Near-Field
Links Transition from Free-Space
See also: Matt Reynolds, Low frequency indoor radiopositioning, PhD Thesis, MIT, February 2003
Loop Link Law
Link depends on loaded Q and size:
See: Umar Azad, Hengzhen Crystal Jing, Yuanxun Ethan Wang, “Link budget and Capacity Performance of Inductively Coupled Resonant Loops,” IEEE Transactions on Antennas and Propagation, Vol. 60, No. 5, May 2012, pp. 2453-2461.
Near-Field
Links in Practice
Example:
Q-Limits & Gain
Gain Relation:
Wavelength > Range
- Slow monotonic variation in signal properties
- Robust link that "ignores" most obstructions
- Easier gain measurement - Q and size
- Ideal free space links frequency independent:
Choice of Frequency depends on:
- Noise Environment
- Near-Field Range (1-3 times radiansphere)
Received Power (dBm)
Near-
Field Links
References:
- J. S. McLean, “A re-examination of the fundamental limits on the radiation Q of electrically small antennas,” IEEE Transactions on Antennas and Propagation, Vol. 44, No. 5, pp. 672–676, May 1996.
- David M. Pozar, “New Results for Minimum Q, Maximum Gain, and Polarization Properties of Electrically Small Arbitrary Antennas,” 3rd European Conference on Antennas and Propagation, 2009 (EuCAP 2009), 23-27 March 2009, pp. 1993-1996.
- A.R. Lopez, “Harold A. Wheeler’s Antenna Design Legacy,” IEEE Long Island 2007 Systems, Applications and Technology Conference, 4 May, 2007, pp. 1-6.
- Oleksiy S. Kim, Olav Breinbjerg, and Arthur D. Yaghjian, “Electrically Small Magnetic Dipole Antennas With Quality Factors Approaching the Chu Lower Bound,” IEEE Transactions on Antennas and Propagation, Vol. 58, No. 6, June 2010, pp. 1898-1906. See Eqn. 2b.
- Herbert L. Thal, “New Radiation Q Limits for Spherical Wire Antennas,” IEEE Transactions on Antennas and Propagation, Vol. 54, No. 10, October 2006, pp. 2757-2763. See Eqn. 8b.
Power Law Exponents
- Modest data rate
- Enhanced propagation
- Range up to quarter wavelength (1MHz)
- Robust in cluttered environment
- No drops, no deadzones, no fading
Maxwell & Antenna Theory
- Enhanced propagation
- Range 20-60m
- Near- & Far-Field Links Converge to Inverse r cubed
“An understanding of the mechanism by which energy is radiated from a circuit and the derivation of equations for expressing this radiation involve conceptions which are unfamiliar to the ordinary engineer.”
– Frederick Emmons Terman, 1932
Near-Field Link
Laws: Gain-Aperture
Far-Field Propagation
- W. Honcharenko, “Modeling UHF Radio Propagation in Buildings,” Ph. D. Dissertation, Polytechnic University, Brooklyn, NY, 1993.
- K. Siwiak, H. Bertoni, and S. Yano, “Relation between Multipath and Wave Propagation Attenuation,” Electronic Letters, Vol. 39, No. 1, January 9, 2003, pp. 142-143.
Significant path gain in near field!
"Maxwell’s field theory was believed to be good at all frequencies, but its language and some of its concepts were totally alien to most engineers of that period."
-Sergei Schelkunoff, 1970
"Forty Years Ago: Maxwell’s Theory Invades
Engineering-and Grows with It," IEEE Trans. Ant. Prop. Vol. AP-18, No. 3, May 1970 p. 309-322.
No frequency dependent path loss in far-field
Efficiency
& Gain
Friis' Law
where for a well-matched antenna, unloaded Q is twice loaded Q.
See:
- Kai Siwiak and Yasaman Bahreini, Radiowave Propagation and Antennas for Personal Communications, 3rd ed. Norwood, MA: Artech House, 2007, p. 359-360.
- Constantine A. Balanis, Antenna Theory Analysis and Design, 2nd ed., New York: John Wiley and Sons, Inc., 1997, p. 762.
Antenna Gain
Generalized Small Antenna Gain
Near-Field Link
Laws: Gain-Aperture
Marconi
Gain Constant Depends Upon:
- Electric vs. Magnetic
- Shape/Form Factor
- Loading
Lodge &
Far-Field
Wireless
- Near-Field Gain Measurement
- Measurement Results
- Theory of Small Antenna Gain
Two link laws
- "Like" Antennas
- "Unlike" Antennas
Gain Constant Value:
- C ~ 4 Ferrite Loaded Loopsticks
- C ~ 2 Air Core Loops
- C ~ 1 Theory, Loops
- C ~ 0.5 NEC Loops
Harald Friis, “A Note on a Simple Transmission Formula,” Proc. IRE, 34, 1946, pp. 254-256
Courtesy, Mount Holyoke College Archives and Special Collections
"It is to be observed that the effective source of the emitted radiation is not at the oscillator itself, but at a quarter wave-length in advance of it. If it is to be assumed to start at the oscillator, the light will seem to travel too quickly; and Hertz found this to be the case.”
Oliver Lodge, Modern Views of Electricity, 3rd ed., London: Macmillan & Co, Ltd., 1907, p. 303.
Tesla's Wireless Power Transfer
Guglielmo Marconi
1874-1937
- High Voltage HF
- Inductive Power Transfer
- Wardenclyffe Tower
Near-Field
Link Laws: Gain-Gain
Tesla's Colorado Springs Experiments
Oliver Lodge
1850-1940
Near-Field
Link Laws: Gain-Gain
Two link laws
- "Like" Antennas
- "Unlike" Antennas
Wardenclyffe Tower
Ferrite Antenna Measurements
Hertz Discovers Far-Field Wireless
Transfer Function
Hans Schantz, “Near-field propagation law & a novel fundamental limit to antenna gain versus size,” 2005 IEEE Antennas and Propagation Society International Symposium, Vol. 3A, 3-8 July 2005, pp. 237-240.
Asymmetry in reciprocity:
- Transmitting transfer function is proportional to the time derivative of the receiving transfer function
- Frequency domain:
Power ~ Signal Squared
Transmit power related to receive power by frequency squared:
Frequency dependent "path loss?"
A.J. Compston, J.D. Fluhler, and H.G. Schantz, “A Fundamental Limit on Antenna Gain for Electrically Small Antennas,” 2008 IEEE Sarnoff Symposium, 28-30 April 2008, pp. 1-5.
- Kanda, Motohiso, “Time Domain Sensors for Radiated Impulsive Measurements,” IEEE Transactions on Antennas and Propagation, Vol. AP-31, No. 3, May 1983, pp. 438-444.
- Hans Schantz, “d/dt, jω, Q, and UWB Antennas,” 2nd International Workshop UWB Radio Communications Proceedings, Inha, Korea, June 24, 2005, pp. 102-110.
Overview:
Heinrich Hertz
1857-1894
Gain or
Aperture?
Transmit Physics
Near-Field
Origins of Wireless
Origins of
Near-Field Wireless
Air Core Antenna Measurements
Accelerating charges are not the source of radiation energy
Energy is stored in near-fields before decoupling and radiating away.
High Frequency Wireless
- Gain best describes transmit behavior
- Aperture best describes receive behavior
H. G. Schantz, The Art and Science of UWB Antennas, Boston: Artech, 2005, p. 176.
H. G. Schantz, “Electromagnetic energy around Hertzian dipoles,” IEEE Antennas and Propagation Magazine, Vol. 43, No. 2, April 2001, pp. 50-62.
Near-Field
Gain Measurements
Gain of Antenna Under Test Given Reference:
Preece's
Inductive Wireless Telegraphy
- VHF
- UHF
- Microwaves
- Radar
- Cellular Communications
- Millimeter Wave
- "Electrically Small" kR ~ 1
Three Antenna Gain Measurement:
- Railway Telegraphy
- Inductive Wireless
- Discovery of Far-Field Wireless
- Re-discovery of Near-Field Wireless
References:
- E. Richards, H. Schantz, J. Unden, K. von Laven, A.J. Compston, and C. Weil, “Electrically Small Antenna Design for Low Frequency Systems,” 34th Annual Antenna Applications Symposium, 21-23 September, 2010.
- IEEE Std. 149-1979, p. 96.
Ghirardi, Alfred A., Radio Physics Course, New York: Farrar & Rinehart, Inc., 1930, p. 330
Motivation: Explore how electrically small antennas( ) work in near-field links.
Specifically:
- Near-Field Links
- Electrically Small Antenna Gain
- Regulatory Near Fields
- NEC Validation
Near-Field
Gain Measurements
William Henry Preece
1834-1913
UWB Antennas
Consider UWB antennas:
- A constant gain antenna transmits a faithful copy of a voltage signal.
- A constant aperture antenna receives a faithful copy of an incident signal.
See: J.J. Fahie, A History of Wireless Telegraphy, 2nd ed. , Dodd Mead & Company, New York, 1901, pp. 135-161.
Comparison
of Results
Evaluation of Magnetic & Electric Antennas
Edison's
Electrostatic Wireless
Edison-
Guillard-Phelps
Link
Validation
Excellent agreement using simulated gain values
Thomas Edison
(1847-1931)
Fields
from Links
Antenna Factor relates Fields to Signal Voltage:
Solve for Gain:
Substitute into Link Law:
Solve for Electric Field Intensity:
Gain
Validation
Numerical Modeling
Simulated gain results
NEC and FEKO - Method of Moments
- Validate gain relation
- Validate link law
- Validate field relation
6DoF Magnetic Motion Tracking
Near-Field Communications (NFC)
Near-Field Magnetic Induction (NFMI)
Near-Field Wireless Technology
Approach:
- Low Frequency AC or Pulsed DC
Polhemus Founded 1969
- See: http://polhemus.com/
Ascension Technology Corporation Founded 1986
- "Flock of Birds" Magnetic Tracking
- See: http://www.ascension-tech.com/index.php
NFC Forum Founded 2004
Data Rate: 106 kbit/s to 424 kbit/s
Frequency: 13.56 MHz
Range: ~10cm or less
[ISO/IEC 18000-3]
Dr. Hans G. Schantz, CTO
Q-Track Corporation
2223 Drake Avenue SW 1st Floor
Huntsville, AL 35805
h.schantz@q-track.com
Twitter: @aetherczar
- Auracomm founded 1995
- RF 10-15MHz
- Wireless Headset/Voice channel
- Range < 2m
- Acquired by Freelinc in 2007 (www.freelinc.com)
Current &
Voltage in Small Loops
Apply Conservation of Energy:
Impedance of high Q resonant antenna:
Fields
Around Small Antennas
Near-Field
Wireless Applications
Wireless
Power Transfer
- Antenna practice employs EIRP
Transmit Power:
Transmit Gain:
- Electromagnetic theory employs dipole moment:
Turns: N
Current: I
Area: A
m
N
Overview
A
Fields from Dipoles, Moments & EIRP
- WiTricity - MIT Demo (9MHz)
- WiPower
- Powermat Technologies
- Qi - wireless power charging
I
High Frequency/Far-Field Ideal For:
- Long Range
- Large Bandwidth
- High Data Rate
Near-Field Links Ideal For:
- Modest Data Rate
- Short Range
- Robust w.r.t. Blockage, Multipath
Field from dipole theory:
Noting that:
Relate moment to EIRP:
Near Field Intensities
Part One:
- Origins of Near-Field Wireless
- Survey of Applications
- Introduction to the Near Field
Part Two:
- Near-Field Links
- Electrically Small Antennas
- Regulatory Limits
Near-Field Electromagnetic Ranging (NFER)
RuBee
IEEE 1902.1
Radio Frequency Identification (RFID)
Data Rate: 1200 baud
Frequency: 131kHz
Range: <15m
See: http://www.visible-assets.com/
Dipole
Fields from EIRP
Encompasses a wide variety of technologies including:
- 10cm read at 120-150kHz
- 10cm-1m read at 13.56MHz
- Range: 20-60m
- Accuracy: 30cm-1m
- Frequency ~1MHz
- Capacity: ~80 tags
at 1Hz update
(www.q-track.com)
Field intensities may be written in terms of EIRP:
Transverse fields (Longitudinal) 6dB stronger....
Conclusion
Near Field
Impedance
Near Field
Phase Relations
Louis Sullivan
1856-1924
Heinrich Hertz
1857-1894
Hertz: Electric Waves, 1893, p. 152
"Form follows function."
What is
the "Near Field?"
See: Charles Capps, "Near Field or Far Field," EDN, August 16, 2001 pp. 95-102
Near Field
Impedance
Conclusion
High Frequency/Far-Field Ideal For:
- Long Range
- Large Bandwidth
- High Data Rate
Near-Field Links Ideal For:
- Modest Data Rate
- Short Range
- Robust w.r.t. Blockage, Multipath
Understand Electrically Small Antennas
Links, Gain, and Fields
Starting Point for Exploring How Near-Field
Links Solve Real-World Problems
Many Near-
Field Parameters
Low Frequency,
Near-Field Links
- Multipath resistant
- Good penetration
- Bends, diffracts around obstructions
- Robust Links
Superposition & Near-Fields