PRC-10 Infantry Radio

UPDATED: 2/20/2022

Some beginning notes on the AN/PRC-10 FM field radio set. AKA the PRC-10 or “Prick 10”.  The basic receiver-transmitter component is the RT-176.  A couple of the many manuals are TM11-5820-292-10 Operators Manual Radio Sets AN/PRC-8. 8A. 9. 9A. 10. 10A and 28. (Sept 1961). Another specific manual is TM11-4065A Field Maintenance (May 1956).  This radio had been used by multiple services over a long period of time so there are a wide range of manuals that apply.

Many other Web resources go into the technical detail of these sets so I will limit this post to basics, observations and actual field operation of the PRC-10 for now.

But first, The Basics.  From the Manual:

Basic Characteristics:
Frequency Coverage: 38-54.9 MC, continuously tunable
Frequency Modulated (FM) voice
Technology: 16 miniature vacuum tubes in circuit
Transmitter deviation: 5-17 KC nominal
Power Source: BA-279 battery (portable) or the AM-598/U vehicular amplifier Power Supply
Battery life: At 4 hours per day, approximately 7 days nominal. Depends upon transmit-receive ratio in use.
Power Output: 0.9 watts
Selectivity: 75 KC at the -6 db points
Sensitivity: 0.7 microvolts. There are independent Volume and Squelch controls on the front panel.
Weight: 24 pounds with battery and accessories, as carried
Planning Range: 5 miles. Depending upon antenna used, siting and terrain, 3-12 miles. (12 miles with an RC-292 antenna at each end)

The PRC-10 was designed for use in the “Infantry Band” or 38-54.9 MC for use in command, control and coordination among infantry units. Related radios were the PRC-8 in the “Armor Band” and the PRC-9 operating in the “Artillery Band”. These categories and frequency ranges were a result of both the technology available at the time as well as the categorization of Army corps into those 3 main branches in that era. “Combined Arms” formations with fully interoperable communications were still off in the future.

In this system, the PRC-10 was the primary replacement for the WWII SCR-300 (AKA BC-1000), the revolutionary FM infantry “Walkie Talkie” radio fielded in late WWII. The PRC-10 was designed to be interoperable with the PRC-6 Handy Talkie, the GRC-7/8 (RT-68) and VRC-7 (RT-70) series of “Old Family” FM sets and backwards compatible with the SCR-300. It was also interoperable with the early FM aircraft sets such as the AN/ARC-44. The PRC-10A had the “flat” versus “stepped” front panel, circuitry differences and a different calibration crystal – otherwise nearly identical.

The PRC-10 began entering service with the US Army in 1951. (My radio carries a 1952 Contract date.) This was during the Korean War but it is not yet clear exactly when, where or in what numbers it began replacing the SCR-300 or other tactical radios in that conflict. While the ground war in Korea was largely fought using WWII vintage weapons and equipment, new technology was just starting to appear. I have seen a photo of a PRC-10 being used in Korea along with a PRC-6 by the 7th Infantry Division in 1953. More research is required here. They were clearly used by US forces in Europe and elsewhere later into the 1950’s and early 1960’s.

The US Army’s Reference Data for Field Radio Communications, ST-11-174, 1 June 1962 lists the PRC-10 Status as “Standard” B. Still issued in the system but probably on its way out. That document also shows the PRC-6 and the new PRC-25 Status as “Standard” A, implying more routine deployment than the PRC-10. Standard “A” is the preferred item, Standard “B” is an acceptable item but “no new procurement’s should be made”. (Ref. 29).

The PRC-10 was given to the South Vietnamese under military assistance programs in the late 1950’s and early 1960’s, primarily to the civilian Self Defense Corps and Civil Guard. They equipped 3,060 squads located in hamlets and villages along with field telephone systems. When paired with GRC-9’s these systems provided a local and regional communications network to the South Vietnamese government in defending itself from the communist insurgency run by Hanoi. (Ref. 3)

They were also used by the South Vietnamese military, and often strapped into a Bird Dog forward air control (FAC) aircraft for infantry coordination. US infantry and military advisors in Vietnam in the early 1960’s also used them in tactical and warning nets but by mid 1965 the wholesale replacement by the PRC-25 was well underway. As expected, the Viet Cong and North Vietnamese Army used many captured PRC-10’s in their networks. (Ref 3)

Above: A couple of PRC-10 radios along with other equipment captured by the Viet Cong / North Vietnamese Army in South Vietnam. I found these radios on display in the basement communications center of the former South Vietnamese Presidential Palace in Saigon. PRC-8/9/10’s show up frequently in Vietnamese military museums today.

Above: At the time, a widely distributed propaganda photo of Viet Cong guerillas operating in South Vietnam. Here using a captured PRC-10 radio while planning their next move.  The man on the left would later become a VC General.  His Komrade operates the H-33 handset connected to the radio; M-1 Carbine nearby.

The PRC-10 was carried in the ST-120A/PR canvas harness, worn on the back. The shape factor of the radio was thinner than the SCR-300 and that kept the weight closer to the soldiers back for comfort and also helped the ability to roll over more easily. The ST-120 harness was connected to the standard pistol belt along with the CW-216(*) accessory bag if carried.

The ST-120 is a clumsy contraption and appears not to be designed for use along with the then-standard combat suspenders and web belt. Too many adjustments, tabs, buckles, straps and parts. Integrated Load Bearing Equipment design still needs some work.

Above: The ST-120A/PR canvas duck carrying harness with period web gear. Canvas LBE equipment in a tropical jungle would prove (again) to be a problem with rot, mildew, fungus etc. Those problems with cotton included additional weight when wet. Even the later PRC-25 retained the cotton duck material. Nylon/synthetics came along later.

The radio was often carried upside down to help conceal the antenna as it attracted enemy fire. The radios were probably also carried inside a field pack alone to save weight and to keep it simple. I don’t think the PRC-10 and the then-new ALICE packs ever crossed paths in Vietnam – they wouldn’t fit very well anyway. These radios were also seen lashed to the wooden pack frame available in those days, probably a good field solution.

Above: The complete set as it might be carried. When all that cotton duck got wet it was very heavy, took forever to “dry”, not good.

Above: The PRC-10 as set up “ground mounted”. The set includes 2 swing-out feet that are attached to the bottom of the battery box. These may be deployed as shown to permit the radio to stand on its own, thus keeping the antenna vertical for optimum range, the stability especially important with the long antenna. Good idea. The previous SCR-300 and the follow-on AN/PRC-25/77’s did not have these feet since they were “thicker” and less prone to tip over when used like this.

Principal operating accessories are the H-33 (*) handset as shown with the coiled cord. Earlier versions of the H-33 had a non-coiled cord. This “old family” handset had the robust U-77 connector, a carbon mic element and rubber frost shields on the mic and earpiece (a lesson learned in Korea). The receiver-end was thinly shaped to fit under the edge of the M-1 steel helmet so the earpiece could be placed against the ear without the necessity of tilting or removing the helmet. That was a problem in the earlier handsets such as the “telephone” type TS-15 used with the SCR-300 and the EE-8 field telephones.

Above: The antennas included the 3 foot AT-272(*)/PRC flexible steel “tape measure” antenna with flexible base gooseneck. The AT-271(*) with base AB-129/PR spring section made up the 10 foot “fish pole” antenna for longer range when its longer length and visual signature could be accommodated.

When installed and in use, the AB-129/PR “Fishpole” antenna base actuates engages a circuit inside the radio. This function operates an impedance matching network to enable the proper loading of the longer antenna. (There is no “switch” involved as in the later PRC-25/77’s.) [Edit]

Important:  When using the short antenna, DO NOT just leave the AB-129/PR base spring in place, even with the fishpole antenna removed and stowed.  That will actuate still enable the circuit which then detunes the output system for the short antenna, reducing range. [Edit]

The 3 foot antenna with its flexible gooseneck is plugged into its own dedicated panel connector. (It should be noted that this gooseneck adapter is not compatible with the PRC-25 or 77 sets – it has a different thread. Likewise, the AB-129/PR long antenna spring-base are also not compatible with the newer radios either. since they don’t require the extended base antenna impedance switch actuator.)  [Edit]

(Antenna circuit clarifications thanks to N9ACC, see comments below.  For a detailed description on how the PRC-10 antenna connections work see K4CHE’s excellent webpost: K4CHE )

These parts were carried in the CW-216(*)/PR accessory bag as shown.  This bag has a February 1956 contract date stamp. The radio was also compatible with the mast-mounted RC-292 ground plane antennas. The Receiver-Transmitter is fitted with a BNC coaxial connector for other external antennas, such as a vehicle antenna.

Above: This is the “Homing” antenna that is used for radio direction finding; the AT-339/PRC and its carrying case. This antenna is plugged into the BNC connector on the PRC-10 and is used in a receive-only mode to determine the direction of a distant transmitter. This function to either find the direction of an enemy transmitter on the frequency to which the set is tuned, or to find your way back home towards a friendly transmitter. The “Homing” function.

I have used the AT-339 DF antenna during a recent “Fox Hunt” at a military radio collectors field event. It was connected to my PRC-6 radio as the receiver operating on 51.0 mc. In hunting the PRT-4 transmitter, the antenna was very effective; it produced sharp nulls and the “sense” function worked well to resolve the 180 degree bearing ambiguity. It’s a good system.

The set was also capable of Radio Wire Integration (RWI) by the use of the GRA-6 system. This permitted the radio to be located in a favorable radio-propagation location with the user communicating with the radio via infantry field telephone wire from a protected or otherwise favorable location – up to 2 miles away.

RWI systems were perfected in WWII and continue to be fielded even today, now known as Net Radio Interface. By the use of Special Purpose Cable Assembly CX-1961/U, two radios may be connected to form a radio relay station operating on 2 independent frequencies, at least 3 MC apart. This setup could be mounted in a favorable location such as a tall tree, mountain top or aircraft to enable communications relay between other distant stations.

Above: Front panel controls and fittings. The continuously tunable frequency can be calibrated against an internal crystal at 1 MC increments. The dial indicator line can be mechanically repositioned on the calibrator frequency with a top-side knob. The dial knob itself can be locked down once the radio has been properly netted.

The function switch can be in the OFF, Remote, ON or Dial Light/Cal positions. The Volume and Squelch controls are evident – this radio has only Carrier Operated “Old Squelch”. Tone Operated “New Squelch” would take another decade to become fielded on the VRC-12 series radios.

The two antenna mounts are evident; SHORT and LONG. They are insulated from the front panel. The BNC Auxiliary Ant. connector with its dust cap is panel mounted. Corner bosses protect the front panel. The single Audio connector is waterproof and rugged, the dial light lamp is accessible under the Lite Cap dust cover.

The set could also be vehicle-mounted by the use of the Amplifier-Power Supply AM-598/U. This chassis provided a mechanical mount for the radio as well as a power supply from the vehicle’s 24 volt system to save the internal dry cell battery. The AM-598 also provided for audio amplification to drive the vehicle’s loudspeaker speaker.

Note that the PRC-10 does not include 2 audio jacks like the later PRC-25 and PRC-77 radios. Thus you used only the handset with no “auxiliary” loudspeaker. The LS-166 speaker was compatible with the PRC-10 but not simultaneously with the handset without a “Y” adapter. For monitoring purposes, intercept or receive-guard service, the LS-166 speaker could be connected to the basic receiver-transmitter alone if necessary.

Power source: For portable, man-packed use, the original BA-279 is long gone. Unobtanium. It was a complex “stack” providing 1.5, 6, 67.5 and 135 volts DC. I have lashed up series-parallel packs made of C cells and 9 volt batteries.

“Inverter” supplies that are made for the PRC-10 are available – they are powered by 6 or 12 volt gel cells while inside the battery box. I believe the French built a purpose-designed DC-DC converter for their PRC-10 radios. The one they designed for the PRC-6 running “D” cells is superb.

I also have an AC power supply built by Motorola that was probably built to power their original VHF Handy Talkie “lunch box” radios in fixed service. It provides all the necessary voltages to the PRC-10 when operating in a “base” situation.

Some observations on the radio’s “non-combat” performance:
I have used this PRC-10 on the 6 meter Amateur Radio band occasionally and it works pretty well. I have talked with a “Radio Set AN/PRC-6” on 51.0 MC over an 8 mile optical line-of-sight path with full quieting at both ends using the 3 foot tape antennas. The crystal calibrator function was more than “close enough” to pre-net with the PRC-6’s crystal controlled transmitter/receiver.

In the Receive department, I am quite impressed with its performance. The frequency stability is adequate but it does drift somewhat over long time periods – probably not a problem when Netted to a wideband transmitter of the same era.  It is OK, considering its IF passband specs.

It is amazingly sensitive, capable of hearing anything my modern, dedicated R-7000 VHF-UHF Ricebox can hear. It makes for a very good “Band Cruiser” in Ham lingo, a “Search and Intercept” receiver in military lingo. Tuning is smooth and the tuning rate seems to be just right. It demodulates both narrow band Ham transmitters as well as wider commercial FM land mobile transmitters; receiver fidelity is good. The California Highway Patrol and local Parks District radios come in just fine.

I particularly like the (carrier-operated) Squelch function which can be completely turned off if needed. The Squelch enables a relay that positively cuts off the audio output in the absence of a carrier and it is a very abrupt drop-out unlike some sloppy, noisy ham gear squelch circuits I have experienced.

The environmental protection is excellent.

I have used the AT-339 DF antenna to get a sharp bearing to the local CHP repeater site and it is quite effective at that.

I bought this particular set from Mike Quinn Electronics in Oakland CA around 1977. I fire it up occasionally and have taken it on Field Days and other camping expeditions. I have never taken it out of its case, inspected, cleaned or adjusted anything inside – it just keeps on working. Good gear.

My buddy went through US Marine Corps advanced infantry training at Camp Lejeune NC in 1968 in preparation for deployment to Vietnam. In that school he participated in continuous field training exercises while equipped with the PRC-10.

Although long obsolete for line units by then, it was still used in training. As a “radio savvy” guy, he reported that the PRC-10 worked very well and was dependable. Seemed like a “real radio” with its continuously tunable receiver and transmitter and adjustable squelch.

By then, the PRC-25 was showing up in Vietnam in large numbers and it overcame some of the PRC-10’s shortcomings as that newer technology became available.

Anecdotal evidence indicates that the radio had “mixed results” in Vietnam service, especially in the hands of a non-radio operator infantryman – its intended “customer”. Although a technical marvel of miniaturization when it was designed in the 1940’s, it still required the infantryman operator to actually “operate it” beyond just squeezing the Push-To-Talk button on the handset.

Bumping the tuning knob off frequency (it has a mechanical lock – if used), improperly netting with your target station, humping (“rucking” for you Army guys) heavy batteries, bumping the Squelch control “full on” etc, would make for a frustrating experience. While being shot at. Works great sitting on a shelf in an air conditioned ham shack however.

Above: The PRC-10 and PRC-6 shown with the PRC-25 (left) that replaced them both.

The PRC-10 functionally replaced the WWII SCR-300 “Walkie Talkie” manpack as well as the SCR-536 “Handy Talkie”. It was a significant improvement over those earlier FM/AM sets. However it was not without its faults, especially when viewed from today’s perspective.

The PRC-10’s principal drawbacks, in hindsight, as viewed in Korea and Vietnam were its “analog” continuous tuning mechanism, “complex” calibration and netting procedures, adjustable squelch and the power consumption inherent with vacuum tubes and complex, multi-section high voltage batteries.  Its WWII predecessor, the SCR-300 had many of the same issues.

They were put into use and made to work but the PRC-25 was a welcomed replacement that required little training for intuitive operation. These advantages included simplicity of operation, a greater number of frequencies available, channelized “detent” direct frequency tuning, better range and the use of mostly transistorized circuitry. More on that later on.

For further information on the follow-on PRC-25, take a look here: PRC-25 Radio