PRT-4 / PRR-9 Squad Radio Set

UPDATED 2/2/20

The PRT-4 and PRR-9 worked together as an attempt at a simple, lightweight Squad radio, albeit one-way. First fielded in Vietnam by the U.S. Army in March 1967, the idea was to extend an infantry platoon and/or squad leaders’ command and control beyond verbal, hand signals and personal contact communications.  They were found to be very useful by Army Special Forces (Reference 1) and they were utilized by Navy Underwater Demolition Teams and the Marines as well.

Interestingly, the recommendation for the adoption of this system, its tactical employment and its replacement of the PRC-6 radio was made in a 1964 document which was written by Colin L. Powell, then a Captain at the US Army School of Infantry at Ft. Benning. Yup, same guy.

Shortly before the Dak To operations in November 1967, the 173rd Airborne Brigade received these new radios with the objective to replace the Korean War vintage AN/PRC-6 “walkie talkies” which proved to be too heavy and cumbersome in the jungle. By late 1967, the PRC-10 had already “come and gone” from Line units and the PRC-25 had become well established – but not as an individual soldier or fire-team communications set.

Using this new system, and depending upon circumstances as described by Captain Powell, the platoon/squad leaders carried both the transmitter and a receiver; squad leaders or individual riflemen carried only the receiver for one-way commands or advisories from the squad leader.  The receiver was clipped to the M-1 helmet or it could be hung from LBE suspenders if necessary.  Units of the 173rd that used them in combat initially reported that they worked quite well.  At the same time, soldiers complained about the lack of a capability to respond to orders since they lacked a transmitter.  Go figure, but it was still much better than what they had. (Reference 3).

Above, a well-worn PRR-9 Radio Receiver (right) and the PRT-4 Transmitter with its telescoping antenna retracted. Shown are the 15 volt BA-399/U Battery for the transmitter and the 6 volt BA-505/U for the receiver.  Note the 1981 and 1982 date codes on the batteries. The FM voice transmitter could operate on 2 separate crystal-controlled radio channels between 47 and 57 MC. Channel 1 set at 450 milliwatts output, Channel 2 set to 50 – 250 milliwatts output (selectable level). Channel 1 planning range was 1600 meters, Channel 2 was 500 meters.  (TM 11-5820-549-35).  As a transmitter-receiver pair, it is designated as the PRC-88. (Field Manual FM 7-7). Total system cost was $1044.

I can find no reference to a crystal kit to re-channel these radios although there is a test set provided for changing frequencies. (ID-1189/PR Indicator, Channel Alignment.) They use a socketed CR-81/U crystal (two per transmitter) and one for the receiver.  The few that I have seen are all on 51.0 MC which is where I use mine on the 6 meter Ham band.

I power my transmitter with two 9 volt batteries connected in series, mounted inside a defunct BA-399 package.  The internal voltage regulator for the low power stages in the PRT-4 can handle the excess 3 volts with no problem. The PA transistor is powered by the higher voltage directly. (A small series dropping resistor would be reasonable at 18 volts however.) Mine works fine without one.

I lashed up four AAA batteries for the receiver absent a viable BA-505/U.  The BA-505/U consists of four N sized cells in series for 6 volts.  (I have also used a 9 volt battery in series with a 200 ohm resistor to power the receiver.)

Planned battery life for the issued BA-399/U was 35 hours, the BA-505/U was 14 hours.  A magnesium receiver battery (BA-4505/U) was also developed with a planned life time of 28 hours.

This transmitter, serial number 21232 bears a contract date of 1967; it was made by the Delco Division of General Motors Corporation.  The transmitter antenna is a black-oxide camouflaged telescoping whip, 24 inches long when extended. The BA-399/U battery box clips on the bottom of the transmitter, its bottom is left open probably to save weight and maybe to drain water (!).  Some transmitter battery boxes also have lightening slots on the front and back.  More on that later.

Above, the circuitry inside the PRT-4 Transmitter.  Note the crystal for Channel 1 (Date code of week 29, 1970), the missing crystal for Channel 2. The silver can in the center foreground is an early integrated circuit consisting of 4 transistors 2 temperature compensation diodes and 5 resistors.  This device provides for the 150 cps tone oscillator, emitter follower and 2-stage speech amplifier. This is reportedly the first use of integrated circuits in a US military field radio.  The circuitry has been moisture-fungus proofed using clear lacquer and a gasketed cover plate.

Meanwhile, back in the jungle (er, rain forest), the set’s shortcomings became apparent.  The receiver antennas often snapped off on underbrush so the soldiers taped them to the plastic helmet liner to protect them.  This reduced the potential range of communications.  Attaching the receiver to the LBE suspenders also reduced the range since the steel helmet was designed to be a part of the antenna system – sort of a “ground plane” – but removing it from the helmet also reduced its range.

Above, the 14-transistor PRR-9 Radio Receiver, also made by Delco, mounted on a standard M-1 Helmet. It can mount on either side – on the left most likely for a right-handed rifleman. In this photo the receiver employs an “ersatz” wire antenna.  This might reflect a typical field repair since the issued AS-1998A/PRR-9 antennas were often broken.  The plastic spiral “trumpet” directed the sound from the speaker directly to the soldiers ear.  Alternatively, the H-269/PRR-9 earphone was supplied.  It plugged into a weather-protected jack on the receiver.

Above, the operating control of the receiver.  The phone jack has a spring-loaded dust cover.  The power ON knob also functions as the volume control.  Interestingly, rotating this control fully clockwise, it stops at a “detent” which disables the carrier operated squelch.  The control then continues to operate as a volume control. The squelch threshold control is an internal potentiometer not accessible by the operator.

The original system was prototyped in 1964 as transistor technology matured.  However, the system did not include Tone Squelch as that system was not in use at the time.  Later on, the PRT-4A was developed and it did include the 150 cps Tone Squelch to make it compatible with the new PRC-25 radios as they were coming online.  The original PRR-9 receivers used a Carrier Squelch and later on the PRR-9 (XE-9) model was fielded that did include a Tone Squelch decoder.

The PRT-4 Transmitter incorporates a 2-position channel switch and a rotating, spring-loaded mode switch which also serves as the Power On switch.  Designed to be held primarily in the left hand while the right hand was holding a weapon, depressing the switch with the left thumb placed it into the voice mode.  Rotating the switch in the “Tone” direction, the transmitter produced a 1200 cps tone (in addition to the 150 cps squelch tone), ostensibly to communicate simple, pre-arranged messages without speaking. “Three beeps: Return to the rally point”.

Using FM technology, my set produces excellent quality, noise free audio.  The microphone gain is very high, enabling “whispering” by the squad leader and the receiver gain is of course variable.  As a pair, they sound great. During field experiments on camping trips to the mountains, the pair has produced good comms in open country at ranges of approximately 1 mile (as designed). About the same range as my UHF “FRS” walkie talkies.

The receivers were also quite fragile as a result of efforts to keep them lightweight.  One significant issue caused by the intent to save weight; both the transmitter and receiver batteries were not fully enclosed.  They are exposed to the elements and quickly degraded in the mud, rain and humidity.

Above; the PRR-9 rectangular speaker outlet and helmet clips are visible.  The BA-505/U is a 6 volt cylindrical tube battery containing four N-sized cells in series.  It was completely exposed to the elements.  This receiver, serial number 7081 was refurbished at the Sacramento Army Depot in Dec 1968.  It had seen a lot of use during its military service.

Navy Underwater Demolition Teams and USMC (presumably Force Recon) did utilize these sets in early 1968 while conducting hydrographic surveys in Vietnam.  Utilizing the experimental “Aqua Darts” to transport swimmers performing Recon and beach and shallow water mapping missions, the AN/PRT-4 / PRR-9 pair was their emergency communications asset. (Reference 97).

The Aqua Darts were powered “sleds” pulling the operator along while equipped with navigation recording devices to correlate hydrographic data with the local terrain.  The radio sets were presumably provided with significant water-proofing  as they were constantly wet in sea water.

At some point, the US Navy had the pair adapted and repackaged to include both the transmitter and receiver into a single unit.  This was possibly driven by a requirement the Marines or Navy UDT/SEAL’s had developed.  It was also confusingly designated as the PRC-88 but it did not see widespread use beyond evaluations as far as I can determine.  The family resemblance is apparent.  Photo from DH4PY.  http://www.greenradio.de/e_prr.htm

In the end, the radios didn’t hold up to the rigors of combat very well and one-way comms were problematic so they began being left behind as operations were launched. Seemed like a good idea at the time.  At least they were light weight.  They were apparently then used in rear-area security work and would have been handy in that role – freeing up better tactical equipment for combat.

Eventually the PRC-25 and PRC-68 (USMC) essentially replaced this system although neither was designed for an individual infantryman as a hand-held “intercom”.  The PRT-4 / PRR-9 was still listed in FM 24-24 Radio and Radar Reference Data printed in December 1983.  Status was listed as “Standard A”.
“The current inventory item is available to fulfill operational requirements.”