Very convenient to use a CB antenna mounted on a vehicle, not only for telecommunication but also to improve radio reception in the FM-band. However we need a device that would operate on one antenna and CBS radio and FM receiver without damaging the last strong signal transmitter.

The duplex option CB/VHF

For this purpose you can use a very simple device - diplexer. His scheme is a combination of 4 series resonant circuits that pass signals of certain frequencies and suppress other signals.

Circuit L1-C1 and L3-C3 are tuned to the mean frequency of si-Bi-band to 27.2 MHz), and L2-C2 and L4-S4 - on top of the portion of the FM band (90 MHz). Coils L1 and L3 wound insulated wire with a diameter of 0.5 mm on the plastic frames with a diameter of 7 mm with ferrite cores. Frameless coils L2 and L4 have 8 turns of silver coated wire with a diameter of 1 mm, is wound on the mandrel with a diameter of 6 mm. the details of the duplex option are placed in a shielded box, the walls which connectors "FM", "CBS" and "Ant".

To configure custom device to the connector ""Ant" connected, SCA, tuned to a frequency of 27.2 MHz, and to "FM" and "CBS" - resistors 50 Ohm.

Controlling the RF voltage on those resistors (e.g., oscilloscope), tunable inductance coil L3 to achieve the maximum voltage on connector CBS", and tuning L1 - minimum voltage on the connector "FM". Then GSS is configured at a frequency of 90 MHz, and, by changing the capacitance of trimmer capacitors C2 and C4, achieve respectively the minimum voltage on the connector "CB" and high on"FM".

These operations it is desirable to do a few time. ensure that the suppression signal with a frequency of 27.2 MHz socket "FM" amounted to not less than 40 dB.


  • "Duplekser CB/UKF", Swiat Radio", N6/2003
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