Antenna Constraints in UWB Applications

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Antenna Constraints in UWB Applications

• Anshul J. Popat
• Stephen S. Mwanje

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AGENDA

• Introduction
• General introduction and classification of
  Antennas
• Design Constraints And Limits
• Practical Antenna Designs

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Introduction to UWB

• Modulation of a narrow pulse signal (picoseconds)
  over the carrier signal resulting in an extremely
  Wideband signal
• FCC has allocated the spectrum from 3.1 GHz to
  10.6 GHz for UWB
• Originally developed for Radar technology, UWB
  has evolved to prove essential in the WPAN and
  WLAN market as a high speed networking solution
  for burst data

Figure 1 Spectrum Over lay of UWB over other
technologies
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Specialty of UWB Antennas

• Broadband antenna design is based on 'frequency
  independent antennas concept, not easy to extend
  to UWB systems for which
• electrical size must be small with high
  efficiency yet the bandwidth is extremely wide
• Antenna pulse distortion must be kept to a
  minimum
• Regulatory requirements dictate strict power
  levels
• Constant radiation pattern through out the band
  of operation is hard to achieve
• Transient effects are no longer negligible as in
  conventional systems in which they are a fraction
  of the symbol time

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Antenna Classification

• Directivity Vs Non Directivity
• High gain or directional antennas concentrate
  energy into a narrower solid angle than an
  omni-directional antenna
• Electric Vs magnetic
• Electric antennas e.g. dipoles and most horns,
  are characterized by intense electric fields
  close to the antenna. They include
• Magnetic antennas e.g. loops and slots, are
  characterized by intense magnetic fields close to
  the antenna

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AGENDA

• Introduction
• Design Constraints And Limits
• Important constraints, considerations and design
  limits of special interest in UWB systems.
• Practical Antenna Designs

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Antenna Characterization

• Single band Vs multi-Narrow band
• Typical UWB antennas in used in the past are
  multi-narrow band
• Instead should be designed to receive a single
  coherent signal with stable pattern and matching
  across the entire operating band
• Dispersive Vs Non Dispersive
• Desire non-dispersive antennas, with a fixed
  phase center. If waveform dispersion occurs in a
  predictable fashion it may be possible to
  compensate for it.
• Desire similar waveforms in all directions.
• A multi-band (OFDM) approach may be considered
  for dispersive antennas.
• Log-periodic antennas are dispersive. By
  contrast, a small element antenna, like a planar
  elliptical dipole tends to radiate a more
  compact, non-dispersive waveform, similar to a
  Gaussian. Thus, small element antennas are
  preferred in many applications.

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Matching, Spectral Control, TF, RL

• In UWB, a good impedance match must be designed
  in from first principles, not added as an
  afterthought.
• Because of the FCC induced regulations, the
  spectra of the pulses radiated by the antenna has
  to be carefully controlled.
• Transfer Function For UWB antennas, the transfer
  function is more important than any of the
  classical antenna parameters. The transfer
  function is the ratio of the output to the input
  and depends on the angular position and the
  frequency of operation (S parameters)
• Return Loss Another important parameter is the
  return loss, which is the ratio of the amplitude
  of the reflected wave to the incident wave.

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Antenna Size and Gain

• We need antennas with small geometrical
  dimensions i.e. the geometrical size is small
  compared to the operating wavelength and can be
  fit into a radiansphere radius of ?/2?.
• Particular consideration should therefore be
  taken in the design of small antennas, as they
  are inefficient by nature and have high quality
  factor.
• Note Electrical size of a small omni-directional
  antenna may be considerably larger than the
  physical area of antenna.
• Antenna gain G is defined in terms of antenna
  aperture A as

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Antenna Size and Gain (Contd)

• fact that electromagnetic energy readily couples
  across the radiansphere range, allows one to
  establish an approximate bound on the possible
  gain from an antenna of a particular physical
  cross-sectional area.
• The maximum possible antenna aperture
  approximately equals the physical aperture plus
  an additional ?/(2p) strip around the periphery
  of the antenna.
• For an antenna with a circular aperture of
  physical radius r, a circular disk of radius R
  r ?/(2p) bounds the antenna aperture. Then from
  Equation 6 one establishes an upper bound to
  antenna gain as a function of physical radius(fig)

Figure 2 (a) Physical aperture and antenna
aperture . (b) Physical aperture and antenna
aperture as a function of physical radius.
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Antenna Size and Bandwidth

• The Chu-Harrington and McLean Limits relate
  quality factor Q (inverse fractional bandwidth)
  of an ideal, perfectly efficient antenna to its
  size denoted by the radius r of the boundary
  sphere.
• The wavelength at either end of an antennas
  operating band may be related to the wavelength
  at the center of the band and the Q factor. Also
  characteristic size of an antennas boundary
  sphere may then be expressed in terms of the
  wavelength at the center frequency (r?C r /
  ?C).
• Differences exist in the predicted results of the
  two limits for non dipole mode (typical for
  element antennas at high frequencies)
• McLeans limit however, (which converges
  asymptotically to r?C 1/(2p)) allows one to
  establish reasonable expectations on antenna
  performance - in the UWB limit, Q ? 0.
• UWB antenna elements preferably span a quarter
  wavelength or so in dimension at their center
  frequency. Miniaturizing antennas further
  requires significant sacrifices in efficiency and
  performance.

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Group Delay

• Group delay, is the measure of a signal
  transition time through a device.
• Due to the short period of UWB signals, the
  impulse response of the antenna is of particular
  interest, because it has the ability to alter or
  shape the transmitted and/or received pulses.
• The antenna is thus analyzed as a filter by means
  of magnitude and phase responses so that we can
  determine the phase linearity of its gain
  response within the frequency band of interest by
  looking at its group delay.
• The group delay variation induced by the
  radiation pattern of the antenna will affect the
  overall receiver system performance, since it can
  bring relatively large timing errors.
• An antenna gain plot without nulls, means a
  linear phase response, hence a constant group
  delay.

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System Performance

• In UWB, Friiss Law must be interpreted in terms
  of spectral power density and one must integrate
  over frequency to find the EIRP and the total
  received power.
• GTX(f) must be the peak gain of the antenna in
  any orientation.
• Regulatory limits are defined in terms of EIRP.
  Thus, we aim for the product PTX (f) GTX (f) to
  be constant and as close to the limit as
  reasonable margin of safety (typically 3 dB) will
  allow.
• Similarly, this power gain product must roll-off
  so as to fall within the skirts of the allowed
  spectral mask.
• Thus,both the antenna designer and transmitter
  designer must work together to achieve a desired
  PTX (f) GTX (f), and shortfalls in one spectral
  response can be compensated in the other

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Converting a narrowband antenna to an UWB antenna
Figure 3 Shortfalls in converting a conventional
Narrow band Antenna into a UWB antenna.
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AGENDA

• Introduction
• Design Constraints And Limits
• Practical Antenna Designs
• Some practical design applicable to UWB PAN
  systems that address the constraints.

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Balanced Antipodal Vivaldi

• A special form of tapered slot antennas with an
  exponential flare profile and a strip line input.
• One side has the input track flared to produce
  one half of the conventional Vivaldi.
• On the opposite side the ground plane is reduced
  to a balanced set/series of lines that is flared
  out in the opposite direction to produce the
  overall balanced structure.
• Typical size is under 100x40x4 mm
• return loss is -10dB and better from 2.5 GHz to
  11 GHz

Figure 4Vivaldi antenna (above), and Balanced
Vivaldi antenna (Below)
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Balanced Antipodal Vivaldi (Contd)

• Phase response is non linear
• Broadly directional radiation patterns.

Figure 5Balanced Vivaldi antenna Left aboveX-Y
Plane time Domain response, Left below Radiation
Patterns - X-Z Plane and X-Y (resp), Right
Typical phase response
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L-Loop Antenna

• Total length of outer limits of the square loop
  antenna should be in one wavelength to ensure
  linearly polarized radiation
• Composed of a single metallic layer,
• Printed on a side of an FR4 substrate with
  dielectric constant er 4.4, loss tangent tan?
  0.02, and thickness of 1 mm.
• A coupled tapered transmission line is printed in
  the same side with a similar metallic layer -
  copper - of 0.018 mm thickness
• Proposed dimensions - 24 x 25 x 1 mm
• Achieves impedance bandwidth in the order of 2
  GHz (3.1-5.1 GHz) for VSWR 1.6

Figure 6L-Loop antenna structure (above),side
view on substrate (Below)
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L-Loop Antenna (Contd)

• Almost stable radiation pattern throughout
  frequency band
• Thus excellent performance for the lower UWB band
  and has the attractive features of small size,
  low-cost, and easy design

Figure 7 L-Loop antenna VSWR (above), Gain
(Below) and Radiation patterns at a) 3.1, b) 4.1,
and c) 5.1 GHz (left)
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Double Sided Printed Bow Tie Antenna

• Conventional Bow tie antenna bandwidth is
  insufficient for UWB applications.
• Modified by printing 2 patches on top and bottom
  of a substrate to create a double sided printed
  Bow tie (DSPBT) antenna
• Typical size is under 40x40x2 mm.
• Without considering dielectric loss and using a
  substrate with a dielectric constant of 6.15, the
  DSPBT shows an average return loss of -10dB from
  2.2 to 9GHz.

Figure 8Bow Tie antenna (above), and Double
Sides Printed Bow Tie (Below)
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DSPBT Antenna (Contd)

• Phase response is approximately linear
• Almost constant omni directional radiation pattern

Figure 9 Bow Tie antenna Above X-Y Plane time
Domain response RightRadiation Patterns - X-Z
Plane (above) and X-Y plane (below)
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Monopole Antenna

• Usually perpendicular to the ground plane, they
  are famous for omni directional radiation in the
  azimuthal plane.
• Can be used advantageously In the printed
  monopole, by making the already existing ground
  plane an active part of the antenna through
  current induction to produce an asymmetric image
  of the monopole
• Using a substrate with a dielectric constant of
  4.7, the Planer monopole shows an average return
  loss of -10dB from 3.1 to 11GHz

Figure 10Monopole antennas structure
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Monopole Antenna (Contd)

• Phase response is approximately linear from 2 to
  8 GHz
• Almost constant omni directional radiation pattern

Figure 11Monopole antenna X-Y Plane time Domain
response (left), and Radiation Patterns - X-Z
Plane (right above) and X-Y (right below)
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Summary

• UWB Antennas should be designed with
  consideration of these constraints form the start
  as opposed to compensating for identified
  shortfalls as is done in Narrow band Antennas
• Consequently, antennas designer and transceiver
  designer should work together to ensure they get
  the appropriate system results.

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• Thank you