For infantry squads in the Vietnam War, the AN/PRC-25 and the closely related AN/PRC-77 was the standard backpacked voice communications radio. Entering service in 1962, it was the first widely deployed radio using primarily solid state rather than vacuum tube electronics, bringing its weight, with battery, below 20 pounds, compared to its predecessor, the AN/PRC-10, weighing in at 26 pounds. 
The AN/PRC-77, is really a slightly upgraded AN/PRC-25, and the two are operationally interchangeable for most purposes.  The PRC-25 while similar in appearance and performance, had better receiving electronics, and replaced the vacuum tube power amplifier to improved battery life and reliability. Between the two, they stayed in service for approximately 30 years. Over 130,000 were built.
Its main successor is the AN/PRC-119, which added SINCGARS frequency agility; the PRC-119 will be replaced by new software-defined radios, initially the AN/PSC-5 and then the AN/PRC-148 Special Forces use, perhaps with a new general version.
Operating in the high HF/low VHF frequency band using frequency modulation (FM), it had a maximum range, using full power and its standard antenna, of approximately 5mi/8km, depending on terrain conditions. Nominal transmit power was 1.5 to 2 watts but it commonly produced four. Later radios allowed lower power settings to conserve battery life and minimize interference; this was one of the limitations of the 25/77.
It was the first U.S. tactical radio that with a frequency synthesizer rather than fixed crystal oscillators, operating in the 30-75.95MHz range, with two preset channels. The sysem allowed a total of 920 channels, spaced 50 KHz apart. The FM transmitter was not frequency agile, but it did not face a sophisticated electronic warfare threat. The PRC-77 was designed to have an external voice encryption device, of the the TSEC/KY-38 or TSEC/KY-57 series, connected to it. There was, however, an upgrade to the PRC-25 that allowed connecting the encryption device, but the nomenclature of this variant, available before the AN/PRC-77 started shipping in 1967, never really standardized.
Both the AN/PRC-25 and AN/PRC-77 were mechanically manpack versions, but could be packaged in alternative ways.
|Form factor||PRC-25 electronics||PRC-77 electronics|
|Vehicle-mounted||AN/VRC-53||AN/VRC-64, AN/VRC-65, AN/GRC-160|
|Dual radio (portable or vehicle)||AN/VRC-125||AN/PRC-77|