TRANSMITTER ON G-807

AIR SIGNAL INDICATOR

Part 2

The direct connection of the antenna with the oscillatory circuit does not lead to a deterioration in its quality factor, since the receiving antenna of the monitor in the form of two pieces of wire 1.5 - 2 meters each, due to its short length (less than 0.01λ), works like a small capacitance and we can assume that the connection of the circuit with the antenna is weak external capacitance. Capacitor C4  - separating, for an operating voltage of 500 volts, serves to protect the device from accidental shorting of the A3 antenna  on the case, it also decouples the antenna sockets and the oscillatory circuit from the rest of the circuit in order to prevent the device from failing if voltage is accidentally applied to the antenna terminals relative to the case (well, the antenna wire fell on the 380 V bus).

Blocking capacitor C5  smoothes the ripple of the RF signal. At the same time, at the upper frequency of the modulating signal of 8 kHz, it has a reactance equal to the input resistance of the current amplification transistor stage, which ensures a blockage in the frequency response by 3 dB.

After the detector, the DC component and the detected modulation signal are separated by the inductor Dr1  and isolation capacitors C6  and C7. The detected low-frequency signal is fed to the bases of two complementary germanium transistors T1  and T2, each amplifying its own half-wave of the modulation signal. Resistors R1  and R2  set the transistors initial bias near the trigger point, providing them with class AB mode and, together with the active resistance of the inductor (Rdr  \u003d 1156Ω), form a collector stabilization circuit for the initial current. The constant component from the output of the detector, through the inductor Dr1  provides power to the transistor amplifier. Electrolytic capacitors C8  and C9  smooth out the pulsations of the DC component and at the same time form an artificial midpoint for the output push-pull transistor emitter follower, providing for it a symmetrical load connection circuit in the form of the primary winding of the output transformer Tr1 loaded on a low-resistance speaker (GR1). Equivalent load resistance (GR1), recalculated to the emitters of transistors, is 1k&Omega.

To indicate the relative level of the output power of the transmitter (the strength of the electromagnetic field), a voltmeter is built into the circuit, which measures the voltage of the DC component from the output of the detector and has two measurement limits of 0 - 5 volts and 0 - 25 volts. The choice of measurement limits corresponds to background, quiet (1.5 - 3 volts) and target, loud (15 - 20 volts) listening to radio broadcasts. Trimmer resistor R5  compensates for the spread in the value of the resistance of the microammeter frame, which, in total with R5  must be set to 3.2 kΩ. Additional resistances for two measurement limits must be: R3  + R4  = 46.8kΩ and R6  + R7  = 246.8 kΩ. To enable precise selection of values, they are composed of two resistors each. If the instrument is to be used for measurements, the resistance values must be selected accurately.

Design.  The monitor is made in a housing from a broadcast loudspeaker. A speaker, a microammeter head, an antenna and voltmeter scale switch, antenna terminals and an output jack are installed on the front panel of the case. Loop tuning capacitor C2  is placed inside the device, tuned to the radio station during the adjustment process and is not regulated during operation. Also, once, when calibrating the voltmeter according to the verified instrument, the tuning resistor R5 is adjusted. The device circuit can be assembled both by surface mounting and on the board, choosing its size and shape according to the available space inside the loudspeaker case.

Details.  Capacitors. C1  – type KSO-1, SGM, KTK or KT-2. C2  - PDA-2 or PDA-3. C3  – KTK, KT-1, KT-2, KD-1. C4  – type KSO-2, KTK or KT-2. C5  – KSO-1, KSO-2 or SGM. C6, C7  - imported film for an operating voltage of 100 volts. C8, C9  - electrolytic firms Jamicon for an operating voltage of at least 50 volts. Resistors. R5  - SPO-0.5 or SP3-9. The rest are MLT-0.125 or C2-23. Diodes D311 are best replaced with older D2E. With the same frequency properties, they have a reverse voltage of 100 volts and this monitor can be safely used with more powerful transmitters. Transistors npn: MP35 - MP38 or MP9 - MP11 with any letter indices; pnp: MP39 - MP41 or MP13 - MP15 with any letter indices. In the case of replacing transistors, it is necessary to select them for the same value of β and reverse collector current, and clarify the values of resistors R1  and R2. The fundamental point: only germanium diodes and transistors will work well in this circuit. The sensitivity of a monitor on silicon semiconductors will be much worse. The loop coil has an inductance of 50 μH, is wound on a piece of electrically insulating PVC pipe with a diameter of 40 mm with PETV-0.63 wire and contains 37 turns wound turn to turn. It is also possible to wind the coil on the frame KR35x44  [2] , then it should have 44 turns of the same wire. Throttle D7-5-0.04 or D15-10-0.05. It is also possible to make a throttle yourself  [3]  with an inductance of 5 - 10 henry, taking into account the bias current of not more than 25 mA.

If initially the broadcast loudspeaker, in the case of which the monitor is assembled, was designed to operate on a 30 or 36 volt network, then its speaker and output transformer (Gr1  and Tr1) are suitable for the monitor without alteration. If the loudspeaker was designed for a 15 volt broadcast network, then the primary winding must be rewound with a wire 1.4 times thinner (but not thinner than 0.1 mm) than the old winding was and lay 1.5 times more turns than it was in old winding. The secondary winding should be rewound with the same wire, and with the same number of turns, as it was wound under the existing speaker. If there is neither a speaker nor a ready-made transformer, then the transformer can be selected from the standard TOT series (for example, TOT61, TOT86, TOT118, TOT124, TOT148) with a transformation ratio n = 0.09 for a speaker with a winding resistance of 8Ω. For a 4 ohm speaker, n = 0.063. For other values of load resistance, the transformation ratio can be calculated: n = √(Rn / 1000). The speaker for the monitor must be selected with the maximum sound output (sensitivity) and with the maximum cone size that fits into the existing case. 4-ohm speakers 4GD8E, 4GD53, 5GDSH5-4 are ideal. Double microtumbler MT-3 can be replaced by TP1-2. It is desirable to use the microammeter head of the M4204 type, but any other one with a full deflection current of the needle of 100 μA will do. It all depends on the size and shape of your broadcast speaker. It is possible to use heads with a current of 200 or even 300 μA, but at the same time, according to Ohm's law, additional resistors will have to be recalculated. At the same time, the sensitivity of the meter will also drop, and the antennas will have to be located closer to the antenna output of the transmitter. Cinch-type output socket, antenna terminals are easily recognizable from Figure 2.

Working with the device.  Two antennas A1 must be connected to the device  and A3  in the form of pieces of wire one and a half - two meters each and stretch them, like a dipole, in opposite directions. The closer the instrument is to the transmitter's antenna terminal, the shorter the antennas are needed. With a long quiet, background, listening volume of the program (Uout  \u003d 0.3 - 1 volt eff.), the constant supply voltage according to the voltmeter can be in the range from 1.5 to 5 volts. If a loud “target” listening to the program is necessary, then the A2 antenna, located closer to the transmitter, is connected to the circuit and, accordingly, the voltmeter scale switches to the limit of 25 volts. In this case, at the output of the transistor amplifier, the voltage Uout  can be up to 5 volts of effective value. I draw your attention to the fact that the device going off scale at the limit of 25 volts can adversely affect the D311 diodes, since their maximum allowable reverse voltage is 30 volts.

When using the monitor as a source for control recording of the air, it must be rigidly fixed on a shelf or wall of the room, select the location and length of the measuring receiving antenna (A1  and A3) so that at a modulation depth of 30% the output voltage is 0.775 volts effective value and firmly fix the position of the antenna. In this case, the voltage of the constant component on the voltmeter should be in the region of 4 volts.

When using this monitor as a field indicator when setting up and matching the transmitting antenna directly at its feed point (sometimes it has to be done on the roof), it should be placed further from the antenna, but in direct line of sight of the instrument scale. By applying a modulation tone signal to the transmitter, in the process of matching the antenna, roughly, you can judge the setting by the volume of the sound, but accurately - by tracking the readings of the device.

The device will also be useful at medium-wave transmitting radio centers, where the intensity of electromagnetic fields in industrial premises can reach up to 5 volts per meter. True, you will have to change its frequency range to 522 - 1440 kHz, winding more turns on the coil and increasing its inductance to 200 μH. In this case, the monitor probably does not need an antenna, and you can reduce its volume by shunting the oscillatory circuit with a kilo-ohm resistor by connecting it between terminals A1  and A3. go between A2  and A3  depending on the field strength, transmitter power and the location of the device. For such an application, diodes will need to be taken D2E, and transistors MP40A and MP37B.

The device allows using it as a single-frequency electromagnetic field intensity meter in the near zones of transmitting antennas and in the premises of transmitting radio centers. In this case, it must be made in a shielded case and a calibrated rigid measuring antenna should be made for it. Accordingly, it will be necessary to compile a table of correspondence between instrument readings and field strength values at both measurement limits. In this case, it will be necessary to draw up instructions for taking measurements and calibrate the device according to an industrial meter.

A variant of the design and layout of the device suitable for measurements and verification is shown in Figure 2. For clarity, the walls of the device case are conditionally shown as transparent. The metal case of the device (for example, from AMG alloy) must be connected to the midpoint of capacitors C8  and C9  (Fig. 1). The case size in this case will be 245 x 140 x 70. The thickness of the front panel is at least 3 mm, the material is D16. The size of the board on which the parts of the device are placed is 105 x 75 x 1.5; material - fiberglass. Coil L1  wound on a frame KR35x44. To improve the sound quality of the monitor, it is advisable to fill all the free space inside the case with mineral wool, and place the speaker in a cotton cloth bag. The size, shape and number of holes in the decorative grille, as well as its material, are chosen by everyone according to their taste. The main thing is that it protects the speaker cone from damage during operation and does not drown out the sound.