Experimental Approach for Determining the Received Pattern of a Rascan Holographic Radar Antenna

Transcription

1 Eperimental Approach for Determining the Receive Pattern of a Rascan Holographic Raar Antenna Masaharu Inagaki Geophysical survey epartment Walnut Lt. Tachikawa, Japan ina_mas@beige.plala.or.jp Timothy Bechtel Dept. of Earth an Environment Franklin & Marshall College, Lancaster, PA USA tbechtel@fanm.eu Vlaimir Razevig Remote Sensing Laboratory Bauman Moscow State Technical University Moscow, Russia vr@aha.ru Abstract Because of the nature of image formation for holographic raar, the role an relevance of antenna raiation pattern are ifferent than for impulse raar, an thus have not been stuie. We etermine theoretical Rascan raiation patterns for ifferent ielectric meia, an compare simulations to eperimental ata from an antenna scanne across a spherical metal target at various ranges/epths. Positions at which the receive signal (phase ifference between object an reference beams) are the same within a single scan, an across scans at ifferent epths provie a 3-D approimation of the effective raiation pattern that closely agrees with the simulations. Keywors-component; antenna; raiation pattern; GPR; holographic raar I. INTRODUCTION Antenna pattern is a funamental specification when analyzing GPR characteristics. Although a raiate wave propagates spherically, the antenna pattern is the angular istribution of signal strength. The signal strength is strongest along the critical angle which may vary with antenna esign, istance from the transmit point, an properties of the meium. In orer to irectly etermine the antenna pattern the strength is measure at the same phase position. In the case of impulse raar, the strength of the pulse is measure in any irection. In the case of continuous wave (CW) raar, the strength of the wave is measure at positions of constant phase. Holographic raar is CW. It prouces a plan view from scanning along many lines. The antenna pattern has not been actually measure only theoretically calculate. A steel ball is a convenient tool to etermine the antenna pattern eperimentally for holographic raar. It guarantees a strong reflection with return path eactly matching the incient path (a perfect return path ). In this paper, the antenna pattern of holographic raar is etermine by eperiments using a steel ball, an is compare with a theoretical simulation. II. Scan lines THEORETICAL APPROACH A. Holographic Raar Holographic raar moves along parallel lines on the surface of the meium to be scanne, an prouces a plan view (See Fig.1). Conventional impulse raars istinguish each reflection coming from ifferent istances by ifferent time-of-flight. However, holographic raar receives all the reflections arriving at the antenna position simultaneously, so epth information is not retaine. Instea, the raar recors an interference pattern of the transmitte an reflecte waves, which is (in low loss material) an accurate plan view image of the target. This is a merit of the raar in terms of getting easily interprete images of the target. B. Simulation of Antenna Raiation Pattern The antenna raiation pattern can be calculate theoretically. Fig. is a three-imensional view by theoretical simulation. The symmetrical shape of the pattern looks a jellyfish. There is little irectional isotropy ecept for weak back lobes. Horizontal plane Burie object Rascan antenna hea Figure 1. Scanning metho of holographic raar (Rascan) /9/$5. 9 IEEE Page 193 of 116

2 Z Phy=18 Theta= Antenna hea Y X Theta=18 Raar image Phy= Figure. Three imensional view of holographic raar antenna raiation pattern by theoretical simulation Figure 3. Relative antenna an target positions an corresponing raar images III. EXPERIMENT An antenna raiation pattern can be elineate by etermining the strength of the electromagnetic fiel at the same phase position (or same two-way travel istance) within the interference pattern. The simplest metho is to place receiver antennas at those positions. However, this metho is not practical for two reasons. One is that positions of antennas cannot be controlle accurately. The other is that the holographic raar antenna is not esigne to be use as only a receiver or only a transmitter. Thus, it is very har to fin the same phase position irectly. However, instea of placing antennas, a reflector can be place at the same position. Then, the antenna can be irectly use for its esigne function. However, if the reflector is a flat plate, it is very har to keep the accurate angle that guarantees the perfect return-path reflection. A conuctive sphere resolves this problem naturally an perfectly. A. Perfect Return-Path Reflection Polishe steel is a near-perfect raar reflector. It reflects an electromagnetic wave with almost one hunre percent efficiency. A spherical steel ball reflects an incient wave an perfectly returns it to the raiate point. It is not necessary to ajust the angular position to guarantee a perfect return-path. A holographic antenna scan (See Fig.1) prouces a GPR recor that looks like black an white concentric stripes (as in [1]). There is a black (or white) circle in the very center. This can be esignate as known or reference phase position. For eample, this is epresse as a gray circle (eepest target conition in Fig.3). In this case, the istance between the antenna hea an a steel ball must be not too close an not too far. Because if the istance is very close, the wave sprea is too small to etermine the antenna pattern. An, if the istance is too far, the reflection is too weak to recognize. When a steel ball is move a little closer to the scanning surface, the same targete event epans a little outwars an forms a gray ring instea of a circle in the raar image (mile target conition in Fig.3). In this conition, to maintain the constant target istance (an phase position), the antenna hea is locate with a little offset from a steel ball. When a steel ball shifts further upwars to contact to the scanning boar, the antenna hea is locate with even more offset to maintain istance/phase. An, in the image, the ring size becomes even larger. The raypath for the reference phase position may be close to horizontal The holographic raar images for a steel ball at some epth beneath a scanning boar have three kins of information, the strength of reflection I, the istance between the antenna hea an a steel ball s an the irectional angle of the steel ball looking from the antenna hea θ (See Fig.4 ). First of all, the strength of reflecte wave is the most important value when etermining the antenna pattern. Each piel in the raar image has a specific igital value. The maimum value among the piels containe in the rings or the circle is aopte as the strength of reflection. The istance is calculate by equation (1). s = + y + ( + r) r Where, s is the istance between the antenna hea an a steel ball, is the position of the antenna hea on the -ais, y is the position on the y-ais, is the vertical istance between the scanning boar an a steel ball, an r is the raius of the steel ball. The angle is calculate by equation (). (1) Page 194 of 116

3 + y 1 θ () = tan + r Antenna hea Where, θ is the irectional angle of a steel ball looking from the antenna hea. Scanning boar y Strength of reflection at a receiver I Actual raiating point Thin plastic plates 3mm each Scanning boar iameter = 43mm z r Figure 4. Parameters escribing the location of an antenna hea relative to a steel ball. s Apparent raiating Center Actual raiating point Figure 6. The eperimental evices. C. Eperimental Metho To etermine the antenna raiation pattern requires a three-imensional ata set. In this case, the horizontal area of the scanning boar is approimately 3cm 3cm, an the maimum epth of the steel ball is 5cm. The eperimental metho is etraorinarily simple. All we nee is to o normal scanning works using a holographic raar antenna hea over a steel ball with various epths. The eperimental evice is shown in Fig.6. A steel ball with 43mm is put on thin plastic plates with thickness of 3mm each. First, a scanning boar is place over the steel ball with zero epth (the top of the ball just contacts the boar), an the scan is carrie out in this conition. Afterwars, removing plates one after another an scanning each time prouces many images at epth increments of 3mm. IV. RESULTS Figure 5. A schematic view of an apparent raiating center. B. Apparent Raiating Center An actual raiating point is the flat bottom of the antenna hea. A wave front is not epecte necessarily to form a perfect hemispherical shape with the raiating point as the center. The shape of the wave front etermines the irectional characteristics. If the shape is close to hemispherical, the wave strongly isperses. If the shape is close to flat, the ispersion is relatively mil. To epress this phenomenon an apparent raiating center is efine as shown in Fig.5. The remote istance of the apparent raiating center from the actual raiating point is an important parameter for evaluating ispersion characteristics of any GPR. The Rascan holographic raar prouces ten images for each complete scan (5 frequencies polarizations). The raiation pattern cannot be recovere from the cross polarization images, because the phase is shifte on arrival of the reflecte wave. The five parallel polarization images, in which frequencies vary from 3.6GHz to 4GHz in.1ghz steps, show similar characteristics. Among these, the highest frequency (4GHz) images were analyze because the highest frequency shoul give the best resolution among five frequencies. A. Holographic Images Fig.7 is a series of 14 images recore by Rascan holographic raar. When =mm, the targete wave front efines a ring. When increases, the ring shrinks inwars, finally becoming a black circle. The maimum signal amplitue appears at =33mm. In further positions (i.e. is larger than 33mm) the strength ecreases. This inicates that the wave front reflects from the top surface of the steel ball when =33mm. Page 195 of 116

4 (mm) (mm) raar has a very weak irectionality, with a symmetrical sensitivity like the jellyfish in Fig.. Depth (mm) θ s Offset (mm) Figure 8. Apparent raiation center Figure 7. Holographic raar images (epth variation) B. Apperent Raiation Center Using the equations (1) an () the istances an the angles to the reflecte point, which is on the surface of a steel ball, have been calculate. The shapes of the rings on the GPR images are almost perfectly circular, inicating that the istances to the ball from any point on the ring are almost equal. The istance an the angle have been calculate by the position of the ring in the image, with the result shown on Fig.8. All the ots have been plotte as a set of values ( s,θ ). The ots lie along an arc that is a part of a perfect circle, with the center of the arc corresponing to the origin. Thus, it has been proven that the actual raiating position an the apparent raiating center share the almost same position for holographic raar Rascan. This means that the holographic C. Anistprophy of Antenna Raiation Pattern Looking through the GPR images over Fig.7, all the rings an the circles seem to have uniform signal strength. However, investigating each value shows that there eists a slight anisotropy. When a reflection position is locate close to the top of a steel ball, the strength of reflecte wave is fairly uniform. However, when a reflection position is locate close to the ege of a steel ball, anisotropy appears. To illustrate this, three cases (=6, 18 an 3mm) are shown in Fig.9. Anisotropy appears on the image from =mm to 1mm. It is isappearing from =15mm to 7mm. Anisotropy completely isappears for >3mm. It is epecte that anisotropy comes from the physical shape of the antenna poles. In case of a ipole antenna, irectional anisotropy eists because the ipole arrangement originally coul not be uniform to any irection. However, the anisotropy is so small in Rascan holographic raar compare with a conventional ipole antenna that the effect will not affect interpretation of images. D. Comparison of Antenna Raiation Pattern Measure antenna raiation pattern has been isplaye on 3D chart (See Fig.1). The actual raiation point is locate at the origin (,, ). The normal irection of raiation is z-ais. All the ata belong to the forwar raiation, because the ata corresponing to back raiation cannot be eperimentally obtaine. The lumpy shell comes from the iscrete points of ata espite of some smoothing treatment. That is not essential. The theoretical raiation pattern for comparison is shown in Fig.. Page 196 of 116

5 Y =6mm X.3..1 Z =18mm =3mm Figure 9. Anisotropy of antenna raiating pattern Both of shapes (theoretical an measure) have quite goo corresponence in overall. When the raiating irection is close to the -y plane, the strength steeply ecreases. This angle Figure 1. Three imensional view of holographic raar antenna raiation pattern using the measure ata position is a little earlier in the measure pattern. As a result this jellyfish-shape pattern, in contrast with the theoretical pattern, seems to be slightly shifte to forwar along z-ais. The measurements were repeate several times, an this result reprouces. This ifference possibly comes from a phase shift in the transition from near fiel to far fiel. V. CONCLUSIONS Using a spherical shape steel ball, the raiation pattern for Rascan holographic raar has been measure. The metho is very simple an it oes not nee any special evices; only the GPR antenna itself an a steel ball. Although the measurements agree in general with theoretical preictions, there is an uneplaine phenomenon. However, this simple metho coul be applie for any other type of GPR to show actual conitions irectly. Because the values are rea from images, there might be a loss of precision relative to irect signal measurement, but this is still simple an convenient metho for etermining the basic shape of a raiation pattern. In this stuy, the raiation pattern is preicte in parallel polarization. In future work, there will also be a possibility for cross polarization to clarify some behaviors in phase shift. REFERENCES [1] M. Inagaki, C.Winsor, T. Bechtel, E.bechtel, S. Ivashov, A. Zhuravlev, Three-imensional views of burie objects from holographic raar imaging, Piers9. Moscow, vol. A47, pp , August 9 [] S. Ivashov, V. Razevig, I. Vasilyev, A. Zhuravlev, T. Bechtel, L. Capineri,, The holographic pronciple in subsurface raar technology, International symposium to commemorate the 6 th anniversary of the invention of holography, Springfiel, Massachusetts USA, October 7-9, 8, pp (references). [3] V.Chapursky, S. Ivashov, V. Razevig, A. Sheyko, I.Vasilyev, Microwave hologram reconstruction for the Rascan type subsurface raar, GPR, Santa Barbara, California USA, April 9-May. (references) Page 197 of 116