PRC-25 Radio Set

UPDATED 19 MAY 2017
The AN/PRC-25 (AKA “Prick-25”) had a relatively short but very successful history. In 1967 General Creighton Abrams, deputy commander of the Military Assistance Command Vietnam called the PRC-25 “The most important tactical item in Vietnam today”. It was still in FM 24-24 as late as 1994 but it was long gone from US active service before then. TM11-5820-398-12 applies, among others.

PRC-25 in Vietnam

PRC-25 in Vietnam

Another day at the office: The PRC-25 in Vietnam, here lashed to a pack board.
Photo: US Army Center for Military History.

(Incidentally, the above photo appears on many sites on the web but is of unknown origin. The radio is variously described as the PRC-10, PRC-25 or PRC-77, it’s hard to tell from the photo. It could be either a 25 or 77 but it is not a PRC-10. The giveaway is the “tight” coiled cord to the handset. The H-33 handsets used with the PRC-10 were either uncoiled (early versions) or coiled with a noticeably larger diameter “coil”. Field radio trivia…..) I’m going with the CMH description as a PRC-25 and probably an H-189 handset.

Many other websites go into this set and system in great detail. Unlike the earlier “Old Family” tactical radio systems like the GRC-3(*) series and PRC-6/10, the development and employment of the PRC-25 is well documented. Primary references include (3), (30) and (31). The research compiled by Dennis Starks is pretty comprehensive. As with other military radio postings on N6CC.COM I will not attempt to duplicate previous research. I will just go into basics, some observations and experiences using this radio set in the military and also on Ham radio frequencies in later civilian use.

Much of the following post applies equally well to the nearly-identical PRC-77 which replaced it.  The PRC-77 was an incremental improvement over the PRC-25 with the change to a transistor power amplifier stage (and simplified power supply module) versus the vacuum tube of the PRC-25.  The main operational difference is that the PRC-77 could operate with an external voice encryption device such as the KY-38 which the PRC-25 could not utilize. Both the ’25 and ’77 use the same batteries although the PRC-77 does not require the 3 volt vacuum tube filament source provided in the standard battery.

Basic Characteristics: (Preliminary)
Frequency Coverage: 30 – 75.95 MC in 2 bands: 30-52.95 MC and 53 to 75.95 MC, each in 50 KC increments. 920 Channels total
Frequency Modulated (FM) “wideband” voice
Technology: Transistorized except for vacuum tube 2DF4 in the transmitter power amplifier
Transmitter deviation: 10 KC nominal
Power Source: BA-386/PRC-25 or BA-4386/PRC-25 battery (portable) or the vehicular amplifier 24 volt Power Supply as the VRC-53 or GRC-125
Battery life: 60 hours with BA-4386/PRC-25. Depends upon transmit-receive ratio in use.
Power Output: 1.5 Watts (Band 2) to 2 watts (Band 1) minimum
Selectivity: (not specified) Defined by measurements made using Test Set TS-723/U
Sensitivity: (not specified) Defined by measurements made using Test Set  TS-723/U
Squelch: Tone (150 cps) Operated “Squelch” mode selected by front panel function switch. Squelch is disabled when the Function switch is in the “ON” position.
Weight: 25 pounds with battery and accessories, as carried
Planning Range: 3-5 miles *

PRC-25 in harness with accesories

PRC-25 in harness with accesories

Above: The PRC-25 mounted in the ST-138/PRC-25 web gear along with an M-1956 “Butt Pack” for some accessories. Shown are the usual antennas and a spare BA-4386 magnesium battery. That newer battery gave over twice the lifetime as the original BA-386 carbon-zinc battery. Good move. The inner plastic bag wrapper around a new battery was often pressed into service to waterproof the handset. Improvise, adapt, overcome.

Battery quirk: When installing a fresh* BA-4386/PRC-25 magnesium battery, the radio will initially fail to work. Huh? Due to some magnesium chemistry Bugga Bugga, you need to keep the radio turned on for maybe 15 seconds and then key the handset a few times to “wake up” the battery. (Be aware: That effect is written into the Battery Specification.) Then noise in the receiver handset comes alive – you’re in business. (*Really not a modern problem since there are no “fresh” BA-4386’s to be had…HaHa)

Speaking of batteries, during the Vietnam War the Viet Cong and North Vietnamese Army elements in South Vietnam stole or captured large numbers of PRC-25’s among other equipment. So much so that they asked their Chicom benefactors to manufacture replacement batteries for them – they did. They made it from China down the Ho Chi Minh trail to support their assault on the South.

I saw many PRC-25’s in Vietnamese military museums in Saigon during a recent visit. During our visit we met with a “retired” Viet Cong General. He told us that he really liked the “RC-25” (PRC-25) radio which he used while running guerilla operations in the Mekong delta and areas southwest of Saigon. The communist government in Hanoi even erected a monument in Saigon immortalizing the PRC-25 in Viet Cong service. The VC and NVA put them to good use, both to coordinate their own operations but also to monitor US and South Vietnamese military comms. Even a little imitative deception on occasion.

Vietnamese Monument to the PRC-25

Vietnamese Monument to the PRC-25

Above: A giant bronze monument in front of the Central Post Office building in Saigon. The worlds biggest PRC-25? By the way, in the background is one of many monuments to Capitalism – which by the way, is alive and well in today’s Vietnam. It now generates TAX revenue for the government rulers after their government-commanded economy collapsed in the mid-1980’s. But I digress…

ST-138 Carrying Harness

ST-138 Carrying Harness

Above: The ST-138 had corner brackets to hold the weight of the radio and stiffeners to better distribute that weight along either side of your backbone. And a vent hole seen here to let the sweat drain out (!). Unfortunately, these stiffeners made the assemblage unable to stand vertically on its own. For temporary fixed-location operation, you needed something to lean it against to keep the antenna vertical. Laying it down so the panel faced you got you a face full of antenna – most of the flex adapters were too stiff to bend up 90 degrees. The shoulder straps lacked simple “D” rings above the adjusters to enable you to hang the handset near your ear. Small stuff – improvise. If this was all you were carrying (LOL!) it was otherwise relatively comfortable.

AN/PRC-25 Infantry Field Radio

AN/PRC-25 Infantry Field Radio

Above: The basic Receiver-Transmitter showing the front panel layout. Pretty self-explanitory; that’s the idea. This particular set is an RT-505B which is visually identical to the basic RT-505/PRC-25 as well as the later PRC-77. Externally, they are all identical except for the data plates. This set had been reworked by the USMC depot in Barstow CA, probably to convert it into the “B” model. That B modification now gave it wideband (300 cps – 20 KC) audio capability for multiplexing several audio frequency inputs simultaneously via the AN/PCC-1 terminal system. The AN/PCC-1 and the associated CU-1857 antenna diplexer (and a second RT-505B) provided for 4 voice channels, 4 teletype channels and one voice Orderwide circuit – all simultaneously. That overall system was known as the AN/TRC-166.

With that wider audio input bandwidth capability the RT-505B could possibly also support an outboard voice crypto capability although I cannot find any reference to that possible configuration. The AN/PCC-1 was a USMC system; there was a similar system build around the VRC-12/RT-524 sets as well providing a vehicle-based multiplex system. The RT-505B Model modifications still support standard, single handset voice operation. The microphones’ inherent frequency response apparently establishing the overall voice bandwidth to be transmitted.

As Bill described in the Comments section below, the Australian Army came up (June 1072) with an interesting new capability for the PRC-25/77. They built an Adapter Set, Morse Transmission, MX-F2 that mounts on the front panel power connector. This adapter enabled the transmission of morse MCW FM via a 2 kc note into the mic circuits. It included a cute CW key that mechanically attached to the adapter directly. The adapter also permitted the transmission of 300 WPM morse when using the AN/GRA-71 Coder Burst Transmission Group keyer system. It also would transmit 300 baud teletype signals with the proper external equipment. Clever. Anyone have any insights into the deployment or use of this system in the field?

My set came from Anker Electronics in San Diego circa 1985. He was moving a lot of DRMO surplus electronics from the Navy and Marine Corps units in SOCAL at the time. Even after refurbishment by the Marine Corps, this RT has seen a lot of service use.

Note the two audio connectors unlike the single connector on the PRC-10. This permits the operator to use a handset and an external speaker like the LS-454 (with modified connector), particularly at a fixed monitoring location. The speaker is quite rugged and quite heavy but many photos of infantrymen on foot in Vietnam show that speaker lashed to the radio so others could hear the traffic when necessary. Ugh.

Incidentally, if that “experienced” RTO tells you you should spit into the handset connector “to improve the connection”, you should whack him upside the head with a dead battery. It just makes a corroded mess. The dust caps and O Rings are designed to keep water OUT of the connectors. Use the eraser on your US Government issued Skilcraft mechanical pencil to clean the mating contacts instead. The environmental protection measures employed are excellent. It is rated to withstand immersion in sea water up to 6 feet deep so it will survive beach landings in the surf aboard an IBS. Or fording a stream or canal. However, it wouldn’t survive being locked-out of a submarine at periscope depth without the added protection of a pressure rated transit case.

RT-505B/PRC-25

RT-505B/PRC-25

The Serial number shown probably reflects the new number assigned during the modification process, not the original serial number. This particular set was repainted USMC Green while in overhaul. It looks and feels “crystalline” and dull like CARC but it is not labeled that way; maybe this was done before CARC became vogue. The Function, Band Switch and Volume knobs are not original and the whip and audio dust covers were missing. In those days, DRMO “De-Mil’d” equipment like this by removing the knobs and meters if equipped. Now they cut them into little cubes with a plasma torch, burn them and run the remains through a shredder. Then they destroy them………..

In the above photo you can see the little flip stops that can engage cams on the 2 frequency selector knobs – which are enabled via a pair of wing nuts on the main knobs. This contraption was designed to allow you to switch between either of two preset tactical frequencies in the field, at night, with your arctic mittens on, while being shot at. Overly complex and confusing to use on a nice day at the beach. We never used that feature – just dial in the Secondary Freq while reading the display. Turn on the dial “Lite” if necessary. If your friendly neighborhood Joint Frequency Coordinator was not listening, you could select “another” frequency by just turning the Band Switch to the other Band, leave the knobs alone. Ahem…

Oh, another handy feature back in the day of analog “Over the Air”broadcast Television – dial in 59.75 mc and listen to Channel 2 TV audio and get a good receiver check (catch the ballgame). Channel 3 was available on 65.75 mc, Channel 4 on 71.75 mc. You could also “say Hi” to anyone in the immediate area who was watching TV Channel 2, 3 or 4… Ahem….

PRC-25 carried in an ALICE pack

PRC-25 carried in an ALICE pack

Above: The PRC-25 in the “radio pocket” of an ALICE Pack Medium, they way we often carried them in the service once the ALICE packs became available. Or sometimes lashed to the ALICE pack frame with LC-2 straps, no Pack attached. The ST-138 works OK if that’s all you were carrying but as usual, the designers were not thinking of integrating it with all the other LBE gear you would be carrying. Here the PRC-25 is wired with coaxial cable from its BNC connector to a “Jungle Antenna” up in the trees.

Note the missing “Z-1” power connector shorting plug in some pic’s. I didn’t have one of those at the time so I just inserted appropriate jumpers into the panel connector to complete the battery circuits directly. My radio is powered by 2 “D” Cells (with a 1 ohm resistor in series) for the 2DF4 filament and a 12 Volt Gel Cell for the rest of the electronics. It all fits in the stock battery box and I can charge the Gel Cell by connecting alligator leads to the Power connector jumper when the Z-1 plug is not used. The 1 ohm resistor in the filament line is needed to drop some voltage (from fresh batteries) to the tube filament when it is cold-started. It is made from 10 feet of 30 gauge magnet wire wound on a high value resistor as a form. Works great. Live long and prosper.

The handset shown above is an H-350 noise cancelling type actually designed for the new digital field telephones. It has a 6-contact U-229 style connector but it works well with the PRC-25, especially the noise cancelling feature. In the service, I had only used the older H-189/U handsets, the newer H-250 had not yet made the scene.

On Antennas: With low powered radios, and especially on VHF like the PRC-25, your elevation and the antenna is EVERYTHING when trying to achieve long range comms.

Ya Think??

Ya Think??

Jungle Antenna connected to the PRC-25

Jungle Antenna connected to the PRC-25

Above: The “Jungle Antenna” – a wire ground plane hung high in the trees to greatly extend the range of the PRC-25 or similar VHF tactical radios. This is the “Deluxe” model with coaxial cable transmission line and connectors. Three bamboo spreaders to hold the ground radial wires “out”. Use local materials.

PRC-25 Jungle Antenna connection

PRC-25 Jungle Antenna connection

Above: If you are using a Jungle Antenna, long wire or half-rhombic made with infantry field telephone wire like WD-1A/TT, make your connections like this. The “ground radials” are connected to chassis ground, here under the screw that holds the BNC antenna dust cap chain. The vertical radiator “Hot” element downlead wire is connected to the antenna jack using the flex base as a “bolt”. Note the knot used to identify the vertical element wire in the pair. “The Knot is Hot”. Do not also install the tape or whip antenna while doing this.

PRC-25 Antennas

PRC-25 Antennas

Above: The usual issued antennas for portable operation. The 10 foot AT-271 and the 3 foot AT-892 themselves are common to the PRC-25 and PRC-77 and also to the older PRC-10. However these base spring and flex adapters are not compatible with the PRC-10. The PRC-10 radio has different threads due to its dual connectors and impedance matching switch arrangement so it has its own unique antenna bases. The CW-216A accessory bag carries all those loose antenna parts and usually a spare handset. Maybe a couple of Pop Flares as well.

AT-984A/G Long Wire Antenna

AT-984A/G Long Wire Antenna

The AT-984A/G long wire antenna seen above is a very handy antenna for extended range ops. Don’t leave home without it. (Or 150+ feet of WD-1/TT commo wire.) It consists of 150 feet of stranded phosphor bronze wire wound on an adaptation of a commercial fly fishing reel for storage and deployment. Strung about 6 feet above ground on the azimuth of your intended receiver target, this antenna can make a big difference. I have worked a 131 mile, nearly line-of-sight path to another PRC-25 from Crandall Peak in the Sierra Nevada mountains to Hawk Hill just north of San Francisco. A ridge in Oakland blocked the optical LOS path but we had solid comms nonetheless. The Hawk Hill radio was using the 10 foot AT-271 fish pole antenna. A nearly ideal path.

During Exercise RIMPAC-84 we ran a coast watcher network in the Hawaiian Islands. With our surveillance center in the mountains above Hanauma Bay on Oahu we had good comms with our PRC-77 equipped team in the mountains on Maui at a distance of 80 miles (mostly over seawater). The PRC-25 manual states that 2 PRC-25’s each equipped with this wire antenna have a planning range between them of 17 miles. High ground is your friend. (Your mileage may vary)

This wire antenna can also be configured as a vertical half-rhombic for even more gain. Terminating the far end with about 600 ohms to ground converts it from bi-directional to unidirectional. An important EW consideration especially if you’re forward of the FEBA but talking to your Boss “Back in the rear with the gear and the beer”.

On “Long Wire” and half-rhombic antenna lengths: Make it about 5 wavelengths long at your lowest likely operating frequency. 10 wavelengths is better (but much more directional – needs careful aiming). Wire length on the right azimuth is your friend. Success can also depend a lot upon the local ground conductivity.

Grounding the PRC-25

Grounding the PRC-25

Above: Grounding the PRC-25; here while testing a half-rhombic wire antenna. Generally speaking, grounding low powered VHF radios like these does not produce any appreciable performance or safety benefit. Especially while using a “balanced” antenna such as a vertical dipole, RC-292 or a Jungle Antenna. This primarily due to the short wavelengths involved. However when using the AT-984A/G long wire or a half-rhombic wire antenna, grounding the radio chassis can help; a counterpoise is even better.
Ideally, to eke out maximum range, the “ground” system would be an array of 1/4 wavelength wires projecting radially away from the radio. That is rarely done with VHF radios but range can be increased by placing the radio on the steel hood of that jeep, a piece of Marston Mat – or on the deck of the ship. As a practical expedient, a simple 12″ tent stake with the paint sanded off and connected with a SHORT jumper can help. (This is “right out of the book” and is a little better than nothing at all in reducing ground losses.) Hammer the stake or a longer ground rod into the bottom of a small hole in the soil and fill the hole with water. Dump in a few salt packages from your C Rats, keep it wet. Run any excess ground wire on the ground directly under the main antenna wire. I carry a big alligator clip for this purpose – quick and simple.

When connecting a long wire antenna (but not a Jungle Antenna) to the PRC-25, the manual says to use the AB-591 spring for the wire “bolt”, not the AT-892 adapter as shown here. It doesn’t actuate the impedance switch in the radio for the longer antenna. I didn’t see much difference on reception.

* On “Planning Range” between sets using the same type antenna. Per the manual the following configurations are addressed with PRC-25’s on each end of a circuit. As with any system like this, range is a strong function of a lot of variables. Your mileage WILL vary.

3 foot tape antennas: 3 miles
10 foot fishpole antennas: 5 miles
RC-292 ground planes:12 miles
150 foot wire antennas: 17 miles
When you desperately need it: 36 feet.

Hint: If the transmissions from your target station are weak and breaking up, momentarily switch from the SQUELCH mode to the ON mode while receiving to disable the 150 cps Tone Squelch. You may now be able to hear him, along with some noise. Go back to SQUELCH mode when you transmit back or he won’t hear you – unless he also knows this trick. The Tone Squelch function costs you a little receiver sensitivity. Sometimes it seems like a LOT.

Also, if no one ever seems to call you anymore (maybe you’re just a jerk) and the Tone Squelch never “opens” you are probably being jammed by the bad guys using the “squelch capture” technique. Simple and effective on untrained operators. How can you tell? Turn the Function switch from SQUELCH to ON (periodically) thereby disabling the squelch function. If no one is talking, you should just hear white noise – hiss. But if the receiver is then quiet (no white noise coming from the earpiece), you are being jammed by a steady carrier which “captures” your receiver, preventing you from hearing the guy you are listening for.

Due to the nature of FM radio receivers, there is a well known phenomenon called “capture effect”. What this means is that if there is a weak and strong signal on the same frequency at the same time, the receiver will respond only to the stronger signal, completely suppressing the weaker one. If the jammer signal carrier is stronger than your buddy, you are hosed – you won’t hear him. Beware. Don’t forget to notify your boss and then send a MIJI report. But I digress…

On Squelch: The PRC-25 is considered to be a member of the “VRC-12 Family”, AKA “New Family” radio systems. As such, it incorporates Tone Squelch which opens the receiver circuit when the distant transmitter transmits a 150 cps sub audible tone along with the operators voice. Good idea, no fussy Squelch Threshold adjustment for the operator to deal with. However the PRC-25 does not also incorporate a Carrier Operated squelch as the VRC-12/RT-524 radios do. (They call carrier operated squelch “Old Squelch” on the front panel control.) With the PRC-25 and 77 you just have Tone Squelch. Be advised. You can turn the Tone Squelch OFF by simply turning the front panel control switch from SQUELCH to the ON position – you will hear noise between transmissions but you won’t miss anything either..

As with the previous generation of “Old Family” field radios like the PRC-10, two PRC-25’s can be connected together to form a relay station. Positioned on high ground, a tall tree or tower or strapped to an aircraft, they can greatly increase the range of infantry comms on the ground. The 2 radios are set to operate on 2 different frequencies about 3 + MC apart or more so they don’t interfere with each other.

PRC-25 Radio Relay Cable Assembly

PRC-25 Radio Relay Cable Assembly

Above: The MK-456/GRC Retransmission Cable Kit includes 50 feet of interconnecting cable CX-4656/GRC. This distance plus the 3 MC or greater frequency separation helps to make sure the transmitter in the pair does not “de-sense” the receiver in the other radio. Set both radios’ Function switches to the RETRANS position and off you go. There is provision for a handset at the relay site for monitoring the circuit. These radios can also be remotely operated by the use of the GRA-39 Radio Wire Integration kit. You can be up to 2 miles away from the radio’s actual location and communicate with it over standard infantry field telephone wire. Put the radio up a tall tree on that hilltop while you are down in the Company Command Post. Very handy. All of the audio and remote control accessories for the VRC-12 series radios are compatible with the PRC-25. A good systems design.

Size Comparison: PRC-6, 10 and 25

Size Comparison: PRC-6, 10 and 25

Above: A size comparison between the PRC-25 and the PRC-6 and PRC-10, both of which which it replaced. This happened around 1963 and into 1965-66 in Vietnam. As noted above, the PRC-77 is essentially identical in form, fit and function to the ’25 but the basic ’25 was not compatible with voice crypto gear. The PRC-77 really is just an incremental modification of the PRC-25; they are physically identical.  The PRC-77 came along around 1967 and was directly compatible with the KY-38 voice encryption equipment. The PRC-25 was “in the clear” only at that point. Incidentally, the KY-38 was a very heavy, clumsy beast. Powered by TWO BA-4386 batteries it was a boat anchor during portable ops. When we used them in training and later in Desert Storm operations we found that the PRC-77/KY-38 system worked but had shorter range than the basic radio. This was also a problem in Vietnam (Ref 31). Ya gotta trade off speed (and range) for security. Choose wisely.

Mt Diablo Comms Experiments with PRC-25

Mt Diablo Comms Experiments with PRC-25

Above: Running comms experiments with the PRC-25, PRC-6 and others from Mt Diablo CA. Easy LOS comms to a PRC-6 on a rooftop in San Francisco, 27 miles west of here.  A fun opportunity to educate Suzi-Q-Public as she walks around bumping into things while she talks to her pink I-Phone….

Some thoughts on complexity. One of the great improvements the PRC-25 brought to the fight when compared with the earlier PRC-10 is simplicity. Channelized “detent” tuning, mechanical non-analog frequency readout, automatic, non-adjustable squelch, mainly transistorized, simple 2-voltage battery versus the PRC-10’s complex 4-voltage battery, minimal controls with no control locks needed, easy to net with Net Control etc. Some of the new tactical radios like the PRC-117, 119, SINGCARS variants etc. offer a bewildering user interface to that wounded corporal who is trying to get air support. Engineers like to design-in many features, they look great on paper, customers often love the initial concept, funding sources will often pay for them. You get a LOT of capability, they “work great” – but. Then there is initial cost, life cycle costs and short MTBF problems with overly complex gear. The WWII SCR-536 and the later PRC-6 probably represent the high water mark in simplicity, especially with the “user interface”.

The exasperated Chief of the Army Signal Corps in WWII stated that “I’m tired of hearing about radios. I want to hear about COMMUNICATIONS!” [sic] Indeed.

The PRC-25 was, and still is, a tough, functional and reliable radio. I have used mine in the field for about 25 years and I have yet to open up the chassis to repair or inspect anything. The ones I used in the service (along with the PRC-77) were equally reliable. They are versatile, work great and will take a beating, including immersion in sea water and other typical field insults. If PRC-25’s are all you have, you have a lot.

That is – if you take care of your equipment and perform your preventative maintenance!

Preventative Maintenance Monthly

Preventative Maintenance Monthly