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PRC 70 Radio

Page history last edited by Michael 2 years, 6 months ago

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     A backpack or vehicular radio, operating in the HF and VHF bands. It has AM and SSB operating modes over the 2 to 30 megahertz "high frequency" band, plus CW (for Morse signalling), FSK (frequency shift keying, for digital data at about 300 baud), and FM from 2 to 76 megahertz ( the upper end is the "very high frequency" band, which is where the usual PRC-68 personal radios operate). When set up as a backpack radio, the range is about 24 kilometers (with the 2 meter long "fishing pole" antenna); when installed as a vehicle radio, range is about 800 kilometers. An 8 meter "long wire" antenna will give longer range, and allow transmissions of Morse code up to 4000 kilometers. Keep in mind that descriptions of range are very general -- time of day, weather, skill of operators, type of equipment, etc. can all greatly affect the actual distance at which a strong, intelligible signal can be received. The radio uses two Project radio batteries (when not connected to external power), which provide 36 hours of power (the radio draws about 7 watts of power when receiving); the battery box is the same as used by the PSC-3 satcom radio. There's a high-power mode that can only be used for a few minutes of transmission at a time for slightly better range (the radio overheats and switches back to low power mode). Transmission power in high-power mode is about 21 to 42 watts; in low-power mode, only 3 watts.

     Two PRC-70 radios can be connected by a special cable to allow re-transmission on two different frequencies.

     "The maximum range from the PRC-70 to the PRC-68 (with whip antennas on both) is several kilometers while the range back (from the "team on foot") is about 2 km."

     Versions built from 1967 to 1977 had several tubes; from 1977 to 1983 a "single tube" version was standard; and from 1983 onwards a "tubeless" version was standard. Note that the original battery for the PRC-70, BB-542/U, had a weight of 3.2 kg; the Project version only weighs about a kilogram.

     The backpack version weighs 17.7 kg, including batteries, accessory bag to hold the 2 meter and 3 meter whip antenna (AS-2974 -- they can be joined together to make a 5 meter tall antenna. but the 5 meter antenna needs to be guyed), a KY-605/U Morse code key, Alice LC-2 pack frame with cargo shelf and two cargo straps, an H-251/U headset (or H-161E headset with microphone), an H-250/U handset, power cable for running the radio from vehicle power, and some other minor accessories. Most Project vehicles include a PP-6148/U power supply, speakers, and an 8-meter long doublet antenna (AS-2975 for 2 to 30 MHz, 2.4 kg including ground rod, balun assembly, halyards and wire ropes) for the PRC-70. The bare radio itself, without handset, batteries or antenna, weighs 9.5 kg; it's 102mm thick, 302mm wide, and 337mm tall. Other accessories may be found in depots and with specialized teams. You can look at the operator's manual here, or here.

 

accessories galore

 

     Other antennas sometimes issued to teams: 

 

AS-2259/GR

 

     A near-vertical incidence skywave (NVIS) antenna. It's seven-section 5 meter pole mast, with four guy ropes, four stakes, a base unit, and some minor bits, all in a big olive-drab canvas roll-up bag. It provides excellent voice communication for up to 1000 km for frequencies from 2 to 30 MHz  -- though 2 MHz to 12 MHz work better, and best results (the 1000 km range) are obtained at about 3.75 MHz, 5 MHz, and 7.5 MHz. Peak emitted power should be no more than 100 watts. It takes about 10 minutes to set up for two people. Mass 7 kg. It should be erected at least 5 meters from any power lines or other high-energy sources (such as radar antennas).

 

AS-2851/TR

 

     This is a VHF log-periodic antenna. It takes 2 people 10 minutes to assemble into a 6 meter tall, guyed pole. Includes a canvas pack a bit more than a meter long, guy lines, ground anchor, stakes, cables, 1 pound ball peen hammer, installation instructions, etc.; mass 16.3 kg, or 17.7 kg including the canvas pack. Can be used for vertical or horizontal polarization; the antenna does not need to be lowered to switch polarization. The top structure looked like a 7-bar home television antenna. The frequency range is 30 to 76 MHz, with a maximum transmitted power of 150 watts; it provides about a 10 decibel increase in power (and reception capability) along its axis -- roughly triple the amplitude.

 

AT-984/G

 

     Known as the "fish reel" antenna, carried in a canvas pouch with ALICE clips. It's used for HF and VHF transmission and reception; the reel holds 50 meters of antenna wire; one end of the wire has a Y-shaped lug that is attached to the radio's antenna terminal, secured by the AT-591 spring mast base. This antenna provides more gain than the usual "whip" antennas. When used at the HF frequencies, it provides skywave (NVIS) characteristics. Mass about 1 kg.

 

AT-784

 

     A hand-held direction finding antenna loop set. It covers 30 to 76 MHz; includes a 2 meter cable. When folded up it's 20 cm by 15 cm by 8 cm; mass 2.5 kg.

 

Antenna Operating Characteristics

 

Operating Modes, Antennas, and Distances

Mode

Antenna

Distance Range

CW (Morse code)

AS-2975 doublet antenna

up to 4000 kilometers

SSB voice, AM

AS-2975 doublet antenna

up to 1200 kilometers

SSB voice, AM

AS-2259 skywave antenna

up to 500 or 1000 kilometers

SSB voice, FM

vehicle whip antenna

up to 800 kilometers

SSB voice, FM

AS-2974 whip antenna

up to 40 kilometers

AM

AS-2974 whip antenna

up to 25 kilometers

FM

AS-2974 whip antenna

up to 25 kilometers

 

     Keep in mind that for transmitting purposes the doublet antenna is somewhat bi-directional:   it has two lobes of full power, one to each side. It also needs to be at least 5 meters above the ground -- preferably closer to 10 meters. At 12 meters above the ground, no further improvement in range will be achieved.

     The NVIS antenna will have a series of ranges where it works, and in between each of those will be ranges where it is weak -- like ripples in a pond.

     The following items might be issued with this radio, especially for base, depot, or large vehicle use:

 

AM-201A Amplifier

 

     This only works on the HF channels (2 to 30 megahertz), but allows power levels of 25, 50, 100 or 200 watts to be transmitted. It draws a lot of power, and produces a lot of heat. It probably weighs 10 or 12 kg, and is the same width and depth as the PRC-70.

 

Digital Message Device

 

      This is the KY-879/P terminal,  a small alphanumeric keyboard, with a one-line 32 character LCD screen. It stores messages, and permits burst transmission.. Memory is sufficient for messages up to 1000 characters transmitted (2000 characters may be received); message rates are 300 baud "low" or 1200 baud "high" -- only the low rate may be used with HF radios like the PRC-70. Has a soft plastic-and-velcro cover, a power cable, and a signal cable; it is powered by four AA batteries, or 12 volts DC through the power cable. The batteries will operate the device for 4 hours, and keep the memory fresh for 3 weeks (about 4 months with MP-AA batteries). This device can also be used with the PSC-3 satcom radio. The device is 25 centimeters wide by 25 centimeters long, and 7.6 centimeters thick; it weighs 3.9 kilograms with the cables, junction box, and soft cover; volume 4.9 liters.

  • Within the Morrow Project, this is normally employed by MARS teams and other "important" teams when they cannot employ a "regular" computer (such as in the field, away from a computer).

  • The device password is of 16 alphanumeric characters. Note however that this is not an encryption device!

 

CN-690 Voltage Regulator

 

     The PRC-70 radio can be damaged by incorrect voltages from the power source (batteries, etc.) -- especially low voltage. This device, about the size of a brick, with a heavy cable, will shut down the radio if the supply voltage drops too low. It's got a vacuum tube inside -- reasonably sturdy, in a rubber shock mount, but don't drop it.

     While not issued to Morrow field teams, this would be supplied to communities using PRC-70 radios from uncertain power sources, such as hand-cranked generators, windmills or water turbines, etc..

     We haven't got the weight, but it's probably a couple of kilograms.

 

PP-6148/U Power Supply

 

     A heavy box, the same rough dimensions as the PRC-70 radio, with a mass of 9.4 kg; it has a 2 meter long power supply cable with a three-prong (grounded) plug, and a short cable to plug into regular U.S. military batteries for the radio. It's used to recharge military PRC-70 batteries (but not Morrow Project batteries) and to operate various military radios without batteries from an external 115 or 230 volt AC power source (frequency may be 50, 60, or 400 Hz). The output is 12 to 16 volts DC, or 24 to 32 volts DC (depending on how a switch and some knobs are set). It's normally found in bases, depots, and large vehicles -- MARS One, Science-1, and the various other overland trains. The manual warns against using the PRC-70 with a whip antenna when in "high power" mode and connected to this equipment -- it can cause an over-voltage condition.

 

 

 

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