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INTRODUCTION

The following observations and opinions pertain to:

a. the concept of field effect 1 monitoring

b. the equipment delivered by the

c. suggestions for further research

STATEMENP OF THE TECHNIQUE

A ahort antenna (e.g., l8" of wire mounted vertically in the air above an insulatirg support) wiil take on an electrical potential which will vary in response to the time variation of the electrical field in its vicinity. In particular, .1

cause cyclic variation of the potential on the antenna which correlates to the JConsider the instantaneous potential on the antenna to be a scalar, uniform over the surface of the antenna (its length is very much smaller than the wave length of the signal, e.g., 60 Hz has a wave length of 3l00 miles).

The amplitude of the signal is a function of the magnitude of the the Lantenne distance, the relative humnidity, the /the degree of static charge of the ∫to pame just a few. The signal frequency range extends from DC to less than l00 Hertz.

5 February 1973

MEMORANDUM FOR:

SUBJECT:

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A major problem with using such an antenna to intercept is its sensitivity to extreneous signal sources. It would be useful to select one.out of a set of potantial sources by means of some appropriate combination of shielding or enhanced directivity of the antenda.

Antenna theory suggests that directivity cannot be erranced by means of antenna configuration for all practical purposes recsuse the antenna size and the spacirg between elements of an antenna array wilì always be vanishingly small compared with a wavelength. The wave ,length of l00 Hz wave is:

$$
\therefore = \frac { c } { f } = \frac { 3 \times 1 0 ^ { 8 } \pi / 8 e c } { 1 0 ^ { 8 } e \times c ^ { - 2 } } = 3 0 0 0 k m
$$

Therefore, shielding or shaping of the field must be used to advantage if possible.

Another present limitation of the technique is the lack of knowł ledge required for reliable interpretation of the signals acquired. information should be reliable given an adequate signal-to-noise ratic, but waveform analysis for detection end evaluation would require considerable adaitional researc\_. (This comment is made prior to receipt of the |Final Report.)

Finally, since the antenna sits in the near field of the source, signals received will fall off as the cube o the distance. If the signsl is tangential at. 1 mecer, the signal will be 18 db down at 2 meters, and 28 ab down at 3 meters.

Signal-to-noise ratio is critical whenever data recording is done on a machine of finite dynamic:range. (The TEAC recorder supplied has a dynamic range of about 35 db.)

TE PROTOTYPE EQUIEMENT

The jequipment received from the Jwas briefly evaluáted. The unit performed much as expected, and was able ffrom to provide outputs clearly ahowing a as distances up to about 1 meter. The signal quality rapidly deteriorates with increasing range, as expected.

The unit has a rather poor battery life, rated at three hours in the instruction manual.

LIMITATTONS OF THE TECHNIQUE

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The operating time couid be approximately tripled by installing larger battery parks in the unused spaces o the case. A suggestion: if battery life is a serious consideration, and the unit undergoes design revisions, replace the circuitry with micropower components. Newly available operational amplifiers can run on as low as l.75 volts, drawing a few hundred microwatts. The present equipment, exclusive of the recorder, requires about l.9 watts. The tape recorder drews about 2.4 watts.

The unit is rather inconvenient to operate gince it requires battery replacement prior to each use, and a carfeul set-up of gain contrcls under conditions closely approximating those expected operationally As previously noted, the signal amplitude is quite range dependent and probably varies considerably with environmental circumstances such as the Automatic gain controls or logarithmic amplifiers should be considered in order to optimize use of the tape recorder's dynamic range.

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