1. Agilent Technologies
Free Space Materials Characterization Shelley Blasdel Begley Application Development Engineer
Abstract
Free-Space Materials Characterization: Free-space techniques using a network analyzer for measuring dielectric and magnetic properties of materials in the microwave to mm-wave frequency range will be discussed. These measurements are often used for design and test of radome, low observables, and absorbing materials, but this non-contacting technique is useful for other materials as well. A featured topic will be Gated Reflect Line calibration, an innovative calibration technique developed by Agilent. System hardware and setup considerations will also be covered.
Bio
Shelley Begley, Technical Marketing Engineer Shelley Begley has over 21 years experience in the HP/Agilent network analyzer business, working in production engineering, electrical metrology, and product marketing. She currently leads a small team of both R&D and marketing, focused on advancing dielectric measurement techniques. She has given countless papers and seminars around the world on the topic.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

2. Agilent Technologies Agenda What is it? Free Space Measurement Systems
Calibration
Measurement Results
Abstract
Free-Space Materials Characterization: Free-space techniques using a network analyzer for measuring dielectric and magnetic properties of materials in the microwave to mm-wave frequency range will be discussed. These measurements are often used for design and test of radome, low observables, and absorbing materials, but this non-contacting technique is useful for other materials as well. A featured topic will be Gated Reflect Line calibration, an innovative calibration technique developed by Agilent. System hardware and setup considerations will also be covered.
Bio
Shelley Begley, Technical Marketing Engineer Shelley Begley has over 21 years experience in the HP/Agilent network analyzer business, working in production engineering, electrical metrology, and product marketing. She currently leads a small team of both R&D and marketing, focused on advancing dielectric measurement techniques. She has given countless papers and seminars around the world on the topic.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

3. What is Free Space Materials Characterization?
Measuring Electromagnetic Properties of Materials
Using Free Space Measurement Technique.
Important for:
Circuit design
Military Applications
Car Radar Applications
New Materials Research
Incoming Inspection
Quality Assurance
Health and Safety
and more…
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

4. Electromagnetic Materials
Radar Absorbing and Stealth Materials
Radome Materials
Electronic Substrate and Packaging Materials
Specific Absorption Rate (SAR) Phantoms
Here’s a look at what materials customers are interested in measuring.
Radar Absorbing and Stealth Materials
Used to control Radar reflections in Antennae applications, such as measurements in an anechoic chamber, or coatings make things like airplanes invisible to enemy Radar
Flexible or rigid sheets, foam, extrusions, or coatings such as paint.
Radome Materials
Used to protect antennae but still pass signal. Can be on a ground based antenna, an aircraft, mobile phones, cars…
Flexible or rigid sheets, solid blocks, and custom shapes.
Electronic Substrate and Packaging Materials
Used for electronics, printed circuit antennae (PCA), etc…
Can be layered or laminate materials, ceramics, ptfe based materials.
Specific Absorption Rate (SAR) Phantoms
Used to test how much electromagnetic energy gets into humans. Typically from consumer telecommunications products, there is also work be done in miltary, car radar applications.
Usually Gel or thick goop.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

5. Transmission Free-Space System
Computer
(not required for PNA)
Network Analyzer
GP-IB or LAN
85071E Materials Measurement Software
With Option 100
Free Space Calibration
The free-space technique is just a variation of the transmission line technique. A typical free-space system consists of a vector network analyzer, two antennae facing each other with a sample holder between them. Again, Agilent provides software that converts the S-parameter output of the network analyzer to dielectric properties.
The free-space configuration overcomes some of the difficulties of the trying to fit samples into transmission line sample holders. And, because the sample is fixtured in space, it is isolated from the other hardware in the system. This can be very useful in a variety of situations.
Sample holder
fixtured between two antennae
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

6. Lightwave Analogy Lens Incident
First a brief look at how a network analyzer works. One of the most fundamental concepts of network analysis involves incident, reflected and transmitted waves traveling along transmission lines. It is helpful to think of traveling waves along a transmission line in terms of a lightwave analogy. We can imagine incident light striking some optical component like a clear lens. Some of the light is reflected off the surface of the lens, but most of the light continues on through the lens. If the lens were made of some lossy material, then a portion of the light could be absorbed within the lens. If the lens had mirrored surfaces, then most of the light would be reflected and little or none would be transmitted through the lens. This concept is valid for RF signals as well, except the electromagnetic energy is in the RF range instead of the optical range, and our components and circuits can be electrical devices, networks and even materials, instead of lenses and mirrors.
Network analysis is concerned with the accurate measurement of the ratios of the reflected signal to the incident signal, and the transmitted signal to the incident signal. Ratioed measurements reduce errors caused by imperfections in the source. Errors caused by differences in these signal paths or any leakage signals within the network analyzer can be calibrated out by the user before measurements are made.. Calibration is typically a simple procedure where three known standards are measured. The coaxial probe and transmission techniques rely on this calibration for accurate measurements.
Bio-dielectrics: Theories, Mechanisms, Applications, Stamford Hall, Leicester, UK

7. Lightwave Analogy Lens Incident Reflected
First a brief look at how a network analyzer works. One of the most fundamental concepts of network analysis involves incident, reflected and transmitted waves traveling along transmission lines. It is helpful to think of traveling waves along a transmission line in terms of a lightwave analogy. We can imagine incident light striking some optical component like a clear lens. Some of the light is reflected off the surface of the lens, but most of the light continues on through the lens. If the lens were made of some lossy material, then a portion of the light could be absorbed within the lens. If the lens had mirrored surfaces, then most of the light would be reflected and little or none would be transmitted through the lens. This concept is valid for RF signals as well, except the electromagnetic energy is in the RF range instead of the optical range, and our components and circuits can be electrical devices, networks and even materials, instead of lenses and mirrors.
Network analysis is concerned with the accurate measurement of the ratios of the reflected signal to the incident signal, and the transmitted signal to the incident signal. Ratioed measurements reduce errors caused by imperfections in the source. Errors caused by differences in these signal paths or any leakage signals within the network analyzer can be calibrated out by the user before measurements are made.. Calibration is typically a simple procedure where three known standards are measured. The coaxial probe and transmission techniques rely on this calibration for accurate measurements.
Bio-dielectrics: Theories, Mechanisms, Applications, Stamford Hall, Leicester, UK

8. Lightwave Analogy Lens Incident Reflected
First a brief look at how a network analyzer works. One of the most fundamental concepts of network analysis involves incident, reflected and transmitted waves traveling along transmission lines. It is helpful to think of traveling waves along a transmission line in terms of a lightwave analogy. We can imagine incident light striking some optical component like a clear lens. Some of the light is reflected off the surface of the lens, but most of the light continues on through the lens. If the lens were made of some lossy material, then a portion of the light could be absorbed within the lens. If the lens had mirrored surfaces, then most of the light would be reflected and little or none would be transmitted through the lens. This concept is valid for RF signals as well, except the electromagnetic energy is in the RF range instead of the optical range, and our components and circuits can be electrical devices, networks and even materials, instead of lenses and mirrors.
Network analysis is concerned with the accurate measurement of the ratios of the reflected signal to the incident signal, and the transmitted signal to the incident signal. Ratioed measurements reduce errors caused by imperfections in the source. Errors caused by differences in these signal paths or any leakage signals within the network analyzer can be calibrated out by the user before measurements are made.. Calibration is typically a simple procedure where three known standards are measured. The coaxial probe and transmission techniques rely on this calibration for accurate measurements.
Bio-dielectrics: Theories, Mechanisms, Applications, Stamford Hall, Leicester, UK

9. Lightwave Analogy Lens Incident Transmitted Reflected
First a brief look at how a network analyzer works. One of the most fundamental concepts of network analysis involves incident, reflected and transmitted waves traveling along transmission lines. It is helpful to think of traveling waves along a transmission line in terms of a lightwave analogy. We can imagine incident light striking some optical component like a clear lens. Some of the light is reflected off the surface of the lens, but most of the light continues on through the lens. If the lens were made of some lossy material, then a portion of the light could be absorbed within the lens. If the lens had mirrored surfaces, then most of the light would be reflected and little or none would be transmitted through the lens. This concept is valid for RF signals as well, except the electromagnetic energy is in the RF range instead of the optical range, and our components and circuits can be electrical devices, networks and even materials, instead of lenses and mirrors.
Network analysis is concerned with the accurate measurement of the ratios of the reflected signal to the incident signal, and the transmitted signal to the incident signal. Ratioed measurements reduce errors caused by imperfections in the source. Errors caused by differences in these signal paths or any leakage signals within the network analyzer can be calibrated out by the user before measurements are made.. Calibration is typically a simple procedure where three known standards are measured. The coaxial probe and transmission techniques rely on this calibration for accurate measurements.
Bio-dielectrics: Theories, Mechanisms, Applications, Stamford Hall, Leicester, UK

10. Network Analyzer Block Diagram
Fixture
Incident
Transmitted
MUT
Reflected
SOURCE
Here is a generalized block diagram of a network analyzer, showing the major signal-processing sections. In order to measure the incident, reflected and transmitted signal, four sections are required:
Microwave signal source for stimulus
Signal-separation devices
Receivers that down convert and detect the signals
Processor/display for calculating and reviewing the results
A reflection measurement is the ratio of the Reflected signal detected at A, over the Incident signal detected at R. A transmission measurement is the ratio of the Transmitted signal detected at B, over the Incident signal detected at R. Ratioed measurements reduce errors caused by imperfections in the source. Errors caused by differences in these signal paths or any leakage signals within the network analyzer can be calibrated out by the user before measurements are made.. Calibration is typically a simple procedure where three known standards are measured.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

11. Network Analyzer Block Diagram
Fixture
Incident
Transmitted
MUT
Reflected
SOURCE
SIGNAL
SEPARATION
Here is a generalized block diagram of a network analyzer, showing the major signal-processing sections. In order to measure the incident, reflected and transmitted signal, four sections are required:
Microwave signal source for stimulus
Signal-separation devices
Receivers that down convert and detect the signals
Processor/display for calculating and reviewing the results
A reflection measurement is the ratio of the Reflected signal detected at A, over the Incident signal detected at R. A transmission measurement is the ratio of the Transmitted signal detected at B, over the Incident signal detected at R. Ratioed measurements reduce errors caused by imperfections in the source. Errors caused by differences in these signal paths or any leakage signals within the network analyzer can be calibrated out by the user before measurements are made.. Calibration is typically a simple procedure where three known standards are measured.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

12. Network Analyzer Block Diagram
Fixture
Incident
Transmitted
MUT
Reflected
SOURCE
SIGNAL
SEPARATION
INCIDENT (R)
REFLECTED
(A)
Here is a generalized block diagram of a network analyzer, showing the major signal-processing sections. In order to measure the incident, reflected and transmitted signal, four sections are required:
Microwave signal source for stimulus
Signal-separation devices
Receivers that down convert and detect the signals
Processor/display for calculating and reviewing the results
A reflection measurement is the ratio of the Reflected signal detected at A, over the Incident signal detected at R. A transmission measurement is the ratio of the Transmitted signal detected at B, over the Incident signal detected at R. Ratioed measurements reduce errors caused by imperfections in the source. Errors caused by differences in these signal paths or any leakage signals within the network analyzer can be calibrated out by the user before measurements are made.. Calibration is typically a simple procedure where three known standards are measured.
RECEIVER / DETECTOR
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

13. Network Analyzer Block Diagram
Fixture
Incident
Transmitted
MUT
Reflected
SOURCE
SIGNAL
SEPARATION
INCIDENT (R)
REFLECTED
(A)
TRANSMITTED
(B)
Here is a generalized block diagram of a network analyzer, showing the major signal-processing sections. In order to measure the incident, reflected and transmitted signal, four sections are required:
Microwave signal source for stimulus
Signal-separation devices
Receivers that down convert and detect the signals
Processor/display for calculating and reviewing the results
A reflection measurement is the ratio of the Reflected signal detected at A, over the Incident signal detected at R. A transmission measurement is the ratio of the Transmitted signal detected at B, over the Incident signal detected at R. Ratioed measurements reduce errors caused by imperfections in the source. Errors caused by differences in these signal paths or any leakage signals within the network analyzer can be calibrated out by the user before measurements are made.. Calibration is typically a simple procedure where three known standards are measured.
RECEIVER / DETECTOR
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

14. Network Analyzer Block Diagram
RECEIVER / DETECTOR
PROCESSOR / DISPLAY
REFLECTED
(A)
TRANSMITTED
(B)
INCIDENT (R)
SIGNAL
SEPARATION
SOURCE
Incident
Reflected
Transmitted
MUT
Fixture
Here is a generalized block diagram of a network analyzer, showing the major signal-processing sections. In order to measure the incident, reflected and transmitted signal, four sections are required:
Microwave signal source for stimulus
Signal-separation devices
Receivers that down convert and detect the signals
Processor/display for calculating and reviewing the results
A reflection measurement is the ratio of the Reflected signal detected at A, over the Incident signal detected at R. A transmission measurement is the ratio of the Transmitted signal detected at B, over the Incident signal detected at R. Ratioed measurements reduce errors caused by imperfections in the source. Errors caused by differences in these signal paths or any leakage signals within the network analyzer can be calibrated out by the user before measurements are made.. Calibration is typically a simple procedure where three known standards are measured.
Reflected
Transmitted
= S11
= S21
Incident
Incident
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

15. Transmission Algorithms
Measured S-parameters
Output
Nicolson-Ross
S11,S21,S12,S22
er and mr
Precision (NIST) er
Fast
S21,S12
These are the algorithms available in agilent software to calcualte permittivity or permeability. Each has it’s optimum uses.
The first algorithm is the Nicholson-Ross model. It utilizes all four S-parameters to calculate permittivity as well as permeability. So this is the model to use if you have magnetic materials. At ½ wavelength the math suffers blow ups. To avoid this the sample should be kept smaller than ½ wavelength, optimally as short as one quarter lamda.
The next two models assume that the material is non-magnetic and only calculate permittivity, not permeability.
The second algoritm, the NIST Precision model, uses full S-parameter data to calculate permittivity. It does not have the math blowups at ½ wavelength. The optimal length is some number of half wave length, so low loss samples can be made longer to get better sensitivity.
The third algorithm is the Fast Transmission model, which is an Agilent proprietary model. Because it only uses transmission confidents, effective directivity errors in the network analyzer do not effect it as much as the other models and it will often converge when the others run into problems.
(85071E also has three reflection algorithms)

16. Before a Measurement Can be Made…
System errors must be corrected for before a measurement is made.
Calibration is required!
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

17. Hard to get broadband absorbers for match
TRM Calibration
Thru
Reflect
Match
One widely use technique has been TRM. This technique uses a thru, reflect standard and matched load standards to calculate the error coefficients. One difficulty is finding a broad band absorbing material for the match standard. Imperfections in the match standard cause residual errors after calibration.
Hard to get broadband absorbers for match
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

18. TRL Calibration Thru Reflect Line
Move the antenna away to compensate for the thickness of the short. Move it back for the next step.
Move the antenna away on a quarter-wavelength and then back in the original position.
A second widely used technique is TRL. TRL uses a thru, reflect and line standard to determine the error coefficients. However, to measure the reflect, the port two antenna must be moved back by the thickness of the metal plate. The line standard is then realized by precisely moving the port two antenna on quarter wavelength. After the calibration the antenna needs to be precisely moved back to its original position. In order to do this accurately enough to get a good calibration, expensive positioning fixturing is required.
For both TRM and TRL calibration the main problem is the third standard (Match or Line). The GRL calibration avoids using the third standard as it will be explained next.
Precise positioning fixtures are expensive
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

19. Gated Reflect Line (GRL) Calibration Two Tiered Process
Two port calibration at waveguide or coax input into antennas removes errors associated with network analyzer and cables. 1.
GRL calibration is a two tiered calibration process. First, we remove the antennae and calibrate the network analyzer using any 2-port method.
ECal, SOLT or TRL Cal done here
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

20. Gated Reflect Line (GRL) Calibration Two Tiered Process
Two additional free space calibration standards remove errors from antennas and fixture. 2.
Reflect
(metal plate of
known thickness)
Line
(empty fixture)
Then, we attached the antennae and sample and sample holder, turn on the calibration done in step one and measure two additional standards, the empty fixture, and a metal plate of known thickness. When the calibration is complete the empty fixture measures as a slice of air the thickness of the metal plate.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

21. GRL Cal Error Model (forward only)
GRL Cal – How it works
GRL Cal Error Model (forward only)
2-port Cal Terms
MUT
2-port Cal Terms 1
S21 Tt
GRL Error Adapter
GRL Error Adapter D Ms
S11
S22 Ml Tr
S12
Coax or Waveguide 2-port Cal corrects errors from end of cable back into the instrument.
Gated Reflect Line Calibration
Looking at a error model of our system before calibration. We have the two port error model, plus the free space fixture, with its antennae and sample holder, that can be thought of as two error adapters between the end of the cables and the space that our Material Under Test (MUT) will occupy.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

22. GRL Cal Error Model (forward only)
GRL Cal – How it works
GRL Cal Error Model (forward only)
2-port Cal Terms
MUT
2-port Cal Terms 1
S21 Tt
GRL Error Adapter
GRL Error Adapter D Ms
S11
S22 Ml Tr
S12
Coax or Waveguide 2-port Cal corrects errors from end of cable back into the instrument.
Gated Reflect Line Calibration
Looking at a error model of our system before calibration. We have the two port error model, plus the free space fixture, with its antennae and sample holder, that can be thought of as two error adapters between the end of the cables and the space that our Material Under Test (MUT) will occupy.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

23. GRL Cal Error Model (forward only)
GRL Cal – How it works
GRL Cal Error Model (forward only)
2-port Cal Terms
MUT
2-port Cal Terms 1
S21 Tt
GRL Error Adapter
GRL Error Adapter D Ms
S11
S22 Ml Tr
S12
Coax or Waveguide 2-port Cal corrects errors from end of cable back into the instrument.
Gated Reflect Line Calibration
Looking at a error model of our system before calibration. We have the two port error model, plus the free space fixture, with its antennae and sample holder, that can be thought of as two error adapters between the end of the cables and the space that our Material Under Test (MUT) will occupy.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

24. GRL Cal Error Model (forward only)
GRL Cal – How it works
GRL Cal Error Model (forward only)
2-port Cal Terms
MUT
2-port Cal Terms 1
S21 Tt
GRL Error Adapter
GRL Error Adapter D Ms
S11
S22 Ml Tr
S12
Coax or Waveguide 2-port Cal corrects errors from end of cable back into the instrument.
Errors from Antennas and Fixture can be thought of as being lumped into a GRL error adapter.
The GRL error adapter is quantified by measurements of reflect and line standards.
Gated Reflect Line Calibration
Looking at a error model of our system before calibration. We have the two port error model, plus the free space fixture, with its antennae and sample holder, that can be thought of as two error adapters between the end of the cables and the space that our Material Under Test (MUT) will occupy.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

25. GRL Cal – How it works Time Domain of Empty Free Space Fixture gate
Transmitting Antenna
Above is a time domain S11 graph of the measured fixture when the fixture is empty
Note that you can identify the various responses as the reflection associated to the transmitting antenna, followed by the low reflection of air and then the reflection associated with the receiving antenna and its associated supporting structure. The GRL places time domain gates around the responses associated with the transmitting antenna. The frequency domain of this gated measurement is O11 of the port one error adapter. The same approach can be used to determine T11 of the port 2 error adapter.. These terms can be then embedded into the original 2-port calibration.
Using this new calibration set the other terms of the error adapters can be calculated from the s-parameter measurements of the Thru (empty fixture or air) and the Reflect (metal plate) standards.
Receiving Antenna
Air
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

26. GRL Cal Error Model (forward only)
GRL Cal – How it works
GRL Cal Error Model (forward only)
2-port Cal Terms
MUT
2-port Cal Terms 1
S21 Tt
GRL Error Adapter
GRL Error Adapter D Ms
S11
S22 Ml Tr
S12
Coax or Waveguide 2-port Cal corrects errors from end of cable back into the instrument.
Errors from Antennas and Fixture can be thought of as being lumped into a GRL error adapter.
The GRL error adapter is quantified by measurements of reflect and line standards.
The original 2-port Cal is modified to correct for the error adapter.
Gated Reflect Line Calibration
Looking at a error model of our system before calibration. We have the two port error model, plus the free space fixture, with its antennae and sample holder, that can be thought of as two error adapters between the end of the cables and the space that our Material Under Test (MUT) will occupy.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

27. GRL Cal Error Model (forward only)
GRL Cal – How it works
GRL Cal Error Model (forward only)
2-port Cal Terms
MUT
2-port Cal Terms 1
S21 Tt
GRL Error Adapter
GRL Error Adapter D Ms
S11
S22 Ml Tr
S12
Coax or Waveguide 2-port Cal corrects errors from end of cable back into the instrument.
Errors from Antennas and Fixture can be thought of as being lumped into a GRL error adapter.
The GRL error adapter is quantified by measurements of reflect and line standards.
The original 2-port Cal is modified to correct for the error adapter.
Gated Reflect Line Calibration
Looking at a error model of our system before calibration. We have the two port error model, plus the free space fixture, with its antennae and sample holder, that can be thought of as two error adapters between the end of the cables and the space that our Material Under Test (MUT) will occupy.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

28. Free-space X-Band System
Here is a picture of a very simple and inexpensive X-band, 8.2 to 12.4GHz, free space system. The fixture is made from a shelving unit purchased at an office supply store. The antennae are held in place on the top and bottom shelf. The middle shelf had a hole cut out to serve as a sample holder. New calibration techniques we will discuss later correct for the reflections of the fixture, antenna and surrounding area, and fairly reasonable results were obtained. Care had to be taken, however, not to bump or move the system after calibration, as it was not very rigid.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

29. Free Space 40 – 60GHz System Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

30. A Simple Free Space Powder Fixture
Powders are also easily fixtured between the antennae. These pictures show a simple freespace powder fixture used in the cornmeal moisture measurement. It is basically a tray made out of expanded polystyrene. The structure holding the antennae in this case is a commercially available shelving unit. The horns are held vertically so that gravity holds the powder in place.
It is easy to imagine a more industrial version, with perhaps a conveyor belt moving the powder between the antennae. Since this method is non-contacting, it is perfect for remote sensing.
Close Up
Powder in Tray Placed Between Antennae
Bio-dielectrics: Theories, Mechanisms, Applications, Stamford Hall, Leicester, UK

31. Free Space 75 – 110GHz Standard Gain Horn System
Here’s a look at a W-band, GHz, setup using standard gain horns. The horns are from Custom Microwave in Colorado for around $ The sample holder was made in house. Fairly good results can be obtained with this system.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

32. Free Space W-band System
Here is a closer look at the horns and sample holder. Obviously, more robust fixturing can be made for industrial use, but even with these easily made, simple fixtures, reasonable results were obtained, which you will see later in this presentation.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

33. Free Space 75-110GHz Quasi-Optical System
Here’s a look at another W-band setup, this time using mirrors to reshape and redirect the beam. More accurate results for e’ can be achieved.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

34. Free Space 75-110GHz Quasi-Optical System
Here’s a closer look at the precision antennae and mirrors.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

35. Free Space 75-110GHz Quasi-Optical System
Here’s a closer look at the precision antennae and mirrors.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

36. Free Space 75-110GHz Quasi-Optical System
Here’s a closer look at the precision antennae and mirrors.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

37. Free Space 75-110GHz Quasi-Optical System
Here’s a closer look at the precision antennae and mirrors.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

38. Free Space 75-110GHz Quasi-Optical System
Here’s a closer look at the precision antennae and mirrors.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

39. Measurement Results
Here is a comparison of the results of the standard gain horn and Thomas Keating Ltd quasi optical setups for the real part of permittivity. The standard gain horn is the dark blue trace and the Thomas Keating Ltd quasi optical setup is the lighter pink trace. The black line is a linear fit to that trace. The ripple on the traces are due to measurement error. The quasi optical table improves the result.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

40. Measurement Results
Here is a comparison of the results of the standard gain horn and Thomas Keating Ltd quasi optical setups for the imaginary pat o permittivity. The standard gain horn is the dark blue trace and the Thomas Keating Ltd quasi optical setup is the lighter pink trace. The ripple on the traces are due to measurement error. The quasi optical table improves the result.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

41. Conclusion
Free Space is a useful method for measuring electromagnetic properties of a of materials.
Innovative calibration overcomes challenges of previous free space error correction methods.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

42. For More Information
Visit our website at:
Thank you for your attention. I hope that the information presented was useful. For more information, please visit our website at There you will find more detailed information about the techniques presented today.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

43. References
R N Clarke (Ed.), “A Guide to the Characterisation of DielectricMaterials at RF and Microwave Frequencies,” Published by The Institute of Measurement & Control (UK) & NPL, 2003
J. Baker-Jarvis, M.D. Janezic, R.F. Riddle, R.T. Johnk, P. Kabos, C. Holloway, R.G. Geyer, C.A. Grosvenor, “Measuring the Permittivity and Permeability of Lossy Materials: Solids, Liquids, Metals, Building Materials, and Negative-Index Materials,” NIST Technical Note
“Test methods for complex permittivity (Dielectric Constant) of solid electrical insulating materials at microwave frequencies and temperatures to 1650°, ” ASTM Standard D2520, American Society for Testing and Materials
Janezic M. and Baker-Jarvis J., “Full-wave Analysis of a Split-Cylinder Resonator for Nondestructive Permittivity Measurements,” IEEE Transactions on Microwave Theory and Techniques vol. 47, no. 10, Oct 1999, pg
J. Krupka , A.P. Gregory, O.C. Rochard, R.N. Clarke, B. Riddle, J. Baker-Jarvis, “Uncertainty of Complex Permittivity Measurement by Split-Post Dielectric Resonator Techniques,” Journal of the European Ceramic Society
No. 10, 2001, pg
“Basics of Measureing the Dielectric Properties of Materials”. Agilent application note EN, April 28, 2005
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

44. mm-Wave Free Space Results
Here is some measurement results using the sstandard gain horn W-band system.
A rexolite sample is used as a verification standard because it has well known dielectric properties into the hundreds of GHz. Published value for rexolite epsilon prime is Results over the Ghz range were within 1 percent.
Once the system was measured, the MUT or in this case Acrylic sheet was measured. Actual value of this sample was unknown, but expected results are somewhere between 2.4 to 2.8.
Rexolite expected value
e’ = 2.53
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

45. Apendix • • • • Section Title Agilent Technical Forum
• • • •
Section Title
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

46. GRL Cal – System Considerations
Determine Sample Position
Determine Sample Size
Choose Metal Plate
Fixture with Metal Plate
There are several practical considerations when setting up a system to performing a GRL cal.
Distance to Sample
The distance from the antenna to the sample can be determined by looking at the time domain response of the metal plate measurement. There should be a low reflection span of time between the last response of the antenna and the metal plate.
Sample Size
The size of the sample and the distance the sample needs to be from the antenna can be determined by analyzing your fixture using time domain. The antennas used determine the minimum sample size. First locate the position of the metal plate in time. When the plate is removed, the difference in the response should be large. You should not be able to see the fixture's supporting structure. At least 50 dB. The greater the better. The sample height and width should be large enough that the beam fits inside. This can be easily checked by sliding the metal plate in from the side and seeing when it starts to show in time domain.
Metal Plate
Another consideration is the selection of the metal plate. Ideally the metal plate should approximately the same thickness as the sample to be measured. This puts the measurement reference planes at the surface of the sample. While this is ideal, any thickness differences are accounted mathematically by the sample holder description entries of the 85071E.
Metal Plate
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Free Space Materials Characterization
15 October 2008

47. GRL Cal - Considerations
Empty Fixture
Choose Number of points to Avoid Aliasing
Minumum Number of Points =
1 + Range * (Stop Frequency – Start Frequency)
Where Range is the needed alias free range in Seconds
Receiving Antenna
Transmitting Antenna
Number of measurement points.
Time domain is used to isolate the reflections off each of the two antennas. Because of this, aliasing must be considered. The alias-free range can be calculated as shown.
Range= (number of points-1)/ (stop frequency - start frequency)
It is important that the alias-free range be greater than the length of the freespace fixture. This includes all paths until the signal is attenuated to an insignificant level. Below is a plot of S11 of the empty fixture with a calibration coaxial/waveguide calibration turned on.
In the plot above, the first series of reflections, after t=0, is the reflections associated with the antenna. Next we see the reflections off the receiving antenna and the supporting structure. The response then reduces to approximately zero at about 20 nsec. Since the measurement was made over x-band ( GHz) the minimum number of points can be calculated as:
Number of points= 1 + Range*(stop frequency - start frequency)=1 + 20e-9*(12.4 e e9) = 85
Based on the required range the minimum number of points requires is 85. In general the more points the better to insure that as little aliasing occurs as possible. The same analysis should be performed on S21.
20nS
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Free Space Materials Characterization
15 October 2008

48. GRL Cal - Considerations
Choose Time Domain Parameters
Empty Fixture
Fixture with Metal Plate
Time domain parameters.
After the initial 2-port calibration is performed, the location of the metal plate in time must be determined. Above is a plot of the empty fixture and a plot of the fixture with the metal plate.
By comparing the two plots, the time position of the plate fall between 2 and 6 nsec. The exact position is not important as long as you specify times where no other reflection has a larger amplitude than the reflection off the plate. This can occur when the reflection off the antenna is larger than the plate.
The final parameters to set are the gate shape and the gate span. These parameters are used during the gated response isolation portion of the calibration. This calibration is separate from the GRL cal and is used to reduce any residual errors. The GRL corrected measurements of the empty fixture and the fixture with the metal plate are gated and used as a separate response /isolation calibration. These terms are applied in the software. The gate span should be set wide enough to include the entire response of the reflection off the plate or the transmission of the empty fixture but narrow enough to minimize the unwanted responses.
Air at 3.5nS
Metal Plate at 3.5nS
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Free Space Materials Characterization
15 October 2008

49. Quasi Optical System Schematic
Additional information available at :
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008

50. MUT and GRL Error Adapters After 2-Port Calibration
Six Unknowns
After the 1st Tier calibration is turned on, the signal flow graph shows just the GRL adapters and a MUT. The Oxx parameters refer to port 1 and the Txx refers to port 2. The goal is to determine all the Oxxs and Txxs and embeding them into the Tier 1 calibration. Each of the error adapters can be modeled by their four s-parameters. Because of the passive nature of these error adapters O21=O12 and T21 = T12. This leaves six unknowns.
021 = O12
O11
O22
T21 = T12
T11
T22
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Free Space Materials Characterization
15 October 2008

51. MUT and GRL Error Adapters After O11 and T11 are embedded into the original 2-Port calibration.
Four Unknowns
Once O11 and T11 are embedded into the original 2-port calibration, the signal flow graph looks like this, with four remaining terms in the error adapter. They will be removed by measuring two additional standards, the Thru and Reflect.
O21 = O12
O22
T21 = T12
T22
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Free Space Materials Characterization
15 October 2008

52. GRL Metal Plate Standard
P11 = P22 = -1
P21=P12=0.
MUT
S11
S22
S21
S12
T22
T12
T21
O22
O21
O12
The plate standard is the metal plate of know thickness. In a perfect freespace system, the metal plate will reflect all energy back, so P11 and P22 are set to minus one. Since all energy is reflected, no energy passes through, so P12 and P21 are set to zero.
Using Mason’s rule, the S-parameters of the port one and two plate standards are established.
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Free Space Materials Characterization
15 October 2008

53. GRL Thru Standard (Air)
A11=A22=0
A21 = A12 =
= frequency
= permittivity of air
= permeability of air.
d= thickness of the metal plate
MUT
S11
S22
S21
S12
T22
T12
T21
O22
O21
O12
The Thru standard is the empty fixture or air and designated here by A. Since in a perfect free space system, S11 or air, or A11 here, would see no reflection back, A11 and A22 are set to zero.
A21 and A12 are equal to the expression above.
Once again, using Mason’s loop rule, we can establish expressions for the S-parameters of the port one and two air standards. With these four equations, the two here and the on the previous slide, the four remaining coefficients of the error adapters can be solved for. These are then embedded into the original calibration. The result is a full two-port calibration with the reference planes at the surface of the metal plate.
The two equations on the previous slide and these equations are used to solve for the remaining coefficients of the error adapters. These terms are then embedded into the original calibration. The result is a full two-port calibration with the reference planes at the surface of the metal plate.
Agilent Technical Forum
Free Space Materials Characterization
15 October 2008