The Co-Conical Field Generation System A 40 GHz Antenna Test Cell

Transcription

1 A 40 GHz Antenna Test Cell David R. Novotny and Robert T. Johnk RF Technology Division 325 Broadway Boulder, Colorado

2 An Effort of the Free-Field Time Domain Project Working in Broadband Measurements 8 Field Generation and Sensor Calibrations? Cone & Ground Plane? TEM Horns? Co-Conical Field Generation System 8 Whole System Testing? Chamber / OATS / EMC facility evaluation? Penetration Studies (aircraft, enclosures) 8 Material Testing & Reflectivity Studies? Non-destructive Dielectric Measurements? Absorber Evaluation? RCS Analysis

3 PURPOSE: PRODUCE A HIGH QUALITY ANTENNA CALIBRATION FIELD FROM 10 MHZ TO 40 GHz IN ONE UNIT TO FACILITATE FASTER TESTING. METHODOLOGY OF THE CFGS: 8 AN EXPANDING, SYMMETRIC TRANSMISSION LINE 8 SINGLE MODE OPERATION 8 CONSTANT IMPEDANCE 8 DISTRIBUTED, BROADBAND, HIGH POWER TERMINATION TO END THE TEST VOLUME.

4 BENEFITS: 8 HIGH FIELD UNIFORMITY / FIDELITY 8 BROADBAND PERFORMANCE - FASTER TESTING FOR WIDEBAND ANTENNAS 8 CONFINED, DIRECTED ENERGY SYSTEM - HIGHER FIELD LEVEL / INPUT WATT < REDUCED AMPLIFIER COSTS 8 FIELDS KNOWN THROUGHOUT THE CELL - REDUCED UNCERTAINTIES 8 PULSED POWER AND CW TESTING 8 CHEAPER TO BUILD, EQUIP AND USE TRADEOFFS: 8 LIMITED TEST VOLUME & REDUCTION IN VERSATILITY

5 Dielectric Support x Absorber Termination θ Outer Conductor θ 2 = 10 0 Inner Conductor θ 1 = WALL SCAN (θ=constant) PARALLEL TO OUTER CONDUCTOR AXIS SCAN ( x = constant) PARALLEL TO CELL AXIS Balsa Support / Impedance Match 2.92 mm Connector

6 8 ANALYTIC MODEL AND NUMERICAL SIMULATIONS E Z c η Pe e = = r sin( θ ) H = η θ θ ln cot tan 2 π ε r jβ r jω t jβ r jω t 0 η 0 Pe e jβ r jω t jβ r jω t Pe e = = r sin( θ ) Pe Fields vary as distance for central axis - larger test volumes 8 TRANSMISSION PERFORMANCE - TIME DOMAIN REFLECTOMETRY - Termination performance better than 20 db : 2 MHz to 40 GHz x x e 8 TESTING FIELD UNIFORMITY PASSIVE SCATTERER TESTS ALONG MAJOR CONTOURS OF THE CELL - Field uniformity better than 1 db

7 Co-Conical Field Generation System 8 Require Broadband and High Power 8 Imperfect absorber 9 avoid an all or nothing design 8 Distributed conical termination shunts the energy over several wavelengths Termination Design Issues 8 Large obtuse angle at the center conductor and Brewster angle at the outer conductor redirect reflections away from feed and test volume 8 Conductive cloth provides full band performance down to DC and is followed by traditional urethane absorber

8 0 TDR shows termination is absorbing 99% of the incident energy Peaking due to poor conductor connections, corrected in current design TEST SETUP -20 Step Generator / Oscilloscope 40 GHz ANA Frequency [GHz]

9 Field Probing 8 Using a passive scatterer to sample the fields within the test volume 8 Reflected signal is proportional to E 2 at the point of the scatterer 8 Cylindrical scatterer:10mm x 0.26mm Dielectric Support θ x Absorber Term ination Outer Conductor θ 2 = Scatterer aligned with E ë and moved within the cell Inner Conductor θ 1 = Wall scans (1/r falloff) and axis scans (constant E) show very good agreement with theory. AXIS SCAN ( x = constant) PARALLEL TO CELL AXIS Balsa Support / Impedance Match WALL SCAN (θ=constant) PARALLEL TO OUTER CONDUCTOR 2.92 mm Connector

10 Analytic Model Finite Element Simulation

11 Axis Scan (high field gradient area) show uniform field volumes (variations may be due in part to probe positioning errors).

12 Wall scan (corrected for 1/r) shows very good agreement with model. ± 1 db field uniformity plots show the lack of moding and indicate low system losses

13 SUMMARY 8 1 meter prototype built and tested - full scale design in progress 8 Numerical predictions match measurements 8 Termination scheme works over the entire frequency band 8 Data shows 40 GHz operation with good field uniformity 8 High quality calibration field in the test volume 8 VIABLE ALTERNATIVE TO AN ANECHOIC CHAMBER FOR BROADBAND TESTING OF LIMITED SIZE DEVICES