Automatic antenna switching is not only convenience, but also safer operation of the equipment. Yeasu transceivers are equipped with a socket on which BCD signals represent the currently selected band. The truth table for the TUN/LIN connector in FT-991 is below:
|D (pin 7)||C (pin 6)||B (pin 5)||A (pin 4)||Dec value||BAND||CD4514 pin|
|0||0||1||1||3||5 & 7 MHz||8|
Warning! For these signals to be available on the connector, in the case of FT-991 in MENU position 141 (TUNER SELECT), the LAMP (Linear Amplifier) option must be selected.
Janusz SP5CIB published a description of his switch, in which he used CMOS decoder type CD4028 to control relays. Unfortunately, in this solution, it is impossible to decode signals for the 50, 144 and 430 MHz bands. The problem is solved only by using a decoder CD4514 with 16 outputs, as shown below.
An additional advantage of the CD4514 IC is the ability to easily control antenna relays using the ULN2803A driver (8 x Darlington), as shown in the simplified diagram below. Security and flirting elements of the CD4514 inputs have been added there, which will be important for a longer cable between the antenna switch and the transceiver.
The CD4514 decoder can also be used to control multi-band antennas using simple diode logic. The last figure shows an example of automatic switching of a 6-band antenna (Hexbeam 20-6 m), a 2-band antenna (W735 80-40 m) and a dipole for a 30 m band.