Research and Development Technical Rep)rtECOM-3504

0 • SIGNAL PROPAGATION AT 400 kHz USING AN

* OAK TREE WITH A HEMAC AS AN ANTENNA

SK. Skrivseth

i November 1971

0 1ISTR IBUT ION STATEMNT FWr

distri btion unlimited. &-- [

C Bd

UNITED STATES ARMY ELECTRONICS COMMAND- FORT MONMO-'JTH, N.J.

, NATIONAL TECHNICAL. INFORMATION SERVICE

-/ Slnglel, ii 21S

UNCLASSIFIEDs ejsity cblssiicatiti

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ORIGINATING ACTIVITY (Corporate autho) (241 REPORT SECURITY CLASSIFICATION

'. U.S. Army Electronics Command UNCLASSIFIEDATTN: AMSEL-NL-R-4 Zb. GRouP

Fort Monmouth, N. J.3. REPORT TITLE

Signal Propagation at 400 kHz Using an Oak Tree with a HEMAC as an Antenna

4. OESCRIPTIVE NOTES (Type Ot rpt and incluatve dates)

Technical ReportI S -AUTHORISI (Firt naie, widEo initial, lost name)

Kenneth A. Skrivseth

. REPORT DATE 7i. TOTAL NO. OF PAGES jib. NO. OF REPSS _Nlovember _ 197/18-

&4. CONTRACT On GMANT NO. MB. ORIGINATORS REPORT NU#.rSER(S)

.LPROJECT NO H6 62701 A448-I j~no62701EOOM-3504

*. Task No. -06 sb. OTHER REPORT NO(S) (Any otmer ninere ilat AW be aselfledthisl report)

DA Work tkdi No. -181S0. DISTRISUTION STATEMENT

Approved for public release; distribution unlimited.

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_ _ _ _ _ _ _ U. S. Army Electronics Command

Fo rt Monmouth, N.J.AINSTRACT

AHE4AC (Hybrid Electromagnetic Antenna Coupler) is used to couple r-f energyin the medium frequency range (400-425 kHz) to an oak tree that is used as anantenna. A discussion of the measured impedance data and the effects of thenearby terrain are presented along with the conclusions reached.

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PPOSN FORM 45.I J N is. IS060,S.L91t FOR AR USE. won UNCLASUse.

NOTICES

Disclaimers

The findings in this report are not to be construed as anofficial Department of the Army position, unless so desig-nated by other authorized documents.

The citation of trade names and names of manufacturers in Fthis report is not to bo construed as official Governmentindorsement or approval of commercial products or servicesreferenced herein.

Disposition

Destroy this report when it is no longer needed. Do notreturn it to the originator.

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TAMLE OF CONTTEMTS

Abstract i

1. Introduction1

2. Test Setup1

* 3. Test Results 2

14. Conclusions3

UIST OF FIGURES

1. Transmitter Test Siteh

2.* Transmitter Circuit Diagram5

~i.3.* Receivar Antenna Mountinga 6

4. Received Signal Yap 7

5.Transmitter Site (Terrain Cross-Section)8

1. if

Reports Control Symbol OSD-1366

TECHNICAL RiPORT ECOM- 3504

SIGNAL PROPAGATION AT 400 kd~z USING AN OAK TREE

WITH A HEHC AS AN A"rENNA

K. Skrivseth

~NOVEMBER 1971

DA Work Unit No. 1H6 62701 A448-06-181

DISTRIBUTION STATEMENT

Approved for public ,tiea. distribution unlimited.

U. S. ARMY ELECTRONICS COMMAND

FORT I'4NMUTH, N.J. 07703

Jo!

jii

ABSTRACT

A HEMAC (Hybrid Electromagnetic Antenna Coupler) is used to couple r-fenergy in the medium frequency range (400-425 kHz) to an oak tree thatis used as an antenna. A discussion of the measured impedance data andthe effects of the nearby terrain is presented along with the conclusionsreached.

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1. Intrcduction

4In the effort to utilize the electrical properties of live vegetationto advantage for jungle radio communications, HEMAC Is! have been showm to beuseful in coupling RF energy from transmitters to forest tree antennas.Prior to this report, most of t ie work was done in the Lebanon State Forest,New Jersey at High Frequei-y. There were indications as early at April 1969that a lower frequency operation of HEMAC's might prove more satisfactory,

f and an initial investigatin was begun in June 1970 in the medium frequencyrange, specifically 400-425 kHz. The investigation was conducted at theEarle Test Area.

As will be shown, an unexpected strong directional transmitted patternappeared at 00O kHz when the HEMAC was utilized at one particularforested site at Earle Test Area.

2. Test Setup

Transmitter

A IEMC was placed at a 3 foot height around the trunk of a largeoak tree at a forested site in the Earle Test Area. Power for the trans-mitter was supplied by a gasoline generator. Figure 1 shows the transmittersite and Figure 2 is a circuit diagram. The HEMAC toroid used in nearlyall tests has 23 turns with a one foot coil diameter and a four foot overalldiameter. RF poezer input to the HEMAC was about 35 watts. Transmissionswere made using amplitude modulation with a pulsed 400 Hz tone, providingabout 50% modulation of the carrier. This enabled the signal to berecognized w th certainty at the receiver.

Receiver

A National HRO-500 Communications Receiver was used along with aJeep-mounted 15 foot whip antenna shown in Figure 3. The HRO-500 has aseparate "Low Frequency Tuning Head" for optimum antenna matching to thereceiver. The receiver was transported in the Jeep to locations where onlya marginal signal could be received. A map of the marginal received signallocations is shown in Figure 4.

IA HEMAC (Hybrid Electromagnetic Antenna Coupler) is a toroid which is usedto couple RF energy between a transmitter and a forest environment. Thetoroid carrying RF current causes a field to be radiated with the aid ofthe tree on which the toroid is mounted. (ECOM Patent Application #19084-.)

3. Test Results

An unexpected pronounced lobe to the south appears in the antennapattern of Figure 4. (The lobe may extend to the southeast or east butthere were no offshore measurements made.) To the south the 400 kflz signalwas received over 30 miles from the transmitter, compared with 5 miles orless to the north. Directional patterns have been observed at HF usingHEMAC's both in Lebanon State Forest and at Earle Test Aroa, but therehas never been this strong an effect. Thus some phenomenon wOhich has astrong directional effect at 400 kHz does not significantly affect HFexperiments performed at the same transmitter location.

Input impedance measurements of the HEMAC mounted on the tree showedthat the HEMAC is mostly inductive in the frequency range 1 to 2500 kUz.There should thus be no current standing waves in the windings of the toroidat 400 kHz. In other words, an RF current of 1 amp rms at the toroidterminals would indicate 1 amp rms of RF current at any arbitrary point inthe windings. A directional effect due to current variations in the HEMACwindings can thus be discounted as a cause for the lobed pattern. Spacingof the windings was kept uniform during all tests described here.

RF measurements made at the receiver sites indicate a background varia-tion of less than 2 db for all sites. In a further attempt to find the causefor the highly directional pattern of Figure 4, the forest height andterrain variations in the vicinity of the original transmitter site werestudied, resulting in two vertical plane cross-sections at the transmittersite (Figure 5). There is little variation in the east-west direction,of forest height, terrain, or elevation. The average forest height isabout 45 feet. In the north-south direction, however, there are significantdifferences in these parameters. It happens that the transmitter site isiadjacent to the border between two types of forest. To the north theforest height averages about 45 feet and the trees are primarily oak. Theground appears damp to the eye, and in fact the low terrain area 150 yardsnorth of the transmitter has standing water for several days following aheavy rain. To the south, the average forest height is about 25 feet,significantly 20 feet less than to the north. The soil is drier and moresandy than to the north. About half of the vegetation to the south ispine.

The transmitter tree was eliminated as the sole cause for the patternlobe. Non uniform tree shape and proximity to other trees are expectedto cause a minor directional pattern, but in this case the transmittersetup -as moved to a second tree about 50 meters east of the originaltransmitter site resulting in no los of signal strength at the southernmost receiver site.

Moving the transmitter and HEMAC about mile southwest of the originaltransmitter site to a large oak tree, however, had a definite effect. Theradiated 400 kHz pattern from this third site was much more omnidirectional.In the vicinity of this third site the forest height and density is faie'ly

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uniform in all directions, and the terrapin is nearly flat. The forestdensity in the area is iower than at the original site, and average treeheight is about 30 feet. The soil is relatively sandy.

4. Conclusions

If the causes for a directional pattern when using a HEMOA at mediumfrequency can be uniquely determined, the property may well be used toadvantage in future work. Some considerations are given below.

The forest height difference between the north and south at the firsttest area (Figure 5) could enhance the 400 kHz signal to the south byseveral means. The effective antenna height to the south should be about20 feet greater than to the north, which would mean enhancement of anyhorizontally polarized field component, at least, to the south. The antennanear field losses should be less to the south, diie to less obstructingvegetation in that direction. These arguments do not, however, explain thealmost omnidirectional character of HF experiments performed at the samesite with similar apparatus.

It may be that a wet low area 150 yards to the north of the originalsite (and extending from there to the east and west) is a factor, acting asa reflecting element (Figure 5). Since a quarter wavelength at 400 kllzin free space is 1.87 meters or 204 yards, a "reflector spacing" of 150 yardswould not be unreasonable for antenna construction. This reflectionpossibly would also account for the differences between 400 kHz propagation fand HF propagation characteristics, since for HF the 150 yard spacing isover one warjelength.

Another possibility is some effect due to variations in ground constantswith depth in the soil, a variation which would necessarily occur both belowthe depth of penetration of HF electromagnetic waves, and still well withinthe skin depth ab 400 kHz. Skin depth is nearly inversely proportional tothe square root of frequency, so the skin depth at 4 14Hz, for example,viould be less than 1/3 the 400 kHz skin depth (idealized for invariantground constants and assuming a moderately good conductor.) 2

Other cases may'well appear if a comprehensive numerical field strengthstudy is made. A numerical study would help eliminate such variables ashuman error and changing background noise with time and location. The scopeof such a study should include such areas as directivity as a function offrequency, and received signal polarization versus azimuth from the trans-mitter site. The study should point towards a clear determination of theusefulness of HEMAC's at various frequencies in helping to reduce theproblems of communications in a dense forest environment.

2For sandy soil (@= 2 x 10 - 3 mho/meter, r = l0,/q = 1.), a typicalcalculated depth of penetration is 8.7 meters for 4 MHz and 18.7 metersfor 400 kHz.

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FIG. 3 PHOTO OF JEEP-MOUNTED 15 FOOT WHIP RECEIVING ANTENNA

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FlG.4 MAP OF RECEIVED SIGNAL LOCATIONS:9 XMTR TOROID TREE

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NORTH SOUTH VERTICAL SECTION THROUGH TRANSMITTER SITE

(NOT TO ACCURATE SCALE)

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FIG.5 TERRAIN CROSS-SECTION IN THE VERTICAL PLANE AT THE

HEMAC TRANSMITTER SITE

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