RADIO SCIENCE Journal of Research NBSjUSNC-URSI Vol. 68D, No.8, August 1964

Measurement of the Attenuation of Radio Signals by Jungles

Jack W. Herbstreit and W. Q. Crichlow

Contribution From the Central Radio Propagation Laboratory, National Bureau of Standards, Boulder, Colo.

(Received March 27, 1964; revised April 10, 1964)

Recent interest in jungle communications has indicated the desirability of publishing quantitative field strength measurements made in jungles by the authors during World War II. The jungle attenuation of radio signals is so great that for satisfactory communi­cations over distances greater than one mile, skywave propagation or elevated antennas should be employed.

1. Introduction

A part of World War II was conducted in jungle territory in the Southwest P acific area. In 1943, the authors were m embers of Dr. W. L . Everitt's Opera­tional R esearch Staff in the Office of the Chief Signal Officer, Department of the Army, Washington, D.C . In this capacity they conducted a communications research study and quantitative field measurements of radio propagation through jungles in the rain forests of Panama and New Guinea. The results obtained are given in Signal Corps reports [Herb­streit and Crichlow, 1943; Bateman, H erbs treit, and Zechiel, 1944; War Department, 1944] but are not generally available. Portions of these results are published at this time because of recent interest in radio propagation through jungles . It is believed that t he conclusions r eached are still valid. The original Signal Corps reports contain measurements of atmospheric noise levels in jungle areas and appli­cations of both the attenuation and nc,ise measure­ments to the evaluation of the performance of particular transmitting-receiving set performance.

. A more comprehensive atmospheric noise study bas been made following these original results and is available [CCIR, 1963a; Crichlow, Disney, and Jenkins, 1957].

2 . Jungle Attenuation Measurements

In order to determine the magnitude of the attenu­ation of radio waves caused by jungle growth, measurements were made in Panama and New Guinea of field strength versus distance at several frequencies . The jungles of both Panama and New Guinea where the measurements were conducted were dense rain forests with many tall trees and thick, almost impenetrable low level foliage of palms, bamboo, and other plant life interspersed. In Pan­ama the average tree height was approximately 50 ft, and the thick foliage 10 to 30 ft. Generally, in order to install the equipment at measuring points in the Panama jungles it was necessary to cut paths through the undergrowth perpendicular to the propagation

path. In the jungles selected for measurements in New Guinea, the trees were appreciably taller, rang­ing upwards to 90 ft ; however, the undergrowth was not quite as thick and inaccessible as that in Panama.

Low power transportable Signal Corps transmit­ters on frequencies of 2005 kc/s, 3010 kc/s, 5880)w/s, 5975 kc/s, 44 1/1c/s, and 98.8 Mc/s were used with vertical whip antennas near the ground on all fre­quencies except 98.8 Mc/s. At 98.8 Mc/s, a half­wave dipole mounted on an 18 ft mast was used with both horizontal and vertical polarization. Cali­brated field strength meters were used at all frequen­cies for the measurements.

In order to obtain a m easure of the radiated power of each transmitter-antenna combination, field strength versus distance m eaS Ul"em.ents were made in Panama over flat open ground (golf course) . The ground conductivity and dielectric constant of the ground were estimated by comparing the measure­ments with theoretical curves of field strength versus distance computed for several electrical ground constants.

N ext, the transmitters were taken into the jungle and measurements of field strength versus distance were made at a number of distances along a radial path from the transmitters. On 44 11c/s, a con­tinuous recording of field strength versus distance, which gives a picture of the large variations of the field strength encountered in the jungle, was made as the transmitter was carried along a narrow jungle trail. Similar measurements of field strength versus distance were made on frequencies of 3390 and 6070 kc/s in the New Guinea jungle using Signal Corps transmitters similar to those used in Panama for these frequencies. FigUTes 1, 2, and 3 show the results of the Panama measurements, and figure 4 the New Guinea results . These results are shown in terms of the field strengths meas UTed for the par­ticular Signal Corps target tr ansmitters used and in terms of transmission loss [CCIR, 1963b] which is independent of transmitting and receiving equipment.

Figure 1 (a) shows measurements of field strength versus distance at 2005 kc/s over the golf COUl"se and also in the jungle. At this location the jungle ended

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at 1.6 miles, and the land was open and clear beyond this point . It may be noticed that the field strength did not fall off very rapidly after the end of the jungle was reached. The presence of skywave is not indi­cated since no fading was noticed at any of the dis-

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FigUl'e 1 (b) gives the results of meaSUl'ements on 597.'5 kc/s. At this frequency, fading was noticed at a little over 1 mile from the transmitter, and the field strength did not fall off at distances greater than 1 mile. This indicates that skywave was becoming an appreciable part of the received signal even though a short vertical antenna was being used on the transmitter.

The meaSUl'ements at 44 Mc/s show that the eleva­tion of the intervening terrain as well as the jungle growth have a marked effect on the strength of the received signal. Profiles of the actual terrain eleva­tion, without regard to the tree heights, are plo tted on figure 2 below the corresponding field strength measurements. Figure 2(a) shows the point to point measurements made in dense jungle, and figure

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2(b) shows data taken along a jungle trail. It may be seen that the field strength roughly follows the ground profile. Both the maximum and minimum field strength measurements at each location are plotted on figure 2, and the solid line drawn through the measurements is through the maximum values. The dashed lines through the jungle measurements on figure 2 (a) al1d (b) are estimates of the field strength versus distance if the jungle were level. It is interesting to note that the same dashed line can be drawn through the point-to-point measure­ments and those made along the jungle trail. It is found that in a particular location a variation of as much as 4 to 1 in field strength is obtained simply by moving the receiving antenna a few feet.

On 98.8 Mc/s the effect of the terrain elevation is also very marked, and the field strength again roughly shows the jungle profile. Considerable dif­ference exists in both attenuation and field strength variation between horizontal and vertical polariza­t ion in the jungle. The vertically polarized fields are attenuated more rapidly and show more variation in strength than do the horizontally polarized fields . Fio-ure 3 shows that the horizontally polarized fields ar; approximately 15 dB less than vertically polarized fields at a distance of 1 mile. Because of the magni­tude of jungle attenuation found at low antenna heio-hts the effect of elevating the antennas above theOjun'gle growth was investigated for a frequency of 44 Mc/s and a distance of 0.4 mile. These measurements indicate that the received field strength increases approximately 12 dB with the transmitting antenna raised to an elevation of 50 ft, the average height of intervening jungle growth being approxi­mately 50 ft . When both antennas are raised into trees one 50 ft and the other 70 ft above the jungle floor ' an increase of approximately 23 dB is obtained over' that when both antennas are essentially at ground level.

Figure 4 shows similar measurements of field strength versus distance made. in the rain for~st jungles near Oro Bay, New Gmnea, on frequencIes of 3390 kc/s and 6060 kc/s. The jungles at this location were said to be typical of the rain forest type of jun~le found in most of New Guinea. The undergrowth in this jungle was not as dense as the undero-rowth in the Panama jungle through which the previoous measurements were made; however the high trees were considerably taller than those in Panama.

3 . Effective Range of Communications

To determine the effective range of particular transmitting and receiving systems, it is necessary to know the magnitude of the required field strengths in each case; and these, in turn, depend upon the levels of the received external noise and the noise generated in ~he receiver, as well as t~e kinds of signal modulatIOn used. Up to frequenCIes the order of 30 Mc/s, atmospheric noise is expected to be the controlling factor at these low latitudes where jungles are encountered, and at higher frequencies galactic or set noise is expected to limit communica-

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tions [CCIR 1963a, 1963c]. Both the received signal level and the received atmospheric noise level are expected to depend to a certain exten t on the polarization and directivity of the receiving an­tennas, as well as their heights above or below the height of the jungle growth. An analysis of these factors in terms of the signal strength required for satisfactory communication for the particular sets used was included in the original Signal Corps reports. This analysis indicated that with the equipment used in the tests, communication via groundwave through the jungle for distances greater than approximately 1 mile is virtually impossible. Because of the high rate of measured groundwave attenuation with distance, considerable increase in power would be required to extend this groundwave range appreciably. However, it was demonstrated that satisfactory communication by means of sky­wave transmission could be obtained at greater distances by using long wire skywave antennas when transmitting on frequencies reflected by the iono­sphere. It should be noted that when using skywave frequencies for jungle communications, interference to and from other stations at considerable distances is possible. Ionospheric predictions are useful for obtaining optimum results.

4. Conclusions

Attenuation due to dense jungle growth is so great that for communication over distances greater than approximately 1 mile the groundwave which is nor­mally employed for these ranges is practically use­less. Jungle communication greater than 1 mile may be obtained by elevating antennas of VHF sets either by raising them into trees or by using hilltop sites. At VHF, horizontal polarization was found to be preferable because of its lower absorption rate. Ranges greater than 1 mile using medium or high frequencies were found to require skywave trans­mission using antennas radiating energy almost vertically on frequencies reflected by the ionosphere.

5. References

Bateman, Ross, Jack W. I-Ierbstreit, and Robert B. Zechiel (1944), Measurement of factors affecting radio communica­tion and loran navigation in SvVPA, Report No. ORB-2-4, Office of the Chief Signal Officer, Washington, D.C.

CeIR Report 322 (1963a), World diRtri bution of characteris­tics of atmospheric radio noise, Documents of the Xth Plenary Assembly, lTU, Geneva; revision of R eport 65, Revision of atmospheric radio-noise data, meeting in Los Angeles 1959.

CeIR Recommendation 341 (1963b), The concept of trans­mission loss in studies of radio systems, ITU, III, Geneva.

CeIR Recommendation 339 (1963c), Bandwidths and signal­to-noise ratios in complete systems, ITU, III, Geneva.

Crichlow, W. Q., R. T. Disney, and M. A. J enkins (1957) , Quarterly radio noise data, NBS Technical Note 18-1 and supplements.

I-Ierbstreit, Jack W., and W. Q. Crichlow (1943), Measure­ment of factors affecting jungle radio communication, Report No. ORB-2-3, Office of the Chief Signal Officer, Washington, D.C.

War Department (1944), T echnical Bulletin SIG 4.

(Paper 68D8-388)