Duplexer CB/VHF. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Duplexer CB/VHF. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Antennas. Measurements, adjustment, coordination

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It is very convenient to use the CB antenna installed on the car, not only for radio communication, but also to improve radio reception in the FM band. However, a device is required that would allow both a CB radio station and an FM receiver to work on one antenna without damaging the latter with a powerful transmitter signal.

Duplexer CB/VHF

For this purpose, you can use a very simple device - a duplexer. His circuit is a combination of 4 series resonant circuits that pass signals of some frequencies and suppress signals of others.

The L1-C1 and L3-C3 circuits are tuned to the middle frequency of the C-B band (27,2 MHz), and L2-C2 and L4-C4 are tuned to the beginning of the FM band (90 MHz). Coils L1 and L3 are wound with insulated wire 0,5 mm in diameter on plastic frames 7 mm in diameter with ferrite cores. Frameless coils L2 and L4 have 8 turns of silver-plated wire with a diameter of 1 mm, wound on a mandrel with a diameter of 6 mm. The details of the duplexer are placed in a shielded box, on the walls of which the "FM", "CB" and "Ant" connectors are installed.

To configure the manufactured device, a GSS tuned to a frequency of 27,2 MHz is connected to the "Ant" connector, and 50 Ohm resistors are connected to "FM" and "CBS".

By controlling the RF voltage on these resistors (for example, with an oscilloscope), by adjusting the inductance of the L3 coil, they achieve the maximum voltage at the C-B connector, and by tuning L1, the minimum voltage at the FM connector. Then the GSS is tuned to a frequency of 90 MHz, and, by changing the capacitance of the tuning capacitors C2 and C4, achieve, respectively, a minimum voltage at the CB connector and a maximum at FM.

These operations are desirable to be done several times. ensuring that the 27,2 MHz signal at the "FM" jack is at least 40 dB attenuated.

Literature

"Duplexer CB/UKF", Swiat Radio", N6/2003

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