V4C7 5 February 1973 MEKOFANDUM FOR: SLTBJECT: INTRODUCTION The following observations and opinions pez-tain to: a. the concept of rield effect monitoring 2 b. the equipment delivered by the c. suggestions for further research STATEMMT OF THE TEC=QUE A abort antenna (.e.g... 18" of wire mouzted vertically irx the air above an insulating support) viil take on an electrical potential which will var_v in response to the 2time variation of the electrical field in its vicinity. In particular, )cause cyclic variation of the potential on the antenna which correlates to the .1 con- sider the instantaneous potential on the antenna to be a scalar, uniform over the surface of the antenna (its length is very2 much smaller than the wave length of the signal, e.g., 60 Hz has a wave length of 3100 miles). The amplitude of the signal is a,function of the magnitude of the the @antenna distance the relative humidity, the /the degree of static charge of the 2 jto nane just a few. The signal frequency range extends from DC i-o"i'es-s than 100 Hertz. :,Il@UTATIONS OF TliE TECHNIGLTE A major problem with using such an antenna to intercept is its sensitivity to extraneous signal so@arces. It would be useful to select one-out of a set of potential sources by means of sozle appro-,oriate combination of shieldin2g or enhanced directivity of. the a-itenna. Antenna theory suggests that directivity cannot be er.!,,L-,,iced by means of atitenna configuration for all practical purposes t-ecause the anterl-ia size and the spacing between elements of = antenna array wil3. alway2s be vanishingly -all compared with a -wavelength. The wave .Ienkth of 100 Hz wave is: c = 3XI08 m/see sec-.L OOOkm Therefore, shielding or shaping of the field must be used to advantage if possible. Another present limita2tion of the technique is the lac@i of know- ledge required for reliable interpretation of the signals acquired. @imformation should be reliable gi-en an adequate signal-to-noise ratic, but waveform analysis for detection and evaluation iwou=ld require considerabl@- additional researc..,. (@!his comme2nt is made prior to receipt of the ]Final Report.) Finally, since the antenna sits in the near field of the source, signf-ls received will fall off as the cube o- the distance. If the sign&l is tangential at 1 me,'@ler, the signal will be 18 db down at 2 meters, and 28 db down at 3 meters. 2 Signal-to-noise ratio is critical whene-,rer data recording is done on a machine of finite dynamic:range. (The TFAC recorder supplied has a dynamic range of about 35 db.@ THE ILR The iequipment received from th-- _jwas brie7-'ly 2evaliiited. The unit performed much as expected, azid @rds able to provide outputs clearly showing from a las distances up to about I meter-," -TEe'-s-;'-gn-ay 'j-dality rapidly deteriorates with increasing range, as expected. The unit has a rather poor battery 9life, rated at three hours in the instruction manual. -2- The operating time could be approximately trinled by instal- ling larger battery parks i.-i the unused snaces o-' the case. A suggestion: if battery life is a serious consideration, and ttie unit undergoes design revisions, replace the circuitry with micro- power components. Newly available operational amplifiers can run on 2as low as 1.75 volts, drawing a few hundred microvatts. The present equipment, exclusive of the recorder, re-quires about 1.9 watts. The tape recorder draws about 2.4 watts. The unit is rather inconvenient to operate since it requires battery v@-placement prior to each use, and a car?eul set-up of gain controls under conditions closely approximating 2those expected operationady@ 'As previously noted, the signal amplitude is quite range dependent and pro- ba'oly varies considerably with envirormental cir:@@=stances such as the Automatic gain controls or logarithmic amplifiers should be considered in order to optimize uee of the tape recorder's dynamic :range. 7 -3-