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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Antennoscope is a high-frequency measuring bridge. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Antennas. Measurements, adjustment, coordination When tuning antennas in amateur radio practice, two types of bridge meters are used: unbalanced and balanced. The former are known as SWR meters and are relatively widespread. The latter are usually called antenoscopes in the literature. They are less common, although they allow you to get some additional (compared to SWR meters) information about the antenna-feeder path of the radio station, the analysis of which can facilitate its tuning. For the manufacture of an antennascope, radio amateurs usually use a successful design, a description of which was given in the popular book by K. Rothammel (Antennas. Translated from German - 3rd ed. Revised and added - M .: Energy, 1979. MRB, vol. 998). The device, which will be discussed in this article, differs from it in a more convenient balancing of the bridge and a more accurate determination of the resistance corresponding to the balance of the bridge. The schematic diagram of the device for measurements in the antenna-feeder path of the radio station, consisting of a measuring generator and a balanced bridge, is shown in fig. 1. The actual bridge with the balance indicator is a separate node that operates in a wide frequency band. The upper frequency limit is determined by the bridge design (parasitic capacitances and inductances) and in the usual version easily reaches 30...50 MHz.
The bridge can be used with an external generator providing a few volts of high frequency voltage across the bridge. For these purposes, the radio station itself is also suitable, but its power level must be reduced to the required values \uXNUMXb\uXNUMXb- with a level control (if any) or an additional attenuator. However, if it is necessary to work in the field, it is advisable to combine the bridge in one device with a generator that is powered by an independent source. It is this version of it, designed for measurements in the CB range, that is described in the article. In contrast to the common designs of antenoscopes in this device, the bridge is balanced by two series-connected resistors, which makes it possible to perform this operation more accurately. The presence of two resistors practically eliminates the possibility of reading the resistance corresponding to the balance of the bridge on the scale of the device. The device for determining this resistance provides for its measurement using an external ohmmeter. The RF voltage from the generator is supplied to the connection point of the resistors R5, R6, which form the upper arms of the bridge. Resistors R7 and R8 connected in series constitute an adjustable arm, and the measured one is the input impedance of the antenna or antenna-feeder system connected to the device through the XW1 "RX" jack. As can be seen from the diagram, the R7R8 adjustable arm is connected to the bridge through the SA1.1 switch. It allows you to connect this shoulder to the XS2 socket to measure the total resistance of the resistors set at the balance using an ohmmeter (multimeter). Into the measuring diagonal is the VD1C9 circuit, with the midpoint of which the VD2R9C8R10 circuit is connected to the PA1 dial balance indicator (R10 is its sensitivity regulator). Another difference between this bridge and the antennascope mentioned above from the book of K. Rothamel is that it is powered by high-frequency voltage relative to the common wire. The lower arm of the bridge (R8), the braid of the coaxial cable and the antenna counterweight (through the XW1 socket), as well as the measuring circuit of the balance indicator PA1, are also connected to it. Thanks to this solution, the influence of the operator's body on the measurement results is reduced. The built-in generator is assembled on a transistor VT1 with an oscillatory circuit L1C1 in the collector circuit and a quartz resonator ZQ1 in the base circuit. From the coupling coil L2, inductively coupled to L1, the RF voltage from the generator is fed to the input of a power amplifier made on the transistor VT2. Resistor R4 limits its base current, inductor L3 is a collector load, and the series oscillating circuit L4C7 serves to set the required level of the RF supply voltage of the measuring bridge. The generator and amplifier are powered by direct current from the voltage regulator DA1. An external source must provide a voltage of 12...15 V at a load current of up to 100 mA. It is connected via socket XS1. LEDs HL1 and HL2 - indicators of power supply to the device and its operating modes. In the "Countdown" mode, i.e. when measuring the total resistance of resistors R7 and R8 with an external ohmmeter, the power is turned off to avoid overloading the PA1 indicator that occurs when the bridge is sharply unbalanced due to the adjustable arm being turned off. The device is made in a metal case with dimensions of 130x80x40 mm. In "field" conditions (on a car, on a boat), it is powered from the vehicle's on-board network, and at home - from a rechargeable battery or a transceiver AC power supply. All controls are displayed on the front panel, and sockets - on the side walls. The remaining parts are mounted on two printed circuit boards made of foil fiberglass, the drawings of which are shown in Fig. 2 (node A1) and 3 (node A2). All connections are made with short rigid wires, the A1 board is placed in a separate metal shield made of 0,5 mm thick brass sheet.
During installation, fixed resistors MLT, capacitors K50-35 (C5), KT, KD and KM (the rest) were used. Variable resistors R10 - SP3-4aM, R7 - dual SP3-3dM with a nominal resistance of 1 kOhm (sections are connected in parallel), R8 - SP2-3a. The nominal resistances of the resistors R5 and R6 (about 200 ohms) are not critical, but it is important that they are the same (tolerance is no more than 5%). The total resistance of the resistors R7 and R8 (600 is selected based on the possible values of the input impedances of most antennas. The ZQ1 quartz resonator is any small-sized one with a natural frequency (or third harmonic) corresponding to the frequency of one of the middle (15-25) channels of the subband (grid) "C "CB. Coils L1, L2 and L4 are wound with PEV 0,31 wire on polystyrene frames with a diameter of 7,5 mm with SCR-1 trimmers made of carbonyl iron. L1 contains 12, L2 (wound over L1) - 4, L4 - 10 turns. Inductor L3 - unified DM-0,1 with an inductance of 20 μH. High-frequency socket XW1 - СР-50-73П8, XS1 and XS2 - any low-frequency screened ones. The SA1 switch is a small-sized toggle switch of any type with two positions and two directions. When repeating the device to work on other ranges, it is necessary to change all the elements that affect its frequency characteristics (quartz resonator, coils and capacitors of oscillatory circuits). When adjusting the device by rotating the coil trimmer L1, the L1C1 circuit is tuned to the frequency of the quartz resonator. Next, set the RF voltage on the measuring bridge. To do this, instead of the latter, a constant resistor with a resistance of 6 ... 120 Ohms with a power dissipation of 130 ... 0,5 W is connected to the capacitor C1 and, by changing the inductance of the L4 coil with a trimmer, an RF voltage of 9 ... 10 V is set on it. In conclusion remove the resistor and restore the connection of the capacitor C6 with the bridge. Before using the device, the slider of the variable resistor R10 must be set to the position corresponding to the minimum sensitivity of PA1 (lower according to the diagram), and the SA1 switch to the "Countdown" position. Then, a power source is connected to the XS1 socket (the HL2 LED should light up), to the XS2 socket - a multimeter in the resistance measurement mode from 0 to 600 Ohms, and to XW1 - a matched antenna with an unknown input resistance RA. Next, the SA1 switch is switched to the "Balance" position (the HL1 LED lights up) and by changing the resistance of the input parts of the variable resistors R7 (coarsely) and R8 (finely), the readings of the microammeter PA1 are achieved, while increasing its sensitivity by reducing the resistance of the resistor R10. The minimum reading at the maximum sensitivity of the device corresponds to the exact balance of the bridge. After that, the switch is returned to the "Countdown" position and the total resistance of the resistors R7, R8 is determined by the reading of the ohmmeter. After completing the measurements, move the slider of the resistor R10 to its original position and turn off the power. In the general case, the input impedance of the antenna-feeder path with SWR not equal to 1 has both active and reactive components. Since the antennascope does not have reactive component compensation elements in the adjustable arm, in some cases the minimum corresponding to the balance of the bridge may not be very deep. Nevertheless, the resistance value obtained as a result of measurements with an antennascope is close to the value of the active component of the input resistance of the antenna-feeder path. It can be used during antenna tuning to assess the degree of matching of the transmitter with the antenna-feeder path and to improve the degree of matching. Authors: L.Nikolsky, B.Tatarko, Tver
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