Promising dual-band antenna-feeder device for a radio center of mobile VHF radio communications..
KOSTYCHEV Alexander Nikolaevich,
KRYUCHKOV Igor Borisovich,
PUGACHEV Vladimir Anatolyevich.
PROSPECTIVE DUAL-BAND ANTENNA-FEEDER DEVICE FOR A RADIO CENTER OF MOBILE VHF RADIO COMMUNICATION
This article examines the general principles of construction and technical solutions for advanced dual-band antenna-feeder devices (AFD) intended for urban or remote radio centers of mobile VHF radio communication systems and providing simultaneous independent operation of several transceivers in two dedicated frequency ranges, conventionally designated range “1” and range “2”. The technical characteristics, design and operation of the device, as well as its components, are provided.
Mobile VHF radio communication systems most often provide the ability to operate in two dedicated frequency ranges, with the frequency values being multiples, for example, the ranges of 450–470 MHz and 915–921 MHz in the TETRA trunking communication system standard. In these conditions, it is advisable to use dual-range AFUs with a circular or narrow-beam diagram. Calculations show that the use of such AFUs provides a significant economic effect due to reduced costs for manufacturing the product (one universal instead of two), the cost of design and installation work, and subsequent technical maintenance of the product.
The use of a dual-band antenna-feeder device (AFD) is significantly more advantageous in technical and economic terms than the use of separate AFDs, since many units are combined and are used simultaneously in both frequency ranges. In particular, there is only one antenna array, which is lighter and takes up less space on the support than two antennas for different ranges with similar electrical characteristics. For the same reason, it is advisable to use a dual-band AFD in cases where the radio center currently uses equipment for only one of the ranges, but both ranges are expected in the future.
With a sufficiently dense road network (for example, in cities), when the correspondent's azimuth can be arbitrary, non-directional radiation (reception) is required, and the antenna of the central station's antenna feeder must provide a directivity pattern (DP) in the horizontal plane close to circular (with some unevenness). Formation of a circular DP in a multi-channel antenna feeder is ensured by using a ring antenna array (RA), the number of radiators of which (within one floor) are located uniformly along a circle of a certain radius and are excited by means of a beam-forming scheme (BFS). As the latter, it is advisable to use the so-called Butler matrix (BM), which is a system of quadrature bridges connected to each other by means of coaxial cables of a certain length, which provide the necessary phase shifts. In the antenna feeder considered here, directional couplers (DC) with a transient attenuation of 3 dB (at operating frequencies) are used as bridges.
With low road density, when it is necessary to establish communication in some, quite specific directions, non-directional radiation (reception) turns out to be excessive, while the use of directional antennas allows to significantly increase the directivity gain due to the compression of the RP in the horizontal plane, due to which it is possible to reduce the number of floors (with the same communication range), i.e. significantly improve the weight and size indicators and reduce the cost. In these cases, it is advisable to use antennas with switchable (commutated) sector RPs, the set of which covers all directions (azimuths), since this makes the AFU more universal (in comparison with the AFU with a fixed directional RP, for example, in areas with road intersections), capable of adapting to the situation.
In this article, we will briefly consider the main problems that must be solved when creating a dual-band multi-channel AFU with a circular RP.
As calculations show, the non-uniformity of the circular RP of a Butler matrix AA increases with the AA radius and the number of emitters. It should be noted here that the AA radius is limited by the mutual coupling between the emitters, which manifests itself in the dependence of the input resistance of the emitters on their number. With too small AA radii, power losses increase significantly due to the mismatch of the emitters.
This significantly complicates the creation of a dual-range antenna feed with a circular pattern in the horizontal plane, since the electric radius of the array in different ranges changes approximately twice (in this case), which leads to a contradiction between the need to minimize the mutual coupling between the emitters in range “1” and ensuring a pattern with low non-uniformity (for a sufficient number of Butler matrix inputs) in range “2”.
When moving from range “1” to range “2”, the electrical distance between the vibrator and the support (reflector) approximately doubles, which affects the pattern in the vertical plane and the input impedance. An increase in this distance (from l /4 and more, l is the wavelength) causes a pattern distortion, which leads to a decrease in the directivity coefficient, a decrease (from l /4 and less) leads to an increase in the reactive component of the input impedance, which complicates matching. Thus, a contradiction arises between matching in range 1” and the formation of the pattern in the vertical plane in range “2”.
The use of a dual-range MB instead of two for each of the ranges in the AFU with circular RP is much more advantageous from a technical and economic point of view. However, the creation of such a MB is a serious problem. The use of phase shifters with a nonlinear frequency dependence of the phase shift based on complex phase-shifting circuits significantly complicates and increases the cost of the MB, reduces reliability, so that it does not allow obtaining any advantages compared to two single-range matrices. MB with phase shifters in the form of cable sections of a certain length is free from these disadvantages. However, the linear frequency dependence of such phase shifters leads to distortion of phase distributions, which, in turn, causes distortion of the RP in the horizontal plane, which is accompanied by an increase in its unevenness. In this case, it is necessary to ensure a reserve for the unevenness of the RP in the horizontal plane of the KAA when it is excited with nominal phase distributions.
Thus, the development of a dual-band multi-channel antenna feeder with a circular RP in the horizontal plane is a very complex task, within the framework of which it is necessary to solve a number of problems on the optimization of the antenna together with the support (in the case of antenna feeders with circular RP — together with the Butler matrix). It should be noted that the «openwork» of the support (in the considered ranges, an «opaque solid support is unacceptable for reasons of mass, wind and ice loads) is a useful quality, since it provides a significant number of variable parameters (the mutual arrangement of the support metal structures, any passive elements and active vibrators, the density of the grid of conductors forming the reflector, the longitudinal and transverse dimensions of the conductors, etc.), whereas in the case of a solid support it would be necessary to limit ourselves to changing its dimensions and shape.
When solving the optimization problem, it is necessary to take into account the amplitudes and phases of the currents induced on the metal structures and passive elements, as well as the mutual coupling between the emitters. This makes it necessary to use electrodynamic methods of analysis. In this case, the calculation of the antenna array directivity characteristics was performed based on a preliminary solution to the electrodynamic problem of current distribution along the conductors (the arrays are systems of linear conductors).
Below we consider a specific domestic development – the Pyramid AFU, which is a stationary six-input device that provides simultaneous independent operation of six transmitters and six receivers on one antenna with a circular RP in the horizontal plane with a non-uniformity of ± 3 dB and with an azimuth-averaged directivity gain (relative to the half-wave vibrator) of at least 8 dB in range “1” and 8.5 dB in range “2”. The Pyramid AFU allows connection to each of the transmitting inputs (receiving outputs) of the transmitter (receiver) of any of the used ranges (“1” or “2”).
The structural diagram of the Pyramid AFU is shown in Fig. 1. The main components are an eight-story AFU containing 8 emitters per floor and a transmitter-receiver combining device (TRUD). The KAR is connected to the UOPP via eight main feeders (MF1, MF2, — MF8).
The inputs of the KAR are the inputs of the RM 1×8 power distributors (8 pcs. in total, only one is shown in Fig. 1, connected to MF1). Each distributor evenly distributes the input signal between its eight outputs and provides power to the vertical line (KAR section) of eight emitters (vertical vibrators) located one above the other. The signal from the distributor to the vibrators is transmitted via lowering feeders. Within one section, the vibrators of floors 2-7 are excited in phase, the vibrators of the 1st and 8th floors (lower and upper) are excited with a small dephasing in order to eliminate interference zeros in the RP in the vertical plane. Phase distribution within the floor is provided by the UOPP (Butler matrix). The excitation amplitudes of all KAR vibrators are equal.
In the UOPP, the path of each transmitting input (inputs PRD1…PRD6) is divided by means of separating filters (SF) into two paths – ranges “1” and “2”, in each of which ferrite valves of the corresponding ranges are installed (B-1 – range “1”, B-2 – range “2”), increasing the isolation between the inputs to the required level. Directly at the input of each transmitting path (before the separating filter) a measuring section (MS) is installed, which together with the computing unit (CU) forms a KBV meter, designed to control the matching.
The diagram-forming circuit is a dual-range Butler matrix MB 7×8-RK (7 inputs, since a power divider is used instead of one of the input directional couplers), in which only 2 phase shifters (with a nominal phase shift of -45°) are tuned to the middle frequency between ranges 1″ and «2» (the actual phase shift is -30° and -60° at the transmission frequencies in ranges «1» and «2», respectively). The remaining phase shifters are tuned to range «2». Such preference for this range is due to the fact that in range «1» the electrical radius of the DA is quite small, as a result of which a larger reserve is provided for the non-uniformity of the radiation pattern in the horizontal plane. The directional couplers in the Butler matrix are made “overcoupled” (i.e. with a transient attenuation of less than 3 dB) at the center frequency between ranges 1” and “2”, so that at the frequencies of the ranges used, a transient attenuation of 3 dB is ensured, i.e. equal-amplitude division of the signal.
Only 6 inputs are used in the MB, the unused input is connected to the ballast load (BL). The second and sixth inputs of the MB are used to connect the receivers for the ranges “1” and “2”, respectively. The separation of the transmitting and receiving paths is carried out by means of diplexers (six-pole filters) D-1 and D-2. After passing the diplexers, the receiving signals are amplified in the 2×6-RK amplifier and are evenly distributed between the six receiving outputs PRM1 — PRM6.
In the AFU (in the computing units) there is a light and sound signaling of an unacceptably low level of coordination of the transmitting inputs of the UOPP. The value of the KBV at which it is triggered is no more than 0.5.
The emitters and other elements of the KAR are placed on a support, which is a system of vertical conductors. Some of the conductors (8 pcs.) are supporting elements of belts, the rest have a lightweight design and perform only an “electrical function” — they form a wire reflector. The horizontal connection of the belts and the designed geometry of the support are provided by horizontal struts — frames, the necessary mechanical rigidity of the support is provided by cross-shaped diagonals. The belt is formed by racks from pipes screwed together at the level of the struts. The connection of the racks is performed with threaded studs and locked with nuts. The racks of the lower five grids of the barrel are selected with a greater wall thickness than the upper ones. The material of the racks is high-strength aluminum alloy, studs, nuts — aluminum alloy. The upper frame has a lightweight design, the upper racks of the belts and the lightning rod are attached to it. The lower posts of the belts and braces are attached to the lower frame; the frame also has places for fastening to the stand. The vibrators are attached to the spacers (in azimuth – in the intervals between the belts), the power distributors are attached to the belts in the center of the support.
The radius of the KAR along the axes of the vibrators is 1.02 m. The distance between floors (the period of the vertical antenna array) is 1.5 m.
All vertically connected support elements have places protected from paint coating, ensuring their galvanic connection. Vibrators have electrical contact with spacers, power distributors — with belt posts. All support trunk elements have paint coating. The color of the coating of the three upper and three lower sections of the trunk is orange.
The vibrator used in this AFU is the result of theoretical and experimental studies that ensured the required electrical characteristics in the frequency bands of both ranges (VSWR no more than 1.2 at transmission frequencies and 1.6 at reception frequencies). The vibrator consists of two L-shaped arms, the vertical sections of which form the radiating surface. Stubs are welded to the horizontal sections of the arms. The central current lead, which is a rectangular plate made of aluminum alloy, and the horizontal sections of the arms form a three-wire line that functions as a balun. The stubs and current lead are fixed using two insulators. The internal cavity of the vibrator is covered with dielectric plates to protect the current-carrying elements from direct precipitation. Tuning plates are provided for adjusting the vibrator during its manufacture. After the vibrator is adjusted, the plates are firmly fixed and are not rebuilt during operation.
On the lower arm of the vibrator there is a socket, which is a design element and is made according to the connection dimensions of the SR-75-202 FV connector.
Fig. 1. Structural diagram of a dual-range antenna feeder with a circular beam pattern
The step-down feeders are based on the RK 75-9-13 cable and differ only in length. The power distributor is a branching of the coaxial path into eight directions, structurally combined with a four-stage transformer. The transformer is designed to match eight parallel-connected distributor loads (input resistances of the step-down feeders) with the wave impedance of the main feeder and is based on a rigid shielded line containing 4 cascade-connected sections with different wave impedances.
The directional coupler (as part of the Butler matrix) is made in the form of two strip lines on printed circuit boards located in a common housing, closed on both sides with covers. The boards are located strictly parallel at a fixed distance, which is precisely set by a set of washers. The distances between the housing covers and the boards are adjusted using strips. The power divider contains a two-band transformer and a splitter. The divider is implemented on a strip board placed in a housing similar to the NO housing.
The ballast load NB 50-RK is made on the basis of the resistor P1-3-50, which, together with the elements of the matching circuit, is installed on the radiator and covered with a lid.
The valve is a 3-input ferrite circulator, to one of the inputs of which a built-in ballast load is connected. Two types of valves are used: IV150-17 (range “1”) and IV150-34 (range 2”). Two valves are installed on one radiator.
The FR isolating filter is a fork-type connection of filters of ranges “1” and “2”. The device is structurally implemented on sections of coaxial cable. Mounting connections are made on two printed circuit boards. The FR housing is similar to the NO housing.
The D-1 diplexer is structurally an assembly of two NO and two filters, combined into a single rigid structure using a frame and connected by cables. A matched load is installed on one of the NO sockets. The filter is a printed circuit board located in a case. The case is closed with covers. A capacitive element is installed on the case. A movable short-circuiter is brought out through slots in the cover. The D-2 diplexer is designed similarly.
The 2×6-RK amplifier includes two amplifiers (ranges “1” and “2”), a control device and a divider. The amplifier of range “1” consists of a rejection filter (tuned to the transmission frequency band), a high-frequency amplifier (a two-stage transistor circuit) and a divider (ensures uniform distribution of the amplified signal power to six receiver inputs and isolation between them). The rejection filter is made on sections of coaxial cable, the divider is made according to a bridge circuit on quarter-wave sections of coaxial cable. The amplifier of range “2” is designed similarly. The control device monitors the current consumption of the amplifiers and, in the event of its deviation from the set level by ± 30% or more, generates an “Alarm” signal (in this case, the single “ALARM” indicator lights up and the relay, the contacts of which are brought out to the remote control plug, is triggered).
The 2×6-RK amplifier is powered by a power supply unit BP, which provides stabilized voltage of +24 V from two independent (main and backup) power sources. If there is a voltage of +24 V in the main channel, there is no voltage on the backup stabilizer. If the voltage of +24 V in the main channel disappears, the backup stabilizer is switched on. The BP has a fuse blown control circuit. When the “MAINS” toggle switch is turned on, single indicators “~220 V” and “+24 V” of the main and “~220 V” of the backup sources light up. If any of the networks disappears or the stabilizer fails, automatic switching to a working power source occurs. The BP is designed as a monoblock. The front panel of the BP contains: the NETWORK toggle switch, the “~220 V” and “+24 V” indicators, and fuse holders.
The KVV meter consists of a section of the measuring IC and a computing unit CU. The operation of the KVV meter is based on the use of the reflectometric method for determining the KVV by the levels of signals of the incident and reflected waves in the measured section of the feeder. Loop coaxial directional couplers are used as sensors of signals of the incident and reflected waves in the measured feeder. They are installed in the IC, where there is also an electronic circuit that ensures the detection of high-frequency signals corresponding to the incident and reflected waves, and the formation of DC signals that are fed to the input of the CU. A digital signal is formed in the CU that controls the digital indicator on which the KVV value is displayed. In the absence of an incident wave, the signal “No Ufall” is also generated (displayed on a single indicator), and when the indicator shows the values of the KVV in the range of 0.01…0.49, signals are generated for the generation of light and sound alarms indicating an emergency situation in the antenna-feeder path. To check the operability of the BV, a special mode is provided, which is activated by pressing the “CALIBER” button. If the BV is in good condition, the digital indicators should display a value of 0.75 in this mode. The BV has an autonomous secondary power source with a voltage of +5 V, -5 V, implemented on a transformer and microcircuits.
Structurally, the UOPP is a stand and a set of sections installed on the stand. All sections are installed in a certain order and connected to each other in a rack (cabinet). The rack is closed with perforated walls on the sides, top and back. The stand is closed with walls along the contour. A protective grounding clamp 3B-S-6×30 GOST 21130-75 is installed inside the stand on the frame.
The dual-band AFU can be placed both on the masts of city and remote VHF radio communication centers, and on the roofs of high-rise buildings.
An approximate option for placing the AFU on the mast of a radio center is shown in Fig. 2.
Fig. 2. Option for placing the AFU on the mast of the radio center
The specified AFU was mounted on one of the facilities in 1998 and is successfully operated to this day.