A device for orienting UHF antennas. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Television antennas

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Difficulties in orienting antennas are encountered by many radio amateurs. This process can be facilitated by special devices - tunable indicators or level meters. In particular, in "Radio" No. 11 of 1996 (pp. 8, 9) a device for orienting antennas in the MB band was described. The article published here considers a similar device for the UHF range.

The development of the terrestrial television broadcasting network in our country is moving along the path of putting into operation new transmitters operating mainly in the UHF band. It is often difficult to achieve high quality TV reception in this band. Most new transmitters tend to be of low power, low height transmitting antennas, often located in different urban areas.

All this leads to the fact that the use of indoor antennas becomes impossible. You have to use effectively directional antennas, place them outside the living space and at a considerable distance from the TV. In turn, this causes additional signal attenuation in the connecting cable, which forces the use of antenna amplifiers. In addition, there is a problem of antenna orientation.

In solving these problems, a relatively simple device proposed for repetition by radio amateurs will help. It allows you to significantly simplify the orientation procedure and approximately determine the level of the received television signal. Its dimensions are small (see Fig. 1, approximately like a pack of cigarettes), so it is convenient to use it when orienting antennas in a variety of places.

The device was developed at the request of readers based on the circuitry of a similar device for the MB range, described in "Radio" earlier. Its scheme is simpler (see Fig. 2), and the dimensions are smaller. The device is a receiver for direct conversion of UHF signals and contains an RF amplifier (VT1, VT2), a local oscillator (VT3), a mixer (VT4), a video amplifier (VT5, VT6) and an amplitude detector (VD1, VD2). The level of the received signal is indicated by the pointer measuring head PA1.

(click to enlarge)

The television radio signal is fed to an RF amplifier, assembled according to a two-stage circuit with deep negative DC feedback through resistor R4. At the input of the RF amplifier, a C1L1C2 RF filter is installed, which suppresses signals below the UHF range. To expand the interval for indicating the level of received signals, two RF connectors are installed at the input. Through the XS1 connector, the radio signal from the antenna enters the RF filter. By applying the same signal to the XS2 connector, you can attenuate it by a factor of ten. The RF amplifier transfer coefficient is approximately 15 dB, and the frequency response unevenness in the frequency range 470 ... 800 MHz does not exceed 1 dB.

The amplified signal goes to the mixer. There is also a local oscillator signal. The resulting video signal passes through the C11L4C12 low-pass filter with a cutoff frequency of 4 MHz to the video amplifier.

The local oscillator is assembled according to the capacitive three-point scheme. Its frequency is tuned by a variable capacitor C8. The local oscillator is connected to the mixer through the coupling coil L3. It operates in the DM V interval. The converted frequency band is from 0,02 to 4 MHz. Since the mirror channel is not suppressed in the presented version of the device, its total bandwidth is approximately 8 MHz, which corresponds to the width of one television channel.

The selected video signal, having passed through the video amplifier, is detected by an amplitude detector, and the resulting voltage is measured by a pointer indicator.

The operating mode of the device is changed by the switch SA1. In its position 4 - "Off." The power supply to the device is not supplied. In position 3 - "Control", a resistor R25 is connected to the battery, through which a current flows equal to that consumed by the device. Through the resistor R26, the battery voltage comes to the dial indicator PA1, which controls its value.

In positions 1 and 2 of the switch, the device operates in indicator mode. In position 1 - "0,2 mV", the battery voltage is supplied directly to all components of the device and the maximum value of the dial indicator readings is 0,2 mV. In position 2 - "2 mV", the supply voltage to the RF amplifier comes through the tuning resistor R17, the transmission coefficient of the RF amplifier decreases and the maximum value of the scale will already correspond to 2 mV. In addition, the sensitivity can be reduced another ten times by applying a signal to the XS2 connector. Therefore, the maximum displayed level is 20 mV, and the minimum level is determined by the sensitivity of the entire device and is in the range of 20...40 µV.

Structurally, the parts of the device are placed in a plastic case with dimensions of 100x65x25 mm. At the same time, part of it serves as a battery compartment, and an area of ​​​​60x65 mm remains for the rest of the parts. The M4761 pointer indicator is fixed here, which has large scale sizes and a relatively small electromagnetic system. A rectangular window measuring 50x25 mm was made for the indicator on the front panel of the case. The pointer indicator itself has been improved: parts of its body have been removed from both sides near the electromagnetic system. If you use an indicator of smaller sizes, for example, M4762-M1, then such refinement is not required.

Most of the parts are located on a printed circuit board made of double-sided foil fiberglass, a sketch of which is shown in fig. 3. The side free from parts is left metallized, it plays the role of a screen and is connected in several places around the perimeter to the common wire of the other side of the board.

The placement of the instrument nodes inside the case is shown in fig. 4. Dial indicator 1 is glued to the bottom of the case, which serves as the front panel. A printed circuit board 2 is glued to the indicator. Near the electromagnetic system 4 of the indicator, on one side of it, a capacitor of variable capacitance 5 is installed on the case, and on the other, a switch (obstructed by the system 4). Under the capacitor, before installing it, it is necessary to place a strip of foil b made of tinned copper, which must be connected to a common wire and the screen of the printed circuit board 2. To eliminate the influence of hands on setting up the device, the part of the board on which the high-frequency elements are placed must be covered with a screen 3 of foil or thin one-sided foil fiberglass, connecting its foil in several places with a common wire.

In addition to those indicated in the diagram, transistors KT382 (VT1-VT4) with any letter index, KT315V, KT315G, KT2102A - KT3102G (\ Ya5, UT6) or similar can be used in the device. Diodes - KD521, KD503. KD509 with any letter index. Capacitors C15, C20 - K50, K53. Variable capacitor C8 - 1KPVM with an air dielectric. The remaining capacitors are KM, KD, KLS. Fixed resistors - MLT C2-33 or C2-10, tuned R17 - SPZ-19. When mounting resistors and capacitors in the high-frequency components of the device, their leads should be shortened to the minimum possible length.

Coil L1 is wound with wire PEV-2 0,2 on a mandrel with a diameter of 2 mm and contains three turns with leads 2 ... 3 mm long. Coils L2 and L3 are wound on the same mandrel and contain, respectively, three and one turns of PEV-2 0.8 wire. Coil 12 is installed between the PCB pad corresponding to the circuit and the output of the variable capacitor C8, and coil L3 is placed close to 12. Coil L4 is a DM-0,1 inductor.

Setting up the device begins with checking the performance of the local oscillator and setting its tuning boundaries. If it is possible to use a frequency meter, then it is connected to the L3 coil. Otherwise, you will have to use a TV, which is tuned to the lowest frequency 21st UHF channel and bring its antenna close to the local oscillator. The rotor of the capacitor C8 is set to the maximum capacitance position and, by bringing together or pushing the turns of the coil 12, the local oscillator signal appears in this channel.

Next, the rotor of the capacitor C8 is rotated to the position of the minimum capacitance and it is checked at the frequency of which channel the local oscillator operates. Sometimes this has to be done approximately, since most modern TVs do not have exact UHF channel number or frequency indicators. You will have to navigate by the signals of working transmitters.

For the variable capacitor ratings indicated on the diagram, the local oscillator is tuned from 470 to about 650 ... 670 MHz, i.e. from the 21st to the 44th channel. If this is not enough, you need to use a variable capacitor with a maximum value of one and a half times the maximum capacitance, and wind the coils L2, L3 on a mandrel of a smaller diameter.

If it is possible to tune the RF amplifier using a measuring device, then this is done by first disconnecting the supply voltage from the local oscillator for a while. By selecting capacitor C5, the minimum frequency response unevenness in the required frequency range is obtained.

Then, after turning on the local oscillator power supply, a signal with an amplitude of 1 ... 2 mV and a frequency corresponding to the middle of the tuning interval is fed to the input of the device from the exemplary generator. The device in switch position 1 is adjusted with capacitor C8 according to the maximum indicator readings. If its arrow goes off scale, then the generator signal level is reduced.

Further, by changing the signal level of the generator, the levels are determined: the first - when the device clearly registers it, that is, when the arrow noticeably deviates, and the second - when the indicator needle is at the maximum mark on the scale. The first level corresponds to the sensitivity of the device. If the second level is within 0,1 ... 0,5 mV, then the indicator scale can be graduated. If it is larger, they increase the transfer coefficient in the IF amplifier by using high-gain transistors.

Set the switch to position 2 and apply a signal from the generator ten times greater than the maximum signal in position 1 of the switch. Trimmer resistor R17 is used to achieve deviation of the indicator needle to the maximum scale mark. Reduce the signal level of the generator and calibrate the scale of the device in millivolts or decibels. And finally, the scale of the variable capacitor is graduated. It is best to do this in the UHF channel numbers.

If there is no need to calibrate the indicator scale or it is not available, then this is not done, leaving it uncalibrated. In this case, the device performs the functions of a relative level indicator, which is quite acceptable for antenna orientation.

In conclusion, a voltage corresponding to the nominal voltage for the battery is supplied from an adjustable power supply, and the resistor R26 is selected so that the arrow deviates to a noticeable scale mark, for example, maximum or average. After that, the voltage is reduced to a level where the parameters of the device noticeably deteriorate, for example, the frequency “goes away” or the sensitivity decreases, and this deviation of the arrow is noted on the indicator scale. During operation, the battery voltage should not drop below this value.

The device is powered by a 9 V battery. The maximum current consumption is 22...25 mA.

It should be noted that the RF amplifier can be used separately to build an antenna amplifier in the UHF range. Using one such amplifier, a gain of about 15 dB is obtained, and two connected in series - 28 ... 30 dB.

Author: I. Nechaev, Kursk

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