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Antennas from Theory to Practice7. Antenna
Manufacturing and Measurements
Yi HUANG Department of Electrical Engineering
Electronics The University of Liverpool Liverpool
L69 3GJ Email Yi.Huang_at_liv.ac.uk
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Objectives of This Chapter

• We are going to see
• what materials are normally employed to make
  antennas,
• what antenna measurements should be conducted and
  how

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7.1 Antenna Manufacturing

• Antennas are normally manufactured using
  conducting materials, low-loss dielectric
  materials or a combination of both
• In addition to conductivity, the selection of the
  material should take the following into account
• Mechanical considerations
• Environmental considerations
• Cost
• Weight

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Conducting Materials

• Copper, brass (an alloy of copper and zinc),
  bronze (an alloy of copper and tin), and
  aluminium are widely used to make antennas.
• Composite materials are becoming popular, e.g.
  PC/ABS alloys are used for making mobile
  antennas.
• One of the most overlooked antenna construction
  considerations is the galvanic corrosion that
  usually occurs when two dissimilar metals are
  brought into physical contact (mated) during the
  assembly process and exposed to the weather.
  There can be serious corrosion at their
  respective contact points.

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Galvanic metals table
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Dielectric Materials

• Dielectric materials are employed to form the
  desired antenna shape (a conductor may be inside
  the dielectric or the dielectric may be covered
  by a conductor/ composite material), to protect a
  metal antenna, or to act as a dielectric resonant
  antenna (DRA).
• DRAs offer a number of good features, including
• small size.
• nearby objects (such as human hands) has limited
  effect on the performance of a DRA
• PCB materials are widely used for making planar
  antennas

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New Materials

• New PCB materials.
• Liquid crystal polymer (LCP) material provides an
  alternative to traditional polyimide film for use
  as a substrate in flexible circuit construction.
• Silver inks and deposited coppers are employed
  for making small and light-weight antennas for
  applications such as RFID
• Electrically conductive adhesives are replacing
  conventional soldering.
• Artificial materials have been developed and
  provide some unique features.

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7.2 Antenna Measurement Basics

• The most important measurements are the impedance
  and radiation pattern measurements
• The most important and useful equipment for
  antenna measurements is the vector network
  analyser (VNA)

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Scattering Parameters

• A two-port network can be characterised by S
  parameters

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• If the network is passive and it contains only
  isotropic and loss-free materials that influence
  the transmitted signal, the network will obey
• the reciprocity principle, which means S21 S12
  or more generally Smn Snm, and
• the law of power conservation

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Network Analyser

• A combination of a transmitter and a receiver.
  Normally it has two ports and the signal can be
  generated or received from either port.
• Scalar network analyser only measures the
  amplitude whilst the VNA measures both the
  amplitude and phase.
• The main parameters that it measures are the
  S-parameters.
• The VNA is frequency-domain equipment it can
  obtain the signal in the time-domain using the
  Fourier transforms. For example, it can be used
  as a time-domain reflectometer (TDR) to identify
  discontinuities of an antenna, a transmission
  line, or a circuit.

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• A typical configuration of a VNA

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What can a VNA be used to measure?

• Transmission measurements
• Gain, insertion loss, insertion phase (degrees),
  transmission coefficients (S12, S21), electrical
  length (m), electrical delay (s), deviation from
  linear phase (degrees), and group delay (s).
• Reflection measurements
• Return loss, reflection coefficients (S11, S22),
  reflection coefficients vs distance (Fourier
  Transform), impedance (R j X) which can be
  displayed on the Smith Chart, and VSWR.

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7.3 Impedance, S11, VSWR and RL Measurements

• The antenna impedance, S11, VSWR and return loss
  (RL) measurements are basically the same when a
  VNA is employed. We just need to measure S11.
• The standard measurement procedures are
• Select a suitable cable and ensure that it is
  properly connected to the VNA a major source of
  errors
• Select the frequency range and number of points
• Perform the one-port calibration and ensure that
  the cable is not moved (or errors could be
  generated)
• Conduct the measurements (ensure no reflections
  back to the antenna office is ok for some
  antennas)
• Record the measured results.

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Effects of package on antennas
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7.4 Radiation Pattern Measurements

• The pattern measurement is much more time
  consuming and more sensitive to the environment.
• The measurement can be made in the near or far
  field.
• The near field system measures the field
  amplitude and phase, and then transferred to the
  far field by FFT (as shown for aperture
  antennas).
• 3D pattern can be easily obtained.
• Not cheap and the accuracy could be a problem.
• The far field system is more popular and there
  are indoor and outdoor facilities.

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Open-Area Test Sites (OATS)

• Outdoor sites where no reflectors, other than
  the ground, are present over a relatively wide
  area. Low cost.

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• The far field condition has to be met
• The reflection from the ground should be
  minimised
• Raising the antennas height
• Place RF absorbing materials between the antennas
• The reflection of the ground may result in

Thus the received power at the antenna under test
(AUT) is a periodic function of hThR and
sensitive to the heights of transmit and receive
antennas, their separation, and the wavelength.
Another problem is that the measurement is
subject to interference and weather.
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Anechoic Chambers

• An indoor environment without echo which is
  achieved by using RF absorbing materials (RAM).

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• Measurement accuracy is limited by the finite
  reflectivity of the chamber walls, the positioner
  and the cables that are used to feed the AUT,
  which are the major sources of errors.
• The lowest frequency of operation is determined
  by the RAM typically its length needs to be
  approximately one wavelength long of the lowest
  frequency.
• A tapered impedance transition from the free
  space to the back of the absorber is to ensure
  broadband absorbing performance.
• Often a VNA can be used, perhaps with an
  additional transmit amplifier to ensure adequate
  signal strength and desired dynamic range.

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• Antenna Near-Field Chambers
• Measuring the amplitude and phase of all field
  components
• Enough samples required
• FFT is used

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7.5 Gain Measurements

• There are several methods of measuring the
  absolute gain of an antenna
• Comparison with a standard gain horn
• Obtain the powers accepted by the AUT and the
  standard gain (SG) antenna. Since the GSG is
  known
• Two-antenna measurement (one antenna G is known)
• Using the chamber path loss info to calculate the
  gain
• Three-antenna measurement (all antennas G
  unknown)
• Using the chamber path loss info to calculate it
  again.

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7.6 Miscellaneous Topics

• Efficiency measurements
• There are a few methods, the simplest one is the
  Wheeler cap method where an electrically small
  conducting cap (a cavity) is used to measure the
  antennas loss resistance. The real part of the
  input impedance can be measured in such as an
  anechoic chamber, thus the efficiency factor can
  be calculated using

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• Impedance de-embedding techniques
• Direct measurement of the impedance of an antenna
  could be a problem when a connector cannot be
  soldered to the antenna feed point and a feed
  line has to be used, which may not affect the
  VSWR and RL, but it will certainly change the
  reading of the impedance.
• The basic idea of impedance de-embedding is to
  make an identical feed line with an
  open/short-circuit load, and then measure its
  reflection coefficient which will be use to
  calculate the antenna impedance at the desired
  reference point.

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Open end
SMA connector
Feed point
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Measure
Then calculate
This is the principle of impedance de-embedding.
We can now use this reflection coefficient to
obtain
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(a) impedance over 8001040 MHz (b) impedance
over 16102090MHz
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SATIMO probe array near field system