The following photograph is from the first years of U.S. involvement in Vietnam. These soldiers are wearing baseball caps and their patches are in color. Later, the patches were olive drab. Obviously, it's some sort of listening equipment, but what kind? And what for?

soldiers are wearing baseball caps servicing antennae

They are wearing the 25th Infantry Division patch. Their equipment is from the 27th Infantry Regiment ("Wolfhounds"). Thoughts on possible location would be helpful.

Other images from same source: photo 1, photo 2, photo 3.


3 Answers 3


I think what you have there may be the AN/TPS-21 battlefield radar:


Further details about the unit are available in the Department of the Army Technical Manual, TM 11-487C-1: Military Standardization Handbook: United States Radar Equipment.

The functional description of the AN/TPS-21 radar is given as follows:

Radar Set AN/TPS-21 is lightweight, portable battlefield surveillance equipment that will search for and detect moving ground targets. The range of the radar set is 100 to 20,000 yds. A characteristic audio signal with a frequency variation dependent upon the speed and direction of the moving target is the 'ontarget' indication. It is capable of an automatic search or 'sector scan' function in which a terrain area, with a depth of 880 yds and a width variable from 30 to 140 degrees or a continuous 360 degrees, is scanned for evidence of moving targets. In addition, the equipment may be manually operated to follow the target and indicate its range and azimuth.

(My emphasis)

If you are interested, the technical specifications of the AN/TPS-21 radar were as follows:

  • Frequency: 9375 plus or minus 30 mc
  • Type of Frequency Control: Automatic electronic frequency control.
  • Type of Emission: Pulse-modulated rf
  • Transmitter Bandwidth: 5 mc
  • Local Oscillator Range: 8500 to 10,000 mc
  • Average Power Output: 2 to 4w
  • Peak Power, Duration and Pulse Repetition Rate: 4 to 7 kw, 6.4 usec, 1600 pps
  • IF Frequency: 30 mc
  • Receiver Selectivity: 3 db down at 5.5 mc
  • Receiver Sensitivity: -98 dbm (min discernible signal).
  • Receiver Output: 50 to 100 mw into a 200 ohm headset.

Electrical Input and Output Data

  • Trigger Input: 30 to 45v across 98 ohms at 1600 pps
  • Video Input: 20v across 1 meg
  • Audio Output: 50 to 100 mw across 200 ohms


  • Rotation: 4 deg per sec
  • Beam Width: 3 deg x 10 deg
  • Gain: 25 db


  • Ambient Temperature: -54 deg C (-65 deg F) to plus 57 deg C (plus 135 deg F).
  • Altitude Limitations: Up to 10,000 ft
  • Humidity Limitations: Up to 95%
  • Power Supply Characteristics: 115v, 400 cycle, single ph; 28v dc

I arrived in Camp Lejeune in August 1969. I was trained on the PPS6 which replaced the TPS21. A PPS6 weighed 42 bounds where the TPS21 was about 120.

It came in 2 large foot locker style cases. Each unit had 2. It was reported that NONE of them were operational.

While at Lejeune I was assigned to try and get one to work. I had about eight of the units delivered to our unit and delved into the manuals over a couple of months. I got 2 of them to work. During EXOTIC DANCER 3 had it displayed during the Commandants drive by inspection.

The TPS21, like the PPS6 worked on the doppler principle A radar transmitted a beam and the antenna captured the return signal and compare it to the previous beam/return. If there was a difference it emitted noise to the head set. With proper training a person could differentiate the number and pace of the target.

When I complete the demonstrated the equipment was returned to their respective commands and never used again while I was there.

I received a commendation and as a reward (??) I received BY NAME orders to an anti-personnel group being formed but at that time I only had a couple of months left on my enlistment. I would be required to re-enlist for an additional 4 years and not ready to commit at that time.

I would do so later but went aviation instead. Served 10 active and 10 reserves. Semper Fi

  • Just casually drop something called "EXOTIC DANCER 3". :) It was a joint exercise in North Carolina spring 1970, for other readers.
    – Schwern
    Feb 9, 2022 at 17:34
  • Could you clarify how this relates to the question? Is the radio set that you used the one pictured in the OP's illustration?
    – Robert Columbia
    Feb 16, 2022 at 19:05

It looks like equipment for artillery sound ranging - which uses the time difference of arrival for artillery fire to calculate either (a) the location of enemy guns; or (b) how close to a target one's own artillery has hit.

enter image description here

Note the semi-parabolic reflectors behind the mikes. These are parabolic only in one plane (slightly off vertical) in all photographs rather than in two planes. Thus the accurate direction of the sound being ranged is unnecessary of course, as that is to be calculated), yet collection occurs in the one plane. I expect that the angle is set to point the mikes just above a rise on the other side of the valley.

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