Modeling of antennas for THz radiation detectors P. Kopyt , W. K. Gwarek Institute of Radioelectronics Warsaw University of Technology Experiment 1 aiming at verifying whether FET detection was sensitive to the polarization of the incident radiation: a commercially available GaAs/AlGaAs FET was subject to linearly polarized 100 GHz radiation. EM modelling (QuickWave 3D) allowed revealing the role of length and orientation of bonding wires in defining the spatial orientation of the two-lobe pattern. a) Device sensitivity measured for 2 values of U GS b) Far field simulations of the device with bonds for properly adjusted impedances Z GS , and Z DG . 1 M. Sakowicz, J. Lusakowski, K. Karpierz, M. Grynberg, W. Knap, W. Gwarek, “Polarization sensitive detection of 100 GHz radiation by high mobility field-effect transistors”, JOURNAL OF APPLIED PHYSICS 104, 024519 (2008) Motivation 100 GHz radiation
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Modeling of antennas for THz
radiation detectors
P. Kopyt, W. K. Gwarek
Institute of Radioelectronics
Warsaw University of Technology
Experiment 1 aiming at verifying whether FET detection was sensitive to the
polarization of the incident radiation: a commercially available GaAs/AlGaAs
FET was subject to linearly polarized 100 GHz radiation.
EM modelling (QuickWave 3D) allowed revealing the role of length and
orientation of bonding wires in defining the spatial orientation of the two-lobe
pattern.
a) Device sensitivity measured for 2 values of UGS
b) Far field simulations of the device with bonds
for properly adjusted impedances ZGS, and ZDG.
1 M. Sakowicz, J. Łusakowski, K. Karpierz, M. Grynberg, W. Knap, W. Gwarek, “Polarization sensitive detection of 100 GHz radiation by high mobility field-effect transistors”, JOURNAL OF APPLIED PHYSICS 104, 024519 (2008)
Motivation
100 GHz
radiation
Antenna is a device which should
effectively couple the EM plane
wave propagating in free space to
a lumped element, eg. input impedance Z0 of a receiver.
E-field component (in space)
Z0
A load – lumped
impedance
EM plane wave
Intermediary
element
Antennas
Antenna realizations
Wideband horns
Dielectric resonator antennas
Open quad ridge horns
Antenna parameters 1
Directivity D(θ,φ) of an antenna measures
the ability of the antenna to direct its power
towards a given direction
Radiation intensity U(θ,φ) is the power radiated by an antenna with the P per
unit solid angle.
Radiated power density Pr total power emitted by an antenna per unit area
rE
1∝
rH
1, ∝
UI – same power but radiated
isotropically
U(θ,φ)
UI
Antenna parameters 2
Efficiency ηηηη defines all loss mechanisms (e.g. ohmic losses of the currents flowing on the antenna wires or volumetric losses in the dielectric)
Gain G(θ,φ) also measures the ability of the antenna to direct its power towards
a given direction, however the normalization is to power delivered to the
antenna terminals.
ηηηη
For a lossless antenna the efficiency factor will be unity. In the case, there is
no distinction between directive and power gain. In real cases ηηηη < 1 (not all delivered power is radiated)!
ηηηη PT Prad
Power lost to heat in
metal and dielectric
Energy stored in
antenna volume
PRPrad
Power lost to heat in
metal and dielectric
Energy stored in
antenna volume
Antenna parameters 3
Effective area A describes the ability of an antenna to extract power from an
incident EM wave.
Diameter(m)
A(m2)
ApertureSize (m2)
Apertureefficiency
0.30
0.45
0.60
0.90
1.20
1.80
0.0375
0.0802
0.1494
0.3121
0.6828
1.3624
0.0707
0.1590
0.2827
0.6362
1.1310
2.5447
0.53
0.50
0.53
0.49
0.60
0.53
The effective area (A) is not equal to the physical
area of an antenna! A is typically a fraction of the
physical area (55–65% for dishes and 60–80% for
horns).
Antenna parameters 4
ZA
Input impedance ZA is the antena impedance as seen by a generator feeding
the antenna: ZA = RA + jXA.
where, PT,max and PR,max are available power from the generator or from the antenna respectively
Under impedance matching conditions:
Impedance matching affects the delivered power.
Antenna parameters 5
Bandwidth BW range of frequencies within which the performance of the
antenna, with respect to some characteristic, conforms to a specified
standards/requirements.
Impedance bandwidth BWZ range of frequencies within which the antena
reflection coefficient ΓΓΓΓ for its input remains low.
Radiation bandwidth BWR range of frequencies within which the antena
radiation characteristics stays within the desired limit. Typically BWZ < BWR
center
lowerupper
Zf
ffBW
−=
fupper flower
fcenter
Elevation [o]
Antenna analysis methods 1
Analytical methods typically possible for antennas of simple geometries, like
linear antennas, selected horn structures, open-ended waveguides used as
radiating structures.
Example: half-wavelength dipole :
Antenna analysis methods 2
Numerical methods employed complex radiating geometries based on the
Finite Difference Time Domain – FDTD (QuickWave 3D, CST); Finite Elements
Method – FEM (HFSS) or Method of Moments – MoM (Sonnet, FEKO).
FDTD method implemented in QuickWave 3D, QWED, Poland
Standard FDTD cells:
air dielectric metal
dielectric media interfaces
metal boundaries
Conformal cells in QuickWave 3D:
Classical examples of application 1
Axially symmetrical corrugated horn for satellite communication:
Hφ at 13.75GHz Design & measurements: P. Brachat, IEEE Trans. AP,
April 1994
6 GHz
-40
-30
-20
-10
020
40
60
80
100
120
140
160200
220
240
260
280
300
320
340
Pyramidal horn antenna for military surveillance:
Design & measurements:
Prof. B. Stec,
Technical Military Academy, Poland
------ vertical plane measured
____ vertical plane simulated
------ horizontal plane measured
____ horizontal plane simulated
Classical examples of application 2
Amplitude and phase imbalance
– from the measured (noisy)
and simulated (smooth) results.www.mma.nrao.edu/memos/html-memos/alma278/memo278.pdf