Physically and Electrically Large Antennas for Antenna Pattern Measurements and Radar Cross Section Measurements in the Upper VHF and UHF bands Vince Rodriguez, PhD Product Manager, Antennas ETS-Lindgren, 1301 Arrow Point Dr. Cedar Park, TX 78613
Outline Introduction and Background The new horn The Open Boundary Quad-ridged Horn. History Lower frequency: Problems with scaling Totally open The Super Open boundary Quad-ridged Horn Computed performance
Outline II The 100MHz to 1GHz Horn, some measured data The 400MHz to 10GHz Horn, comparison of the OBQH and the SuperOBQH The 60MHz to 1GHz Horn. Absorber effects on an SuperOBQH Conclusion
4 To download and follow up www.ets-lindgren.com/event-argentina www.ets-lindgren.com/event-brazil
BACKGROUND INFORMATION 7-18-2006 5
Introduction Over the last 7 years of attending AMTA Symposia became aware of the need for a dual polarized antenna for antenna measurement at low frequencies At those frequencies tapered chamber are the preferred range. Log periodic array antennas not suitable for tapered chambers Decided to improve available 400MHz to 6GHz antenna using lessons from S-Ku band horn presented at AMTA 2005 in Newport
For more Background V. Rodriguez An Open-Boundary Quad-ridged Guide Horn Antenna for Use as a source in Antenna Pattern Measurement Anechoic Chambers IEEE Antennas and Propagation Magazine, Vol. 48, No. 2, April 2006 V. Rodriguez Recent Improvements to Dual Ridge Horn Antennas: The 200MHz to 2GHz and 18GHz to 40GHz Models 2009 IEEE International Symposium on EMC. Austin, TX Aug 17-21 2009. V. Rodriguez Improvements to Broadband Dual Ridge Waveguide Horn Antennas 2009 IEEE International Symposium on Antennas and Propagation and USNC/URSI National Radio Science Meeting. Charleston SC June 1-5 2009. V. Rodriguez, A new broadband Double Ridge guide Horn with improved Radiation Pattern for Electromagnetic Compatibility Testing,16th international Zurich symposium on Electromagnetic compatibility, Zurich, Switzerland, February 2005.
Introduction Existing design has been used successfully in Tapered chambers as a feed. Has a range of 400Mhz to 6GHz Goal was to improve this design using the tricks learned in developing the S-Ku band horn* *V. Rodriguez An Open-Boundary Quad-ridged Guide Horn Antenna for Use as a source in Antenna Pattern Measurement Anechoic Chambers IEEE Antennas and Propagation Magazine, Vol. 48, No. 2, April 2006 A Diagonal Dual Polarized Horn Antenna, US Patent number 6,489,931
New Horn: Design of the UHF to S band antenna One goal was to get good shielding for the cavity so when the antenna is mounted to the shield of the chamber it does not compromise the shielding effectiveness of the enclosure. A cylindrical cavity makes it easier to machine a single block of aluminum for the cavity. A cylindrical cavity was modeled The ridge profile is based on the same as in the previous design, but the curve was modified to have a larger aperture that will allow for better radiation at lower frequencies
New Horn: Design of the UHF to S band antenna The measured pattern for the DDPH at 3GHz. As it can be seen it exhibits a notch in the center of the main beam. This is caused by reflections from the Lexan (polycarbonate) sides. These effects were described in: 1. V. Rodriguez-Pereyra New Broadband EMC double-ridge guide horn antenna RF Design. May 2004, pp. 44-50. The new antenna at the same frequency showing a well formed beam.
New Horn: Design of the UHF to S band antenna The model from CST MW studio was send to Solidworks for mechanical design. Once the mechanical design was finalized it was time to build a prototype and to start measuring the performance of the horn.
The antenna was characterized per the IEEE/ANSI C63.5 method where the antennas are calibrated over a ground plane and one of the them scanned from 1 to 4m to isolate the effects of the ground plane, that accounts for some of the difference between polarizations Measured Performance
The measured VSWR show how close both inputs are even when they are not exactly at the same location as feeds would intersect. Only at high frequencies is the difference apparent Measured Performance
The antenna exhibits a good isolation between the two ports when the S21 is measured between them. Measured Performance
Measured Performance
Measured Performance
the S to Ku band Horn (courtesy of Imbriale et al.) (courtesy of Imbriale et al.) The antenna will be used in a cryogenically cooled cell to reduce the thermal noise. Low noise amplifiers located right at the feed of the antenna.
Use of the 300MHz to 6GHz and the 2 to 18GHz designs
The Design is Scalable 300MHz-6GHz 700MHz-10GHz 2GHz-18GHz
7-18-2006 21 THE 100MHZ TO 1GHZ HORN THE INTRODUCTION OF THE SUPER OPEN BOUNDARY QUADRIDGE HORN CONCEPT
The problems with scalability Simply scaling the design may work electrically as the next slides show But mechanically is a nightmare Very heavy 5.5ft 5.ft 5.5ft Very expensive to machine
The weight issue can be solve by hollowing the ridge which does not appear to affect the performance Scaled Model
The VSWR barely changes as the ridges are hollowed. The gap between the ridges is critical and reducing it improves the performance Performance of scaled horn
The cavity problem The scaled cavity is massive. At 18 inches across and a total 40 inches including the mounting flange it is extremely expensive to manufacture and it also makes the antenna extremely heavy 18 40
Totally open or Eliminate the cavity Study the possibility of an octagonal cavity Which had been used in other horns including dual ridge units covering 200MHz to 800MHz
Totally Open No change in VSWR when the cavity is changed. Decided to go ahead and totally eliminate the cavity. The Open Boundary Quadridge Horn goes to its most extreme evolution with a totally open horn.
Totally open or Eliminate the cavity
RF Performance: input parameters 3.5:1 1.9:1
RF Performance: gain As with all the Open Boundary horns this one shows a very flat gain behavior for most of its range. The lack of feed cavity has not affected its performance.
RF Performance: HPBW
Computed patterns
Computed patterns
Pattern 100 MHz
Pattern 200 MHz
Pattern 300 MHz
Pattern 400 MHz
Pattern 500 MHz
Pattern 600 MHz
Pattern 700 MHz
Pattern 800 MHz
Pattern 900 MHz
Pattern 1000 MHz
Mechanical Specifications Height: 74.17 inches (1.88m) Width: 74.17 inches (1.88m) Depth: 68.52 inches (1.74m) Approximate Weight: 152 Lbs (69kg) Two (2) N-type Connectors Aluminum construction with dielectric supports
The Horn itself
VSWR Measured results (VSWR) Better than the predicted results Frequency (MHz)
Gain (near Field) (db) Measured results (Near Field Gain) 2m test distance) The Purpose of this measurement is to check the difference between the ports Frequency (MHz)
X-port isolation (db) Measured results (Cross Port Isolation) Frequency (MHz)
7-18-2006 49 HIGHER FREQUENCIES SUPER OPEN BOUNDARY QUADRIDGE HORN ANTENNAS
Upper Frequency Unit 700MHz to 10GHz OBQH Equal size S-OBQH 400MHz to 10GHz Eliminating the cavity reduces lowest usable frequency
comparison OBQH 36.07cm 36.07cm 36.58cm 5.1kg 700MHz to 10GHz S-OBQH 36.07cm 36.07cm 36.58cm 3.4kg 400MHz to 10GHz
Performance
7-18-2006 53 60MHZ TO 1GHZ SUPER OPEN BOUNDARY QUAD RIDGE HORN FOR RCS MEASURMENT
7-18-2006 54 Low frequency RCS Pulses are used so LPDA not suitable To reduce ringing very good return loss is required, less than 2:1(-10dB return loss) for the range of interest Super Open Boundary Quadridge Horn concept is used to solve the problem.
Lower Frequency Unit 60MHz to 1GHz SOBQH Size 3.65m x3.65m x 3.35m 500kg Compare with the 700MHz to 10GHz OBQH 36cm by 36cm by 36.5cm 5kg 3.65m 12ft 3.35m 11ft
Factory Test:Transmission measurement 7-18-2006 56
Factory VSWR measurement 7-18-2006 57
X-port coupling
Installed in a chamber
Measured data on site during installation and tuning The horn is reassembled in-situ Horizontal port VSWR before and after improvement Vertical port VSWR Before and after improvement Freq S11 h s11 V 66MHz -13.5-20.5 676MHz -10.4-11.1 950MHz -10.5-10.6 1GHz -12.5-12.4
7-18-2006 61 SUPER OPEN BOUNDARY QUADRIDGE HORNS NESTED IN ABSORBER FIELDS
Absorber effects This antenna is designed to be used either in open area test sites or in chambers In chambers it must operate in close proximity to the absorber Model of slightly customized model for lower VSWR
Absorber effects on boresight gain Using absorber complex permittivity values (assuming constant across range)
Absorber effects on VSWR Using absorber complex permittivity values (assuming constant across range)
Conclusions The Open Boundary Quadridge horn concept is extended to lower frequencies While electrical scaling is a simple approach mechanically it can be a nightmare A totally open horn is introduced where not only the flare of the horn is open but the feed cavity is also left open The concept is easier to scale mechanically to lower frequencies
Conclusions The antenna performance is very similar regardless of the type of use Free space Shield mounted Shield mounted with absorber Additionally the absorber permittivity changes make little difference.
Follow-up Information Vince Rodriguez, PhD / vince.rodriguez@ets-lindgren.com www.ets-lindgren.com/event-argentina www.ets-lindgren.com/event-brazil 67