Radio Wire Integration with field sets

A Brief Overview, Updated 7/31/2024

The Control Group AN/GRA-6 Radio Wire Integration (RWI) system enables a remotely located soldier carrying a radio to communicate with a distant subscriber on a field telephone system via a base radio set.  Or vice-versa, using this equipment as a Half-Duplex, unattended “phone patch”.

See TM11-5820-489-10 (Operators Manual) for details.

It is very much an early, battle-tested, simplex “cellular telephone” system. The high powered radio, often in a vehicle, with its efficient antenna and robust power source can be located on favorable (high/controlled) terrain feature to greatly extend the radio range.   Say out to a distant soldier in the field with a low powered Walkie/Handy Talkie.

The “GRA-6” system then connects that high powered/elevated radio to the wired field telephone system from its favorable position distant from the actual soldier or field telephone user.

Another typical application is in running a command post from a position with good cover and concealment, but connected by telephone wire to its distant radio that itself may be exposed to DF/enemy fire.

The AN/GRA-6 system got its first combat test during the Korean War and was certainly used in Vietnam. By the Korean War, it had largely replaced earlier WWII RWI sets that operated with both AM and FM tactical radio circuits. Those systems included the RM-29/39, RC-261 and others.

When the “New Family” of VRC-12 equipment entered service in the mid-1960’s, the AN/GRA-6 system was replaced by the AN/GRA-39 which provides the same function in those newer systems. RWI is now known as NRI, Net Radio Interface these days.

The “Old Family” PRC-47 SSB radio can easily be remotely operated by the GRA-6 system:

This setup allows you to operate the radio, including turning it On or Off remotely from your tent/sleeping bag. How cool is that?

Below:  Operating the GRA-6 field telephone while talking, via remotely located HF radio, to our buddy located over 150 miles away.

Here the Assistant Radioman was receiving comparative reports indicating that it sounded exactly the same as the local radio set operated directly with an H-33 handset. In the above campsite deployment the radio was only about 100 meters back into the forest, we ran short of wire.  It could be up to 2 miles away.

Above: The C-434 Local Control (left) is used “Local” to the radio set. The C-433 Remote Control (right) is used at the “Remote” operator location. Shown here connected with 150 feet of WD-1A/TT infantry field telephone wire for testing purposes.

Two cables stored in a compartment at the rear of the Local unit plug into the radio(s) to be controlled remotely (not seen above). The Local Control unit can also be directly plugged into the MT-297/8 shock mount supporting the GRC-3/8 sets while installed in a vehicle. In that installation the distant Remote Control unit can select either of the two installed radios as needed.

The Mil Spec for WD-1A/TT telephone wire (the “Zip cord” type) includes a requirement for the conductor resistance to be less that 23 ohms per conductor, per 1000 feet. (46 ohms per 1000 foot shorted-end loop). Also known as “slash wire” in the military, it has over 200 pounds breaking strength.  Good to know.

Above: The C-434 Local Control set seen here sitting on the seat underneath the A.R.C. Type 12 Aircraft set (which is not involved). The VRC-7 radio, (AKA the RT-70 as the Receiver-Transmitter), is mounted in the vehicle using its battery and antenna system. The C-434 plugs into the handset jack on the radio for remote control operation. The GRA-6 system will also work with the other “Old Family” radios such as the GRC-3/8, PRC-10 and PRC-47 etc.

Ready to go for a 2 mile hike down to the camp – with wire comms back to the vehicle radio system.

Below is the other end of the field RWI set in camp, here controlling the VRC-7 VHF FM set mounted in the truck parked at an advantageous location. The Remote Control C-433 system shown below on the left permits remote operation of “Old Family” tactical equipment over infantry field telephone wire. The remote set may be up to 2 miles away from the controlled radio. (That AN/TRC-77 HF CW set was also located at this field site but not connected to the RWI system.)

Above: The GRA-6 Remote Control set, C-433/GRC seen here on the left, can also provide power ON/OFF functioning of the remote radio(s). In the TEL mode, it functions as a normal duplex sound powered field telephone (both handsets keyed) with the Local Control at the radio location. The addition of the J-654/G Audio Junction Box plugged into either Control set enables the use of a handset and external LS-166 speaker simultaneously.

Although this system uses sound-powered voice circuits, each unit contains two BA-30 (D Cell) batteries needed to bias the carbon microphone elements in each H-33/PT Handset.

The Remote unit also uses a BA-414/U 45 volt battery to send switching current down the line to actuate relays in the Local unit for the purpose of controlling the ON/OFF power to the radio (if needed) and operating the PTT relay. Four or five 9 volt batteries snapped in series (the number depending upon the wire length) will provide that voltage absent an unobtanium BA-414 45 volt battery.

Above: Meanwhile down in the Commo Bunker. The AN/VRC-10 (RT-68 Receiver-Transmitter) being remotely controlled by the AN/GRA-6 RWI system. Run that commo wire out to your distant OP or TOC.

This system can also be integrated into a larger area field telephone system via standard field switchboards such as the SB-22. This enables any telephone subscriber to connect to distant units via telephone/radio.

The GRA-6 system works great; the sound quality on both transmit and receive is very good. It was well designed and very handy as a “phone patch”.

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UPDATE:  Some time after the VRC-12 series radios were introduced (and probably before the AN/GRA-39 RWI set was available) there was a need to operate this “New Family” VRC-12 equipment with the older AN/GRA-6 Control Group set.  To do this a special interface cable assembly was introduced, the CX-7474/U.  This connected to the GRA-6 U-77/U radio plug with a U-78/U “female”, the other end had a U-229 to connect to the PRC-25 or RT-524/VRC-46 for example.

So absent a CX-7474/U, I modified the C-434 Local Set so the GRA-6 would work directly with New Family radios using the U-229 connectors while maintaining its ability to continue to work with the Old Family U-77 radios.  Example; as Set 1 and Set 2 respectively.

I modified the C-434 remote unit by removing one of the two U-77 cable connectors and replacing it with a U-229 connector.  The other U-77 remains available as before.

The H-33 old family carbon microphones have sufficient output to overdrive the new family dynamic mic circuit speech amplifiers. Hence the inclusion of a 45 db attenuator in the CX-7474/U to compensate.  So I also installed two resistors (180 ohm shunt, 12K series) in that internal C-434 mic circuit now driving this revised radio cable mic line.    See the PRC-25 Depot Manual for CX-7474/U cable details.

New U-229 wiring as follows:  Red D Mic, Green A Ground, White B Spkr, Tan C PTT, Yellow (power control, not connected)

I can now use the GRA-6 to operate my PRC-25 or PRC-174 (or PRC-47 etc.) remotely over up to 2 miles of infantry field telephone wire.  It works great; I think the AN/GRA-6 set is under appreciated these days.  Very handy.

Note: With the above modification the GRA-6 cannot remotely turn ON/OFF the New Family radio sets as it can with the Old Family sets like the PRC-47.  The New Family radios were not designed to have that power-control capability over RWI.