From A Reader: PRC-64 HF Test Report From Jungle Conditions, 1967

PRC-64 Test Report In Jungle Conditions

Since you teach classes using equipment in said scenarios, I’m forwarding it to you in case you have never read it.
A little old, but in my humble opinion (Signal Corp–Vietnam and 50+ year ham), the principles are generally the same, and the equipment “needs” mostly still apply “today”.
Luckily, today we have equipment like the KX3 and CTX10 that are far better in selected aspects than the equipment they were researching/designing (the PRC-64) when this was written.
Also note the interesting table contained in the article showing the attenuation of HF signals by frequency in a jungle (heavy foliage) environment.


The above is an excellent reference to have and not simply for the results of vegetation loss in dense forest- an environment which if you’ve trained with me on the east coast you know we have- but also for the preface written by an Australian Army Major defining communications needs for long range patrols. It was his points, contained below, that outlined a larger communications model for troops operating in a jungle environment and why HF is a must-have option for long range patrols.


1. Counter-insurgency operations generally follow a definite pattern, which is developed to counter the efforts of insurgents as they progress through the regular phases of a guerrilla force build-up. In brief, a commander, charged with combating guerrillas and extending control over an area, must move his forces into the area and establish local bases of operation.

2. These bases must be of sufficient size to dominate their local area. The primary task will be to control the civil population and isolate it from the guerrillas. Once this is achieved then the security forces must conduct offensive action against local guerrilla units. Bold, aggressive patrol action must then be maintained to ensure that the enemy does not effect a come-back. In some cases it may be necessary to arm friendly elements of the local population as village guards and police auxiliaries. These will require quick support by the military if they are to be effective. They can be invaluable in gaining time for the deployment of the regular forces if reliable communications and mutual confidence exist.

3. The gaining, analysis and dissemination of intelligence data is always a major effort in counter-insurgency operations. The requirements for communications facilities by intelligence officers are very heavy. In this respect, communications to civil police are very important and the military must be prepared to provide communication support to local civil authorities.

4. Another major communications requirement comes from the extensive use of air support for counter-insurgency forces. To offset the mobility of guerrilla forces in difficult terrain every possible use is made of air-mobile forces. Aerial re-supply of patrols, casualty evacuation as well as offensive air support require communications facilities.

5. There are many sub-divisions of the communication system which will be employed to meet these requirements. However, for the purposes of examination of communication problems peculiar to counter-insurgency operations, it is proposed to divide the system into only two categories. These may be classified as “In Jungle” and “Out of Jungle” communications.

6. Where both ends of a communication link are located outside the jungle there is no particular problem in providing the required scale and grade of service. When one or both ends of a link are located in the Jungle there are a number of specific problems which are capable of solution, providing that the problems are recognised. These problems are:

  • a. Portable radio equipment cannot generate sufficient power to”bull-doze” a signal directly through the jungle vegetation.
  • b. Mobile groups within the jungle have to be combat ready at all times. They cannot carry large amounts of radio gear or put up elaborate antennas. They require maximum assistance from communication equipment based outside the jungle.
  • c. There is a distinct difference in the communication requirements of short range and long range jungle patrols operating in counter-insurgency roles.
  • d. A means of alerting security forces when civilians are menaced by guerrillas is a most important requirement of the communications system. To obtain timely, secure and accurate information of this nature is a major problem.

7. The problems outlined above are all capable of solution. However, it must be realized that the solutions will not be achieved by producing some hardware which can be issued to combat troops as the answer to all jungle communication problems. We certainly can improve some items of equipment, to make them more suitable for this environment, but the answers come through adaptation of the available equipment, to meet the requirements of the mission, the terrain and the local enemy situation.

8. To examine the first problem. Here is a table showing the loss at radio frequencies owing to jungle foliage:

Loss per 0.1 Mile in Frequency in Megacycles [also known as Megahertz] Dense Jungle Foliage

2mc / 12.6 db

3mc / 15.4 db

5mc / 20 db

10mc / 28 db

30mc / 48 db

50mc / 63 db

100mc / 86 db

9. These figures bring out the point that low power radio sets cannot push a signal THROUGH the jungle. So we are forced to accept this and look elsewhere for the solution. If both ends of a radio link can raise their antenna ABOVE the jungle there is no problem. However, we know this is an unacceptable requirement for the mobile patrols. So we have to examine the communication arrangements at the local base. The aim will be to have the base equipment as efficient as possible to compensate for the inherent inefficiency of the jungle equipment. If we can achieve this we will have the answer to problems a and b, above.

10. If we analyse the difference in communication requirements between the types of jungle patrols we may then design the base radio installation to cater for three of our basic problems, and perhaps also provide a solution for the fourth. We have already placed these patrols in two categories.These may be defined as:

  • a. Short Range Patrols. Those patrols operating within a 15 mile radius of the local base.
  • b. Long Range Patrols. Those operating at ranges in excess of 15 miles.

11. The short range patrol is generally operating within the range of artillery and mortar support weapons of the local base. The requirement is principally for voice communication with some telegraphy. There is a need for 24 hour reliable circuits for close supervision and coordination by the base commander. The ideal communications would be VHF (FM) radio with”whisper facilities” and morse capability to provide constant, noise free,high quality links.

12. The long range patrol of necessity operates beyond the immediate close support area of the local base. It must be self-sufficient in all aspects and able to stay out for extended periods. The distance factor,coupled with the high attenuation of the jungle foliage, dictates the use of radio sets operating in the HF band and using horizontal wire antenna.Communication is impossible when the patrol is moving but is required at any time when the patrol is halted. The patrol base must have a high efficiency antenna and maintain ceaseless listening watch to compensate for the difficulty of operation experienced by the patrol. The use of such antennas as the “Shirley Array” of phased and spaced dipoles has been found to be very effective. The use of high speed keyers to give “burst transmission” is desirable for long range patrols.

13. The problem of communicating between civilian communities and the local military base requires a new approach in communication methods. The need is for some type of “fire alarm” to be distributed to each community which may be concealed and triggered surreptitiously. Construction of such a device appears to be well within the state of the art at the present time.

14. Now that we have examined the problems we might consider more solutions. It appears mandatory to elevate the base end of all radio links well above the jungle canopy. The most convenient way to do this would be to situate the base on high ground which overlooks the operational area.This will seldom be possible. To put up masts is seldom a practical solution as the height required for effective range is about 500 feet. The best solution appears to be the use of balloons.

15. In counter-insurgency operations there is no requirement to conceal the location of the static operational bases. In fact, the opposite is true, as the aim is to dominate the area by a show of overwhelming military strength. Therefore, the use of large balloons anchored over the bases would be acceptable and, as they would have to be equipped with air-craft warning lights on the cables and balloon body, there might be a bonus gained from the moral effect on the civilian population. In some cases these devices would also be very useful as navigation markers for jungle patrols and light aircraft who may be in a position to obtain a visual fix on the balloon warning lights.

16. The balloon equipment including gas generators, cables, winches,anchors, etc., should be available in easily transported packages. Normal transportation would be by truck but it would be advantageous to have the packages of convenient size and weight for transportation by light aircraft or helicopter. From investigations carried out to date, it appears that there would be no difficulty in obtaining commercial balloon equipment capable of lifting payloads of 70 to 80 pounds up to heights of 500 feet.This greatly exceeds the payload requirements for communication balloons which could vary from about 25 pounds to a maximum of about 40 pounds.To obtain a relatively stable platform for the radio equipment, the balloon would require aerodynamic shaping to give lift under heavy wind conditions. The “KYTOON” type equipment commercially available from a number of sources in the USA would appear to be very suitable.

17. Every effort should be made to use only the normal current field radio equipment for this requirement. For short range patrols the existing FM equipment would be ideal for this purpose, as the performance, size and weight characteristics of such FM radio sets as AN/PRC-25 would ensure adequate short range coverage. It may be necessary to develop a set of connectors from existing lightweight cables and plugs to operate the equipment by remote control. However, it should first be tried with existing remote control equipment, such as AN/GRA-6, to eliminate any time-consuming development not strictly necessary.

18. With the addition of the balloon equipment the short range patrol communication problem will be greatly simplified and may well be completely solved in many areas. If the balloon also carries the receiver for “village alarm” transmitters located in the surrounding countryside it would be a large step forward in providing constant surveillance of the area. With the development of a suitable alarm system, the problem of alerting security forces to guerrilla action will be considerably reduced.

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Although dated by the named equipment, the concepts and challenges are definitely not and the points should look familiar to anyone who’s been to the RTO Course and expounded upon in the Advanced RTO Course. Modern options no doubt can fill the same role. The reality is that most us will not be fighting in a desert like we have over the past generation, we cannot expect a ready infrastructure to resupply us, and learning to implement common, off-the-shelf equipment in unconventional ways (that means not living off in la-la land). Jungle or dense Woodland fighting is a different animal. For that reason I have a feeling a lot of the old lessons will be coming back.

Conversely, the points about signal loss in dense vegetation (which as the chart illustrates goes up with frequency) works to the advantage of the guerrilla as well. Their own communications, when used with directional antennas and short bursts of data, become that much harder to intercept.

13 thoughts on “From A Reader: PRC-64 HF Test Report From Jungle Conditions, 1967

  1. n2382

    One Hertz (Hz) is defined as one cycle per second. Using Hertz to express frequency simply eliminates the need to say ‘per second.’ PR

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  3. I was the commo NCO for my NC National Guard unit, handling the PRC-77, and the crypto gear too. We set up the shelves on the ALICE pack frames to take the PRC-77 and the cryto unit together. Worked well, as long as we set them up properly.

    There is an official ALICE shelf that folks should look for.

    1. Badger

      A friend suggested once that shelf my shelf is not too shabby either if one needs to hump a 5-gal can into someplace either. He turned out to be right. 😉

  4. Mudboy

    Mission critical long range patrols had a radio relay team hidden on the peak of an LOS mountain to ensure comms back to base.

  5. Badger

    Great article and thanks for the download link as well. Good stuff.

    As to RF-eating “jungle” my locale is very much in that category in terms of patrolling (on an overcast day it can seem like triple-canopy & “night” comes quick). Also, with a near ideal base location as discussed above, and dominating the surrounding topography. Not everyone is that blessed.

    My impression is that our densely forested areas here are nearly as bad as tropical, and that conifers eat RF worse than deciduous trees. Evergreens just vacuum up your signal something terrible. No science, just my perception over time. Get the wire high, use a narrow-band mode for max bang-for-buck (Morse in CW mode works great but most won’t go the learning curve) and, with a thought-out CEOI, you can do good things with less power in short amounts of exposure time. If you don’t have that dominating base, find a significant piece of terrain and put up a relay, even temporarily if you have a big enough area that you have people out there beyond workable HT range. Most homesteads don’t fall into that probably, but stuff to think about.

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  7. Toxic Deplorable B Woodman

    Such memories. I came in soon after the PRC-25 (tube amp module) was replaced by the PRC-77 (transistor amp module). The GRA-6 was still in use as a “wired remote” system. My job (31E) was to operate and repair all of the above. Good times!
    BB knows what I’m talking about.

  8. Jefferson Thomas

    For cw burst transmissions, remember that the faster the carrier wave cycle on-off the wider the necessary bandwidth. (Bn = BK) For the same quality waveform (K) the change in bandwidth is proportional to the wpm. By spreading the bandwidth, the transmission power gets spread. Something to consider. Also, by using a computer generated cw signal (as opposed to hand-keying) the quality of the waveform can be held consistent and keep bandwidth down. I’m not saying you don’t have to learn cw especially because your laptop/tablet will break, fall in the water, launch itself over a chasm, or decide it needs to update NOW. Just saying that the RTO needs to be proficient in digital commo ops. Go with the best available at the time. Another application of PACE.

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