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?
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.
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
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.
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.