Construction of the high-frequency path of the base station of the radio communication system
How to build a base station for a professional radio communication system?
In this article, we will consider three options for the layout of the equipment of a single-channel base station and the option of building a multi-channel base station.
Repeater with receiving and transmitting antennas
The simplest version of the HF path of a base station with a repeater is when the receiver and transmitter work on two separate antennas. The separation between the receiving and transmitting paths is ensured by the spatial distribution of the antennas. The method of calculating the spatial spread of antennas is described in the article: https://horwin.info/statti/metodyka-rozrakhunku-neobkhidnoho-pro/.
It is more convenient to do this with dipole antennas, placing them on one mast one above the other. Or one collinear antenna placed on the top of the mast and one dipole placed on the mast below. Dipole antennas for radio communication are most often chosen precisely because of their convenience of placing several antennas on one mast and a wide working band.
In the conditions of low risk of interference from EW systems, the use of two antennas without a duplexer allows you to quickly change the frequency of the base station, while reconfiguring the duplexer requires the presence of a device and experience.
Methods for calculating the required spatial separation between the receiving and transmitting antennas are given below.
One antenna and a duplexer
Small duplex filters
Often, the location of the base station does not allow to ensure the spatial diversity of the antennas. In this case, one antenna and a duplex filter (duplexer) are used, which separates the receiving and transmitting paths.
The use of small-sized duplexers is possible when the radio communication system uses duplex pairs of frequencies with a difference of ≥5 MHz in the VHF range and ≥10 MHz in the UHF range. At the same time, a combination of sufficient signal quality with low cost and compact dimensions of single-channel and multi-channel base stations is ensured.
Duplex filters on volumetric resonators are used for smaller dispersion.
Duplex filters are calculated for the maximum power of the transmitter and differ in design. They are more often produced in versions up to 50 W or up to 100 and more W.
Let’s consider options for building the HF path of a single-channel station
Option 1
A single-channel base station with a repeater power of up to 50/100W and no other transmitters in the immediate vicinity. The band-pass duplex filter (DF) provides a sufficient level of decoupling of the receiving path of the transmitter signal to ensure its maximum sensitivity. Small in size, small frequency difference.
Option 2
The same single-channel base station with a power of up to 50 W, antennas of other base stations operating on neighboring pairs of frequencies are located in the immediate vicinity. Under such conditions, extraneous interference from other stations will definitely appear. The problem will be solved additionally by a band-pass filter (PRF). This type of filter will be able to provide the appropriate maximum sensitivity in the receiving path. At the same time, bandwidths and rejections are selected.
Option 3
Combining stationary radio stations into one antenna.
The repeater of the base station is organized into two mobile radio stations as a receiver and a transmitter. Two simplex radio stations with a frequency difference corresponding to the parameters of the duplex filter can be combined into one antenna. All parameters of radio stations in all modes are provided.
Multi-channel base station with common antenna
In conventional and trunking radio communication systems, multi-channel base stations are most often built. A simple, compact solution for the construction of two-, three-, and four-channel base stations of radio trunking systems is the use of AFP racks (antenna feeder devices) with hybrid combiners. This allows you to use one antenna for independent operation of all channels of the site. Such a solution when constructing the AFP path is possible in the case of selecting duplex pairs of frequencies with a small difference between adjacent channels (the range of difference is determined by the parameters of the duplex filter and the combiner addition bridge).
The functional diagram of the stand is shown in the figure.
Components of the AFP stand:
• ferrite insulators,
• 2.3 harmonic filters,
• hybrid combine harvester,
• small duplex filter,
• band-pass filter,
• receiving distribution panel with a compensating low-noise amplifier
Ferrite insulators (FI – ferrite isolator) provide decoupling between transmitting channels in order to avoid intermodulation in the output stages. Intermodulation interference of a powerful signal of a nearby transmitter can fall on the frequencies of receiving paths and significantly worsen the quality of reception of repeaters. It is recommended to use double isolators to ensure a good level of resolution. This element of the radio system is one of the most complex and expensive components.
2nd and 3rd harmonic filter (FLP filter) to reduce out-of-band transmitter noise for receivers of other bands.
The hybrid combiner (HC – hybrid combiner) consists of an assembly bridge and an absorbing load, ensures the summation of transmitter signals with observance of phase relationships, which ensure resolution between channels. A high-quality hybrid is a combiner with 3 dB attenuation and a large resolution. Such a decoupling, together with the decoupling of ferrite insulators, ensures powerful and uninterrupted operation of transmission paths.
A duplex filter (DF – duplex filter) ensures the operation of one antenna of the transmission and reception paths.
A band-pass filter (PRF – pass-reject filter) provides additional suppression of transmitter signals at the input of the receiving path.
The receiving distribution panel (MC – multicoupler) is used in multi-channel radio communication systems. It amplifies the received signal, compensates for level losses from other devices in the path, and divides it between the receiving inputs of repeaters. This provides the necessary decoupling between the inputs of the receivers.
Structurally, all equipment is compactly placed on a 19″ shelf.
The photo below shows the rack of the HF tract of the 2-channel base station.
Regardless of whether it is a single-channel or multi-channel base station, or a fixed subscriber radio station, the HF path is of crucial importance in radio coverage and communication quality. And this directly depends on the quality of its elements – duplexers (DF), band-pass (PF) and band-pass filters (PRF), combiners, isolators, etc.. Of course, at the same time, the antenna must be installed correctly, and the connectors must be well mounted and waterproofed on coaxial cables.