Past, Present and Future of Tactical Radios – Part 6

In part 5, I closed noting some shortfalls of the otherwise very remarkable AN/PRC-25 and the contemporary tactical radio sets:

  • Use of vacuum tube in the RF output component
  • Lacking support for electronic encryption
  • BA-386 power and life limitations
  • PRC-25 Used at squad levels where not intended by doctrine
  • Maintenance/logistic support chain

The first two issues were not new to the Army.  The intent (in my opinion) was to develop the follow on AN/PRC-77 as the “perfect” radio to address these issues.   But requirements from Vietnam hit before that radio was ready.  Externally indistinguishable from the PRC-25, the PRC-77 featured a new RF output component, removing the last vacuum tube.    The PRC-77 added circuitry to allow use  of cryptographic devices then being fielded in 1968.

AN/PRC-25 or 77 - can't tell from here!

The cryptographic devices issued to line units were the KY-38 NESTOR (more pictures here).  The KY-38 was as large as the PRC-77 and weighed nearly the same – effectively doubling the RTO’s load.

KY-38 - from Jerry Proc's website

An obvious question is why impose this device upon the warfighter in the first place?  Simple answer – because he asked for it.  Cold War planners assumed Soviet forces would use electronic intelligence gathering to harvest valuable information from combat radio networks.  The existing communications security practice, dating back to World War II, involved changing frequencies and call signs at intervals, code words, challenge-authentication tables, shackle codes, and other verbal tricks.  But these were mere obfuscation of information.  And such obfuscation consumed valuable time on both ends of the conversation.   The Army wanted a “green box” solution to provide end-to-end encryption of the signal.  In collaboration with the National Security Agency, the Army fielded the KY-8 for vehicle mounts, the KY-28 for aircraft, and aforementioned KY-38 for backpack use.

Further experience in Vietnam underscored the threat.  In December 1969, a unit in 1st Infantry Division uncovered an enemy radio intercept team using captured US equipment alongside Chinese and commercial radios.  Journals found in the stash included operational information ranging from the time/location of air strikes to unit statuses, all gleaned from the radio nets.  The nature of the logs indicated this was not a singular occurrence.  The communists learned the obfuscation techniques and, using today’s term, hacked the American radio nets.

The NESTOR series suffered from several drawbacks:

  • KY-38 at roughly the size and weight of the PRC-77 doubled the RTO’s load.
  • Special X-Mode cable between the radio and KY-38 supplied in limited quantities.
  • Battery powered KY-38 increased battery consumption rates.
  • Complex process to load the crypto variables.
  • Device Overheating and reliability issues.
  • Need to wait a second or two when starting transmission for the encryption to synch before speaking.  (A precious combat second…)

Clearly this was not the solution to address the radio-intercept threat.  In the field, units used the devices sparingly, and as result still faced the threat of enemy intercept.  Once again, the equipment lagged behind the requirement.

Regarding batteries, the issue of a magnesium-cell BA-4386 partially addressed the battery life problem.  The BA-4386 lasted longer than the older alkaline types.  But patrols still needed substantial quantities of these “bricks” – displacing rations, water, and ammunition in the packs.  With PRC-25/77s pushed down to squad levels, the operational tempo of war, and the introduction of the comsec equipment, battery supply barely kept up with demand.

The Army issued PRC-25/77s down to the squads in place of failed radios designed for that echelon.  The AN/PRC-34/36 set, developed concurrently with the VRC-12 series and PRC-25, failed to meet performance goals as a PRC-6 “handie-talkie” replacement.    The Army then developed the AN/PRT-4 and PRR-9 combination.  With a PRT-4 hand-carried transmitter and a PRR-9 helmet mounted receiver, the overall system was cumbersome.  Tested in Vietnam, the PRT-4/9 lacked range and difficult to operate.  A Navy derivative, the AN/PRC-88, combining both units into a single box, also failed.  Frustrations with these projects eventually lead the Marines to develop their own radio which eventually became the AN/PRC-68 (leading to the AN/PRC-126).  But that product was not in the field until well after Vietnam.

Regarding maintenance support and logistics the issue was hardware reliability rates.  Now I would not dispute the “it worked” perception for the infantryman on the line.  And certainly the PRC-25/77, as the VRC-12 too, were much more reliable than previous Army radios.  But, in order to sustain the warfighter in Vietnam, the Army maintained huge quantities of repair parts and replacement (float) radios.  Radio users and repairmen also adapted to meet the problems.  When shipment of individual repair modules resulted in damaged parts, further straining the system, the logisticians introduced air-cushioned “jiffy bags.”  When supplies of whip antennas in theater ran out, modification kits allowed the use of PRC-10 whips on PRC-25s.  Handsets were so sensitive to moisture that most operators resorted to encasing them in plastic bags.

The PRC-25/77 and VRC-12s, for all their solid state reliability, continued to tax the maintenance and supply systems.  To sustain a goal of 100% readiness of combat radios in the line units, the theater maintained as much as one-third more devices (some sources say even more) as ready replacements.  Large numbers of personnel deployed to theater to simply repair and reissue radio systems.

Consider those support requirements in context of the times.  The Army fought Vietnam, until the last phases, with a draftee army and with a “sky’s the limit” budget.   After Vietnam, the Army had to reorganize as a leaner, but still lethal, force.  This translated to, among dozens of other things at the warfighter level, the need for radios requiring less specialized maintenance and fewer supporting personnel.

In front of this “draw down” cycle, the perception leaving Vietnam was that technology had evolved to the point that even the “knuckle-draggers” could operate the radios.  From about 1972 on, all soldiers received basic radio operations training, not just those assigned as communications operators.  This presented a significant opportunity to trim down an infantry division, as in the TOEs up to 10% of the personnel were communications staff.

In the Army’s “dark age” from 1975 to 1982, the calls for a smaller force structure, better communications security, a squad radio solution, and more reliable hardware – all drove the Army to look for new radios after Vietnam.   Further another factor, not mentioned until now, was the emerging digitization of the battlefield.  After 1973, the digital facet to modern combat could not be ignored.  I will turn to the Army’s next major radio development, relating how SINCGARS arrived on the scene.

7 thoughts on “Past, Present and Future of Tactical Radios – Part 6”

  1. In 1985, when I was in OCS, the Army was still using authentication tables. I have no idea if they are still being used or not.

    1. Yes, the Army continued use of the “obfuscation” techniques (for lack of a better term to identify what all those SOI pages were used for). By 1990, clearly the technology had proved the “green box” solution. But obfuscation was still part of the training, under the premise that hardware does break in combat (ya know!).

      I received the full authentication table training in ROTC and basic. But cannot recall a single time I used it in the real world. Thankfully it was a practice we discarded since our equipment worked.

  2. NESTOR (KY-38) was the one with all those three-position pins, wasn’t it? There were Comm Chiefs who didn’t sleep in the field ever, between about 1975 and 1993. Breathe on one of those pins and you moved one, and had to re-set.

    And the difference between the PRC-25 and PRC-77 was the inclusion of the crypto key module. Some of my older 77s had the hole where they were cut open, and the module retrofitted. They were then dubbed “AN/PRC-77” but were really retrofitted 25s.

  3. I never used the KY-38 NESTOR. I was always in units with the KY-57 VINSON, which worked pretty well, but was pretty training intensive. Plus, it was a real bear for an RTO to carry in addition to a PRC-77.

    The PRC-25 still had a vacuum tube in there somewhere, which was not the case with the -77. It wouldn’t surprise me to learn that some were remanufactued -25 chasis’ though.

  4. All I remember from 1972 & 73 is that backpacking a 77 and 38, along with 3 extra batteries (ba4386?) and your other stuff was a big drain on my skinny body. We still used the authentication cards/book every day to stay in practice.

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