkie-talkie antenna, and the signal source output is adjusted to the relay station to receive noise (relay station is in the non-forwarding state), and the launch of the relay station is started at this time. Noise should not change significantly.
V. How to build a relay station network scheme for relay stations
1, secondary relay relay program
Explanation: (The frequency in the figure is assumed to explain the communication mode of the network)
This solution uses three relay stations with built-in diplexers to network areas A and B.
Intercom set up 2 channels:
1CH TX 150.000 RX 155.700 is used in Zone A
2CH TX 155.800 RX 150.100 is used in Zone B
Repeater A's frequency setting:
TX 155.700 RX 150.000
Repeater B1 frequency setting:
TX 150.100 RX 155.700
Repeater B2 frequency settings:
TX 150.000 RX 155.
2, secondary relay relay program two
Description: (frequency hypothesis in the figure)
This scheme uses a standard repeater (with built-in diplexer) in Area A, and uses two half-duplex radios in the B-zone. The combined repeater uses two payers in Area B. Antenna, intercom set up 3 channels:
1CH TX 150.000 RX 155.700 used in Zone A
2CH TX 155.800 RX 155.100 Talks in Area B and Area A
3CH TX 155.700 RX 150.100 self-relay in zone B
Half-duplex radio 1 frequency setting:
TX 150.000 RX 155.700
Half-duplex radio 2 frequency settings:
TX 150.100 RX 155.800
3. Two-area automatic roaming scheme
Explanation: Each of the A and B areas of this scheme uses a repeater station and a half-duplex station. The half-duplex station uses the half-duplex working mode of transmitting and receiving the same frequency, and only links the two areas.
Intercom sets two channels, frequency:
1CH TX 150.000 RX 155.700
2CH TX 150.000 RX 155.800
And set up these two channels to scan fast, because these two channels TX frequency are the same, no matter which district can be searched and can carry on the conversation, so no need to switch the channel manually in any district, thus reach the function of automatic roaming.
4. Two-area automatic roaming scheme II
Note: The two zones of the program A and B all use the relay station of the half-duplex radio combination and mutual control, and the radio is programmed with three channels and performs a scan search.
1CH TX 150.000 RX 155.700 Zone A self-relay
2CH TX 150.000 RX 155.800 Zone B self-relay
3CH TX 150.000 RX 150.100 A and B zones communicate with each other
Each half-duplex radio transceiver frequency is close to the duplexer's allowable bandwidth, so use duplexer to send and receive isolation.
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