Sensor and Simulation Notes Note September Micro-Impulse Radiating Antenna (MIRA)

Transcription

1 Sensor and Simulaion Noes Noe 578 Sepember 7 Miro-Impulse Radiaing Anenna (MIRA) D.V. Giri Pro-Teh, -C Orhard Cour, Alamo, CA 957 Dep. of ECE, Universiy of New Mexio, Albuquerque, NM 873 Absra In his noe, we desribe he eleromagnei design onsideraions of a small (perhaps he world s smalles) or Miro-Impulse Radiaing Anenna (MIRA). This auhor had designed, analyzed, fabriaed and esed a 3.66 m diameer IRA (perhaps he world s larges) in 99 [- 3]. This very firs IRA was alled he Prooype IRA. We are presening he design of a Miro- IRA wih a diameer of 5 m for fuure appliaions in he areas of deeion of hidden objes of m sizes. A his poin, he exlusive ineres is in design and analysis of his Miro-IRA.

2 Conens Seion Page Inroduion Transien Soure Consideraions Near, Inermediae and Far fields Fabriaion Consideraions Summary Referenes 8 Aknowledgmen I am hankful o Prof. Yanzhao Xie, of Xi an Jiaoong Universiy for suggesing his problem.

3 . Inroduion An Impulse Radiaing Anenna (IRA) is a paraboloidal refleor fed by a pair of oplanar onial ransmission lines. Eah ransmission line has a haraerisi impedane of Ohms. The ne anenna impedane is Ohms when he wo feed lines are onneed in parallel. The firs one buil ira 995 was alled he Prooype IRA and had a diameer of 3.66 m. Many IRAs have been buil sine, and here have been variaions on he original heme as well. The objeive of his noe is o design and analyze a very small IRA wih a diameer of 5 m.. Transien Soure Consideraions The [diameer D/speed of ligh ] needs o be muh larger han he rise ime of he exiaion pulse. In oher words, need many rise imes in he diameer. Or [D/( r )] should be >>. If he diameer is 5 m, his orresponds o a ime of ravel of 66 ps. Consequenly, we should use a pulse soure whose riseime is small ompared o 66 ps. We find PSPL 5 [] ould be useful a leas for iniial modelling. The adverised oupu has he following speifiaions. Volage ampliude - 9 V Fall ime a he beginning of he ransien waveform 5 ps Pulse duraion ns Rise ime a he end of he pulse 5 ps [D/( r )] [66 ps/5 ps]. Tha means rise imes in he diameer, whih is a good number. The oupu of he ransien pulse is of he box ar ype [Appendix B; Waveform 7 in 3] and an be analyially modelled by V ( ) V + exp( α ( ) + exp( α ( ) (Vols) () In his waveform, i is possible o independenly hoose he fla op ampliude, duraion, he fall ime a he beginning of h pulse, and he rise ime a he end of he pulse 3

4 We have hosen peak ampliude of - 9V, -9% fall ime of 5 ps, pulse duraion of ns and a rise ime a he end of he pulse of 5 ps. In praie, i is diffiul o bring he volage bak o zero from is peak, as fas as i an be aken from o peak. The volage waveform of equaion () has an analyial Fourier ransform given by ~ p p jω p p jω V ( ω, α ) s exp( jω ) s exp( jω ) α α α α (V / Hz) () where ω π f is he radian frequeny. The numerial values we have hosen are: V 9V ; a.76 x ns ns resuling ina fixed ( 9%) rise ime of 5 ps resuling in a pulse duraion ( ) ns a.9 x^resuling ina fixed ( 9%) fall ime of 5 ps The resuling ime domain waveform is shown ploed in Figure. Figure shows he deails of he fall ime of 5 ps a he beginning of he waveform. Figure 3 has he deails of he rise ime of 5 ps a he end of he waveform, and Figure has he magniude sperum. V() in Vols 3 V( ) ime (se) PSPL 5 B Pulser Figure. Modelled oupu of he PSPL 5 B pulser

5 V( ) Figure. The fall ime (5 ps) a he beginning of he ransien waveform V( ) Figure 3. The rise ime (5 ps) a he end of he ransien waveform 5

6 7 8 V(f) [Vols/Hz] V( f) f Frequeny (Hz) Magniude Sperum of Pulser Figure. The magniude sperum of he ransien waveform I an be observed ha he null frequenies in he magniude sperum orrespond o he reiproal of he pulse widh and is harmonis. Afer a areful onsideraion of his ransien soure, we have deided no o use his pulser, beause of low ampliude, and also he nulls or no volage a frequenies of approximaely MHz and is harmonis. A seond opion for he ransien soure was found o be FID SP - offered by FID Tehnologies [6]. The pulse waveform is a double exponenial wih an ampliude of kv, -9% rise ime of ps and a FWHM of ns, This waveform is modeled by V ( ) V o (+ a) e d erf ( π / ) < ( π / ) d V ( + a) e erf d > d (3) erf (z) is he omplimenary error funion given by 6

7 z erf ( z) erf ( z) exp( ) d p () d ( + j ω π ) ~ V (+ a) d d V ( ω ) e ( + jω ) (5) This analyial model of he pulser is sill haraerized by hree numbers and has oninuous derivaives. This model an be explained as follows. Consider a Gaussian waveform. An inegraed Gaussian is an s-shaped waveform. When his s-shaped waveform reahes is peak, we add an exponenial deay faor o i. Suh a proess is represened by he ime domain expression in (3). FID SP- pulser oupus are well represened by his model. The parameers are: V kv,.5, α.9, d rise ime ps Wih he above se of onsans, we will ge an exponenial deay ime of. ns and a FWHM ns. The resuling ime domain waveform is shown ploed in Figure 5. Figure 6 shows he rise porion; Figure 7 shows he derivaive of his pulse waveform. Figure 8 has he magniude sperum. V ( ) Peak Volage kv FWHM ns 9% Rise ime ps Figure 5. Modelled oupu of he FID SP-pulser from FID Tehnologies; The peak ampliude of kv and he FWHM of ns an be seen in his Figure. 7

8 V ( ) % of peak a ps 9 % of peak a 3 ps Therefore -9% rise ime ps Figure 6. The rise ime ( ps) a he beginning of he ransien waveform.5 DiV( ) Figure 7. The ime derivaive of he of he ransien waveform 5 VF ( f) f Figure 8. The magniude sperum [Vols/Hz] of he ransien waveform of FPG SP- pulser From Figure 7, we see ha he peak value of he derivaive of his waveform is V/s. This means he maximum rae of rise of his waveform is given by 8

9 maximum rae of rise e mrr V dv d (for an ideal exponenial rise) peak peak (6) For he urren pulser waveform he maximum rae of rise is given by kv/[ (V/s)] ps, same as he -9% rise ime. We also noe ha he far field for his anenna sars a a disane of [3] Far field sars a (approx.) [D / ( r )] (5 m) / ( x 3 x 8 x ps).83 m (7) 3. Near, Inermediae and Far Fields Mikheev e. al, [5] have proposed a simple mehod for alulaing he near, inermediae and far fields of an IRA anenna. Basially, his mehod uses he onial ransmission-line fields refleed in he paraboli mirror. If he anenna was a fla plae, he onial ransmission-line would have an idenial mirror image in he fla plae, resuling in he feed line and is image having he same expansion angle. However, sine he anenna is paraboloidal in shape, he image is also a onial ransmission line wih a differen expansion angle. The various geomerial parameers for boresigh field alulaions are shown in Figure 9. Figure 9. The geomery for boresigh field alulaions 9

10 The oal eleri field a any poin on he boresigh axis, a a disane of r from he foal poin is given by D r V D r F V r r V r r V f r E g )) os( ( ) os( ) sin( ) sin( ) os( ) sin( ), ( g g g π (8) where he geomeri impedane faor f g is he raio of he anenna inpu impedane Z o he haraerisi impedane of free spae Z, or f g ( / ) Z Z. I is noed ha for a paraboloidal refleor, sin( ) os( ) + D F (9) Our field ompuaions are shown ploed in Figure.

11 3.5 Far field sars a approximaely.83 m E (., ).5 E (., ) E (.3, ) E (., ) E (.5, ) prepulse duraion is F/ 6.7 ps Figure. On axis, verial eleri field a disanes of m, m, 3 m, m and 5 m The magniude spera orresponding o he above emporal alulaions are shown in Figure. 6 Efield speral magniude [V/(mHz)] M (., f ) M (., f ) M (.3, f ) M (., f ) 7 M (.5, f ) f Frequeny ( Hz) rae rae rae 3 rae rae 5 Figure. Magniude spera of he on-axis verial eleri field

12 From he field ompuaions shown and unshown above, we an ge he field quaniies lised in Table. Table. Summary of Boresigh Verial eleri field (peak values) # Boresigh Disane r E peak kv/m Vp r E peak kv V p / V wih V kv 5 m.. m m m m m m m m m m As saed earlier, we an onlude ha a a boresigh disane of abou 3 m, we are already in he far field zone for MIRA.. Fabriaional Consideraions There are many fabriaional hallenges in building suh a small IRA. Previously we have fabriaed a m diameer IRA These hallenges inlude and are no limied o finding mirooaxial ables of Ohm impedane and feed poin issues of mainain he fields a he feed poin and avoid aring. However, hese onsideraions will ome o he forefron only during fabriaion and esing and need o be overome. The pulse oupu is ino an SMA onneor of 5 Ohm haraerisi impedane. Sine a differenial oupu is needed o drive a full IRA like he MIRA, we will need a ransmission line balun ha ransforms he 5 Ohm impedane o he required Ohm impedane a he anenna erminal. The balun is mahed o he 5 Ohm soure a one end and he Ohm anenna a he anenna erminal. This is shown in Figure.

13 Female SMA Conneor (5 Ω inpu) Splier Ω able - V o To pulser soure Vo (ino 5 Ω) Ω able To Ω IRA feed sruure +V o Figure. Deails of he 5 Ohm o Ohm balun onneing he pulser soure o he IRA feed sruure. Figure 3 shows a side view of one of he feed arms. The feed arms ome o a poin a he feed poin and onne o he ener onduor of he impedane ransformer. I is noed ha in Figure 3 we are showing jus he op half of he anenna, and we are designing a full IRA. Foal Lengh F Figure 3. Side view showing he upper fla-plae feed arm 3

14 We noe he numerial values o be D 5 mm D / 5 mm F 7.5 mm F/D.35 () d [ D / (6 F)] 8.93 mm (F + d) 6.3 mm (F - d) 8.57 mm Noe ha here are wo launher plaes and hey are imaged in he ground plane. Eah launher plae along wih is image is a ransmission line wih Ohm impedane. The wo lines are in parallel for a ne impedane of Ohms, whih mahes he impedane ransformer. Z Anenna imπedane Ω ( launher πlae above ground) Z Zo K( m) K( m ) K( m) 6 π K( m ) Ω Solving for m; > m.559 m ( m). () K( m) π / m sin ( θ ) dθ K( m ) π / m sin ( θ ) dθ aranm / o an 63. o D aran ( ) F d 7.7 () aranm / o an 78.95

15 ( o ) 7.88 degrees ( ) 7.66 degrees I is observed ha he line denoing he angle o inerses he rim of he refleor. I is no exaly, bu very lose o he biseion of angles and. Anoher view of he launher plaes in relaion o he refleor is shown in Figure. One again, we are showing only he op half of he full IRA in Figure. Figure. End view of launher plaes and he half refleor The launher plaes are seen in Figure, oriened in suh a way ha he blokage from hem is minimized. In Figure, b, b and b are he radii of he edges and he ener of he launher plaes, orresponding o he angles b, b and b respeively. These radii are given by b D b b 5 mm b b m /.6 mm () b b m / 8.9 mm 5

16 The alulaions shown above are useful in he fabriaion of he 5 m MIRA. Nex we urn our aenion o he deails of fabriaing he launher plaes. The deail of he feed plaes is shown in Figure 5. Maerial: Aluminum Thikness r mm Quaniy: F Lengh of Launher Plae Foal lengh F 7.5mm Full Widh of Launher Plae a (D/7) 5 mm / 7 7. mm a Figure 5. Launher plae geomery The dimensions and he maerial of he launher plaes are also shown in Figure 5. The lengh of he launher plae in Figure 5 is seen o be se equal o he foal lengh of he refleor. The reason for his is as follows. If he launher plae is of lengh F, hen he refleion from he end of he launher plae (if any) will ome a exaly he ime when he large impulse ours and he minor refleion from erminaor imperfeion ourring a he same ime will ge swamped ou by he large impulse. If he pulser is urned on a ime, i akes a ime of r 6

17 (r/) o reah he observer In he ime inerval of r o r + (F/), here will be some energy ha goes away from he foal poin o he observer and he effe of he presene of he anenna is no ye seen in his ime frame. Call his as prepulse and he impulse happens a ime r + (F/). The reason why prepulse and impulse are of opposie signs is beause he eleri field reverses in sign when i his he mealli refleor. So, he prepulse is seen o be negaive lasing for a ime (F/), when he signal reahes he observer. (F/) for our ase is seen o be ( x.75 m) / (3 x m/s ) 6.67 ps The negaive prepulse a he observer loaion is seen o las for 6.67 ps. This an also be seen in Figure. We also noe from Figure 5 ha he full widh of he launher plae where he erminaion sars is given by a (Diameer D/7). In oher words, we have se Diameer / (full widh of he launher plae) (D / (a)) (5 mm/ 7. mm) 7 (3) This raio of 7 is needed o ensure ha he impedane of eah launher plae o is image in he ground plane is Ohms [See Table. on page in 7]. Two suh lines in parallel resuls in ne impedane of Ohms for he 5 m MIRA. We now urn our aenion o he Terminaion a he end of he launher plaes ha ineronnes he launher plae o he refleor, as shown in Figure. One of wo launher plae Figure. Conneing he launher plaes o he refleor 7

18 The ne DC resisane of he erminaion needs o be Ohms. We may use one or more parallel resisors depending on available spae in fabriaion. 5. Summary In his noe, we desribe he eleromagnei design onsideraions of a small (perhaps he world s smalles) or Miro-Impulse Radiaing Anenna (MIRA). This auhor had designed, analyzed, fabriaed and esed a 3.66 m diameer IRA (perhaps he world s larges) in 99 [- 3]. This very firs IRA was alled he Prooype IRA. We are presening he design of a Miro- IRA wih a diameer of 5 m for fuure appliaions in he areas of deeion of hidden objes of m sizes. A his poin, he exlusive ineres is in design and analysis of his Miro-IRA. Deailed design and fabriaional onsideraions are presened noing ha i will be a hallenge o fabriae suh a small MIRA. Referenes Cied referenes suh as: Sensor and Simulaion Noes, Ineraion Noes, are available a he URL: hp://ee-researh.unm.edu/summa/noes/ and also from he auhor. []. D. V. Giri, e. al., A Refleor Anenna for Radiaing Impulse-Like Waveforms, Sensor and Simulaion Noe 38, July 995. []. D. V. Giri, and C. E. Baum, Refleor IRA Design and Boresigh Temporal Waveforms, Sensor and Simulaion Noe 365, February 99. [3]. D. V. Giri, High-Power Eleromagnei Radiaors: Nonlehal Weapons and Oher Appliaions, published by Harvard Universiy Press,. []. Pioseond Pulse Labs., now a par of Tekronix. Visi hps:// [5]. O. V. Mikheev e al., New Mehod for Calulaing Pulse Radiaion from an Anenna wih a Refleor, IEEE Transaions on Eleromagnei Compaibiliy, volume 39, number, February 997, pp 8-5. [6]. hp:// [7]. C. E. Baum, D. V. Giri, and R. D. Gonzalez, Eleromagnei Field Disribuion of he TEM Mode in a Symmerial Two-Parallel-Plae Transmission Line, Sensor and Simulaion Noe 9, April