MK-456(*)/GRC Retransmission Cable

© Brooke Clarke, N6GCE

Box at
                  end of CX4656A
The part at the top is an rubber cap on a U-229 socket.
A handset can be connected here.
                  Retransmission Cable


Retransmission is the military version of civilian repeaters.  But retransmission works both ways.  When Radio #1 receives a signal Radio #2 transmits or when Radio #2 receives a signal then Radio #1 transmits.  This two way operation is needed when you want to allow two squads to intercommunicate.  Each squad operates half duplex on a single frequency, i.e. they transmit and receive on the same frequency.  So if Radio #1 is set to transmit and receive on the frequency of Alpha squad and Radio # 2 is set to transmit and receive on the frequency of Beta squad then both squads can communicate with everyone on the same frequency they normally use.

This is supported in the PRC-25 and PRC-77 by the AUDIO connector pin "E" which has the retransmission key singal that goes to ground when the receiver squelch opens.

The Mk-456 can also be used to make a retransmission system using either a (PRC-25 or PRC-77) and an (RT-246 or RT-524) by means of the RETRANS connector on the latter radios.  Ref Fig 4-8 in TM 11-5820-498-35 for the AM-2060.

Note that since the AUDIO connector is being used the system only works with plain text transmissions, not encrypted.

Problems with Retransmission

There are a number of pitfalls that need to be overcome for retransmission to work.  For example if both radios are set to the same frequency once one of them starts to transmit it will never stop because it would hear the signal from the other radio.

Single Radio spurious frequencies

There are some frequencies where a radio used in normal operation will work either poorly or not at all because of spurious interference inside the radio it's self.  These frequencies are listed for the PRC-68 family of squad radios.  These frequencies should be avoided for all the radios that will be used in the retransmission system.

High level spurious frequencies

In a retransmission situation where there is going to be an off channel strong signal there are frequency combinations to avoid becasue of spurious signal generation.  These are documented for some radios like the PRC-25.


If a radio that is in receive mode on frequency A has a strong signal applied to it's antenna at frequency B the sensitivity of the receiver on frequency A may be degraded.  Depending on the design of the radio this effect can be nonexistent to very serious.  For most military radios that support retransmission there is data on the minimum frequency separation needed for retransmission.

Radiated Signals

The signal strength can be reduced by separating the radios and the Mk-456 cable is 50 feet long for this reason.  The radios should not be used closer together than the cable length.

Conducted Signals

The Mk-456 also has RF isolation chokes in the wires so that RF will not be transmitted along the wires at a higher level than through free space.

Well shielded cable needs to be used ot keep RF off of the conductors.


One way to have the two radios physically near each other is to use a diplexer that allows a single antenna to be used by two radios.  The CU-2194 was designed to do this for military VHF low band radios.

Antenna type and Location

The retransmission system is just like a single radio as far as antenna type and location is concerned.  For example using the AT-271 3 meter whip on the PRC-25 or PRC-77 will provide more range than using the 3 foot AT-892 tape measure antenna.  Using the OE-254 antenna will provide more ranget than using the AT-271.  Operating from a hill top will provide a lot more range than operating in a valley.

Frequency Splits

The PRC-25 and PRC-77 can only operate half duplex, i.e. they transmit and receive on the same frequency.  Newer radios like the PRC-68B, PRC-126, PRC-128, PRC-136, etc. can receive on a frequency different from the transmit frequency for a single channel.  This allow some dreative retransmission schemes.


Designed for the PRC-25 should work the same on a PRC-77.
MK-456(*)/GRC Retransmission Cable Kit consists of the following items:
  • CW-502/PRC Cotton Duck bag
  • CX-4656/GRC 50 foot cable with coil box (series RFC on each line) and network box
  • spur chart of non compatible frequencies on page 108 & 109 of TM 11-5820-398-12.pdf
Has no provision for external DC power, but that could be provided though the POWER connector.
MK-456 Cable connects between the AUDIO connectors on two PRC-25 radios.

This unit is described in TM 11-5820-398-12.pdf on pages 57 to 60, including a poor photo and a schematic diagram.


TM 11-5820-101-12
TM 11-5995-202-20P
TM 11-5820-498-35

VIC-1 Retransmission

Another way to make a retransmissin setup is by means of the VIC-1 C-2299 Retransmission Box.  It can be used either stand alone or in conjunction with the AM-1780 Amplifier.  This requires using the AM-2060 Audio Amplifier and the MT-1029 Mount.

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[an error occurred while processing this directive] page created 27 July. 2003.