Adding a RX antenna on the YAESU FT-991 / FT-991A radio

The FT-991 is a nice small size and lightweight radio (4.3 kg / 9.5 lbs), it is perfect for portable operations.
I used it on my FH/HB9AMO and 5U9AMO DXpedition and in the CQWW CW contest. It can be carried along with a laptop computer as hand luggage in airplane. The weak point of this radio, is it does not have a separate RX antenna capability. Therefore i decided to add this feature.
Material needed
One chassis panel mount BNC female connector.   -   One mini ON/OFF or SPDT toggle switch.
One 12 volts mini SPDT relay.   -   One feet or 30 cm of mini coaxial cable like RG-188 or RG-316

On the radio, the HF+50 MHz antenna TX/RX relay is located on the HF-PA unit board. From there, a small coaxial cable carry the RX signal to the Main Unit board.
The modification consists in cutting this small coaxial and inserting in this line a SPDT mini relay which allows to select the received signal from the main antenna or the signal from the newly added BNC connector. The relay is activated by a mini switch fitted at the back of the radio. See schematic below.

On the back side of the radio a BNC connector is added to connect the RX antenna and a switch is fitted to select between RX antenna or main antenna. See the implementation below:

Inside the radio the small relay is glued onto the back side the radio. See the cabling sketch below:

You can also solder two 1N4007 diodes back to back to the RX antenna connector to protect your receiver from strong signals.