Modification to allow separate RX and TX Antennas
This modification will allow the use of separate receive and transmit antennas on the Kenwood TS-850S/AT.
Caution: When using split antennas with this modification you must use an external relay to either open the RX antenna lead or ground it. This modification has no protection for the front end of the receiver. When using the jumper there are no problems as the original protection is used.
The following parts were used in the modification. Use of other parts may require different spacing on the jack placement.
RCA Phono Jack: Mouser PN 154-4315
RCA Phono Plug: Mouser PN ME171-8113 (This is a crimp/solder connector for RG-58)
Place the radio upside down and remove the bottom cover. With the back of the radio towards you the board on the right will be the RF unit. Remove the shield over it and then remove the screws holding down the board. At the rear of the radio are 2 coax cables and an orange 2 wire connector. Unplug all three and then raise the RF unit and fold it back over the radio (See picture) No other cables should have to be unplugged. Be careful with the ribbon cables.
Notice the 2 small (1/8") holes at the right top corner of the rear panel. These are pilot holes for mounting the RCA jacks.
This is a close-up of the holes. They are 1/2" below the top edge. The right hole is 1 1/8" from the right edge. The second hole is 1" over. The pencil line can be erased after drilling the pilot holes. The jacks I used needed a 9/32" hole and a drill was used to enlarge the pilot holes to this size.
After drilling the 9/32" holes the jacks are mounted. Note the orientation of the ground lugs and center connectors. Mount the RF unit back into the radio and extend the cables back toward their original location. The coax cable with the orange marker is then pulled across and cut leaving enough to connect to the first jack. For this radio it was about 1/3 of the way down the orange marker. It is stripped and connected to the first jack as shown. The cut off piece is then soldered to the other jack and plugged back into the RAT jack. Study the picture closely and there shouldn't be any problems. The other coax cable should be plugged in the jack labeled White. If you get it in the wrong place there will be no power out in TX. Plug in the orange 2 pin plug and then re-install the shield.
This is a picture of the jumper cable I made using the Mouser Phono Plugs and RG-58 with a little heat shrink for strain relief. It is 7" long so it doesn't have to make a sharp bend.
This is the jumper installed and ready. The jumper is used for normal operation. When using a different antenna for RX it will be plugged into the jack labeled RX Ant. Don't forget to use a relay or transistor to ground or open the separate RX antenna from the radio in TX.
THERE IS NO PROTECTION FOR THE RECEIVER WHEN IN TX MODE WHEN USING THIS MODIFICATION. TRANSMITTING WITHOUT TAKING PRECAUTIONS TO OPEN OR GROUND THE SEPARATE RX ANTENNA JACK WILL CAUSE DAMAGE TO THE RADIO.
Clif Holland KA5IPF
This modification has been done in my shop and works fine. Your results may vary. If desired we will modify your radio for $50 including the jacks and jumper (plus return shipping). We are currently working on an internal modification to protect the RX front end when using split antennas.