This adapter allows the use of 50 Ohm antennas on the PRC-68 family of squad radios. It mates with the 3/4 x 14 threads at the base of the radio antenna connector and contacts the hot connection where the normal antenna screws into the radio ANT connector. The adapter has a BNC(f) connector that can be used to connect to a cable terminated with a BNC(m) connector or an adapter that ends with a BNC(m) connector.
Side View -
Bottom View - The intent of the design was to have a spring inside the adapter make contact with the top hot metal of the ANT connector on the radio, but the solder holding the spring contacts the top hot ANT instead. You can see the circular indentation on the top side of the solder in this image. This is probably due to differences in the BNC UG-1094 connector. If the BNC connector was longer than the prototype, the solder contacts before the 3/4" thread bottoms. It really does not matter, both contact methods work well.
1.1 Improvement in the Design of AdaptersThe antenna adapters now on the marker have no compliance. They have a "spring" in their design with the idea of compliance, but they have been designed or built in such a way that the "spring" is not functional. The spring does nothing and can be removed without changing anything. This means that if the adapter is in place for a long time or over a range of temperatures it may open, creating a high VSWR. For the spring to work the housing should bottom on the radio.
These probems are fixed in my 68AA antenna adapter.
The antenna adapter built into the OF-185 works on a different principal. It has a center contact pin that has a shoulder about 0.4" is diameter. The contact pin is sitting on a Delrin/teflon/plastic part. When the adapter is screwed onto the radio there is some kind of spring action in the back of the adapter and the housing bottoms on the radio ground. This is a well designed and manufactured antenna adapter. It has these numbers on it: 58167 01186. It is also much more expensive to make.
1.2 Need for a Resistor or RF ChokeThere are a number of cases where you would like to connect a PRC-68, PRC-126, etc. radio to a 50 Ohm antenna or equipment and that equipment has no internal DC return. In this case the radio will not work because it must see less than 2,000 Ohms DC to ground. The way to fix this is to have a 1,000 Ohm resistor or RF choke in the antenna adapter. This is done somewhere in the cable in the OF-185 that connects to the radio and has no connection in the OF-185 box, it just terminates on the back panel with a chassis mount BNC(f) connector. My guess is that the resistor is in the antenna adapter it's self because it's not likely that the cable was made special for this purpose and there is little or no room in the BNC connector for a resistor.
The following items are 50 Ohm devices at RF yet will not work with the PRC-68 family radios directly because they are DC opens:
Antennas that are DC ShortsJust screw the adapter onto the antenna connector and set the ANT switch for "50" and connect a 50 Ohm antenna that has a DC resistance of less than 2,000 Ohms. The OE-254 meets these requirements, but the Hamstick, RC-292 and AS-1729 do not.
Antennas that are DC OpenTo use antennas that are DC open like the RC-292 an RF choke needs to be placed from the center conductor to ground. This choke should have an RF impedance at 30 MHz (and above) of more than 1,000 Ohms. It also should have a DC resistance of less than 2,000 Ohms. A 1.8 K Ohm resistor would fit this description as would a choke made form a torroid.
Assembly of After Market AdapterSimply screw in a UG-1094A BNC(f) chassis mount connector ( Amphenol 31-221).
Clean the copper on the printed circuit board with a pencil eraser and solder coat around the hole.
Drop the PCB into the housing and solder to the center pin.
Photo showing parts and a completed adapter.
Shown are the Aluminum housing of the adapter, and the UG-1094 BNC(f) connector and a washer that can be used to replace the spring.Back to Brooke's Military Electronics Information, Squad Radio, PRC-68B, PRC-126, Home page.
You need either the free WHIP! or free Volo viewer to see the drawing. PRC68AC.dwf
Antenna Adapter NSN 5985-01-097-7337 $37.24 each 1998.
page created 7 Jan. 2001.