Design and Analysis of Low Frequency Strut Straddling Feed ... · Wavelength Array (LWA), Low...
Transcript of Design and Analysis of Low Frequency Strut Straddling Feed ... · Wavelength Array (LWA), Low...
![Page 1: Design and Analysis of Low Frequency Strut Straddling Feed ... · Wavelength Array (LWA), Low Frequency Array for Radio Astronomy (LOFAR). All of these telescopes use dipole like](https://reader034.fdocuments.net/reader034/viewer/2022043007/5f946039ec94bd01ed11c8d9/html5/thumbnails/1.jpg)
Design and Analysis of Low Frequency
Strut Straddling Feed Arrays for EVLA
Reflector Antennas
Mahmud Harun*, S. W. Ellingson
Virginia Polytechnic Institute & State University
URSI NRSM – Jan 7, 2011
![Page 2: Design and Analysis of Low Frequency Strut Straddling Feed ... · Wavelength Array (LWA), Low Frequency Array for Radio Astronomy (LOFAR). All of these telescopes use dipole like](https://reader034.fdocuments.net/reader034/viewer/2022043007/5f946039ec94bd01ed11c8d9/html5/thumbnails/2.jpg)
Expanded Very Large Array (EVLA)
EVLA consists of 27 25-meter (82 ft) diameter Cassegrain
reflector antennas
• Frequency bands in the original design (1981):
� 21 cm (1.5 GHz)
� 6 cm (5 GHz)
� 2 cm (15 GHz)
� 1.3 cm (23 GHz)
• Additional low frequency capabilities implemented:
� 90 cm (327 MHz): completed by 1989
� 4 m (74 MHz): completed by 1997
![Page 3: Design and Analysis of Low Frequency Strut Straddling Feed ... · Wavelength Array (LWA), Low Frequency Array for Radio Astronomy (LOFAR). All of these telescopes use dipole like](https://reader034.fdocuments.net/reader034/viewer/2022043007/5f946039ec94bd01ed11c8d9/html5/thumbnails/3.jpg)
New Low Frequency System(LFS) Proposed
for the EVLA*
� Contiguous coverage to frequencies as low as 50 MHz
� Additional feeds required
� Additional feeds should not increase blockage and effect
high frequency observation
� Focus of this presentation is to design low frequency feed
for covering frequencies < 100 MHz
* J. Ott et.al., “Pushing the Limits of the EVLA: An Enhancement Program for the Next Decade,”
Astro2010 Activity White Paper, April 1, 2009. Available online:http://www.nrao.edu/A2010/.
![Page 4: Design and Analysis of Low Frequency Strut Straddling Feed ... · Wavelength Array (LWA), Low Frequency Array for Radio Astronomy (LOFAR). All of these telescopes use dipole like](https://reader034.fdocuments.net/reader034/viewer/2022043007/5f946039ec94bd01ed11c8d9/html5/thumbnails/4.jpg)
What is special at low frequencies?
At low frequencies….
� Galactic noise can dominate internal noise of the system.
� Improving impedance mismatch efficiency (IME), i.e., the
matching between the antenna and the connected electronics
does not necessarily improve sensitivity
� Simple dipole-like antennas can offer large usable bandwidth from
a sensitivity perspective.
For example: Eight-meter-wavelength Transient Array (ETA), Long
Wavelength Array (LWA), Low Frequency Array for Radio
Astronomy (LOFAR). All of these telescopes use dipole like
antennas based on this principle.
![Page 5: Design and Analysis of Low Frequency Strut Straddling Feed ... · Wavelength Array (LWA), Low Frequency Array for Radio Astronomy (LOFAR). All of these telescopes use dipole like](https://reader034.fdocuments.net/reader034/viewer/2022043007/5f946039ec94bd01ed11c8d9/html5/thumbnails/5.jpg)
What is special at low frequencies?
In a reflector antenna available power spreads away from
the focal region as frequency is decreased.
-12
-12
-12
-12
-9
-9
-9
-6
-6
-6
-3
-3
-12
x (m)
y (m
)
-3 -2 -1 0 1 2 3-3
-2
-1
0
1
2
3
-12
-12
-12
-12
-12
-9-9
-9
-9
-9
-6
-6
-6
-6
-6
-3
-3
-3
x (m)
y (m
)
-3 -2 -1 0 1 2 3-3
-2
-1
0
1
2
3
-15
-10
-5
0
100 MHz 50 MHz
Distribution of power density (dB) in the focal plane of a reflector antenna (comparable in size
to an EVLA reflector) wrt power density at the focus
![Page 6: Design and Analysis of Low Frequency Strut Straddling Feed ... · Wavelength Array (LWA), Low Frequency Array for Radio Astronomy (LOFAR). All of these telescopes use dipole like](https://reader034.fdocuments.net/reader034/viewer/2022043007/5f946039ec94bd01ed11c8d9/html5/thumbnails/6.jpg)
New Feed Design: Strut Straddling Feed Array
New design:
Dipole Feeds mounted between adjacent struts.
Advantage:
Presumably reduced blockage
2 key factors at low frequencies:
• Usable sensitivity over a large bandwidth with dipole antennas.
• Spreading of the available power away from the focal region.
Signals from the dipoles
are multiplied by
combining coefficients, bn
and then combined
![Page 7: Design and Analysis of Low Frequency Strut Straddling Feed ... · Wavelength Array (LWA), Low Frequency Array for Radio Astronomy (LOFAR). All of these telescopes use dipole like](https://reader034.fdocuments.net/reader034/viewer/2022043007/5f946039ec94bd01ed11c8d9/html5/thumbnails/7.jpg)
Background: An Electromagnetic Model of
an EVLA Antenna
Wire Grid Model
for MoM Analysis
� Origin at focus
� Axis along –z-axis
� z=0 is the focal
plane
Required for Evaluation of
Performance
![Page 8: Design and Analysis of Low Frequency Strut Straddling Feed ... · Wavelength Array (LWA), Low Frequency Array for Radio Astronomy (LOFAR). All of these telescopes use dipole like](https://reader034.fdocuments.net/reader034/viewer/2022043007/5f946039ec94bd01ed11c8d9/html5/thumbnails/8.jpg)
Baseline of performance :
Existing 4-m System on the EVLA
•Crossed dipoles (3/16 in.) in front
of subreflector
•Aperture efficiency has been
estimated to be 15-25%
•Sensitivity reduced by 6% at 1.5 GHz
•As a result dipoles are intermittently mounted
![Page 9: Design and Analysis of Low Frequency Strut Straddling Feed ... · Wavelength Array (LWA), Low Frequency Array for Radio Astronomy (LOFAR). All of these telescopes use dipole like](https://reader034.fdocuments.net/reader034/viewer/2022043007/5f946039ec94bd01ed11c8d9/html5/thumbnails/9.jpg)
-15
-10
-5
0
5
10
15
20
Copol G
ain
(dB
)
-150 -100 -50 0 50 100 150-20
-15
-10
-5
0
5
10
15
20
θ (degrees)
Copol G
ain
(dB
)
Aperture Efficiency Estimation using MoM
Ae = 29%
Ae = 11%
Asymmetric pattern
due to tilted
subreflector
Support wire
co-polarized
Support wire
cross-polarized
Support wires
Support wires
Dipole
![Page 10: Design and Analysis of Low Frequency Strut Straddling Feed ... · Wavelength Array (LWA), Low Frequency Array for Radio Astronomy (LOFAR). All of these telescopes use dipole like](https://reader034.fdocuments.net/reader034/viewer/2022043007/5f946039ec94bd01ed11c8d9/html5/thumbnails/10.jpg)
Analysis of Sensitivity of Existing
EVLA 4-m System
• Our model of the existing EVLA 4-m system
• Sensitivity analysis in terms of system
equivalent flux density (SEFD)
– Source power flux spectral density (W m2 Hz-1)
which yields signal-to-noise ratio (SNR) of unity at
the output of the system.
ZL=100 Ω
Tinternal = 250 KAntenna
temp. TA
ZL and Tinternal are selected to be
comparable to other low
frequency systems that employ
dipole like antennas, e.g., LWA
![Page 11: Design and Analysis of Low Frequency Strut Straddling Feed ... · Wavelength Array (LWA), Low Frequency Array for Radio Astronomy (LOFAR). All of these telescopes use dipole like](https://reader034.fdocuments.net/reader034/viewer/2022043007/5f946039ec94bd01ed11c8d9/html5/thumbnails/11.jpg)
50 55 60 65 70 75 80 85 904
4.2
4.4
4.6
4.8
5
5.2
5.4
5.6
5.8
6
Freq (MHz)
SE
FD
(lo
g10 J
y)
Performance Baseline: System Equivalent
Flux Density (SEFD) Estimation of the
Existing EVLA 4-m System
Performance of new design
will be compared with these
results
Uniform Tsky assumed
100 kJy
1000 kJy
Subreflector in full
retracted position
![Page 12: Design and Analysis of Low Frequency Strut Straddling Feed ... · Wavelength Array (LWA), Low Frequency Array for Radio Astronomy (LOFAR). All of these telescopes use dipole like](https://reader034.fdocuments.net/reader034/viewer/2022043007/5f946039ec94bd01ed11c8d9/html5/thumbnails/12.jpg)
Proposed New Low Frequency (< 100 MHz) Feed for
the EVLA :
Strut Straddling Feed Array
Signals from the
dipoles are
multiplied by
combining
coefficients, bn
and then
combined
74 MHz resonant
dipoles (3/8 in.) are
located between
adjacent struts
![Page 13: Design and Analysis of Low Frequency Strut Straddling Feed ... · Wavelength Array (LWA), Low Frequency Array for Radio Astronomy (LOFAR). All of these telescopes use dipole like](https://reader034.fdocuments.net/reader034/viewer/2022043007/5f946039ec94bd01ed11c8d9/html5/thumbnails/13.jpg)
Strut Straddling Feed Array Design:
Location of Dipoles
Dipoles positioned between adjacent struts.
0.5 1 1.5 2 2.5 3 3.5 4
4.8
5
5.2
5.4
5.6
5.8
distance form the focal plane
log
10S
EF
D(J
y)
Which plane?
Lowest SEFD
at z=-1.0 m
bn selected to maximize SNR
4 dipoles combined
![Page 14: Design and Analysis of Low Frequency Strut Straddling Feed ... · Wavelength Array (LWA), Low Frequency Array for Radio Astronomy (LOFAR). All of these telescopes use dipole like](https://reader034.fdocuments.net/reader034/viewer/2022043007/5f946039ec94bd01ed11c8d9/html5/thumbnails/14.jpg)
Strut Straddling Feed Array at z=-1.0 m
Freq. Mag.
bn
Phase
bn
(rad)
50 0.9
1.0
0.9
1.0
0
0.2
0
0.2
74 0.9
1.0
0.9
1.0
0.2
0.0
3.3
3.1
88 1.0
0.9
1.0
0.9
0.1
0.0
0.1
0.0
50 55 60 65 70 75 80 85 904
4.2
4.4
4.6
4.8
5
5.2
5.4
5.6
5.8
6
Freq (MHz)
SE
FD
(lo
g1
0 J
y)
Existing EVLA 4-m System Model
Single Ring @ z=-1.0 m
with Optimized Beamforming Coeffs.
![Page 15: Design and Analysis of Low Frequency Strut Straddling Feed ... · Wavelength Array (LWA), Low Frequency Array for Radio Astronomy (LOFAR). All of these telescopes use dipole like](https://reader034.fdocuments.net/reader034/viewer/2022043007/5f946039ec94bd01ed11c8d9/html5/thumbnails/15.jpg)
Strut Straddling Feed Array at z=-1.0 m
50 55 60 65 70 75 80 85 904
4.2
4.4
4.6
4.8
5
5.2
5.4
5.6
5.8
6
Freq (MHz)
SE
FD
(lo
g10 J
y)
Existing EVLA 4-m System Model
Single Ring @ z=-1.0 m
with Optimized Beamforming Coeffs
Single Ring @ z=-1.0 m,
Combined with Fixed Length (Equal) Cables
![Page 16: Design and Analysis of Low Frequency Strut Straddling Feed ... · Wavelength Array (LWA), Low Frequency Array for Radio Astronomy (LOFAR). All of these telescopes use dipole like](https://reader034.fdocuments.net/reader034/viewer/2022043007/5f946039ec94bd01ed11c8d9/html5/thumbnails/16.jpg)
Double Ring of Strut Straddling Feed
Array
Possible improvement to
single ring: Double ring of
dipoles
Location of the dipole rings:
z=-1.0 m and z=-2.25 m
![Page 17: Design and Analysis of Low Frequency Strut Straddling Feed ... · Wavelength Array (LWA), Low Frequency Array for Radio Astronomy (LOFAR). All of these telescopes use dipole like](https://reader034.fdocuments.net/reader034/viewer/2022043007/5f946039ec94bd01ed11c8d9/html5/thumbnails/17.jpg)
Double Ring at z=-1.0 m and z=-2.25 m
50 55 60 65 70 75 80 85 904
4.2
4.4
4.6
4.8
5
5.2
5.4
5.6
5.8
6
Freq (MHz)
SE
FD
(lo
g10 J
y)
Existing EVLA 4-m System Model
Double Ring at z=-1.0 m & z=-2.25 m
with Optimized Beamforming Coeffs.
![Page 18: Design and Analysis of Low Frequency Strut Straddling Feed ... · Wavelength Array (LWA), Low Frequency Array for Radio Astronomy (LOFAR). All of these telescopes use dipole like](https://reader034.fdocuments.net/reader034/viewer/2022043007/5f946039ec94bd01ed11c8d9/html5/thumbnails/18.jpg)
Double Ring at z=-1.0 m and z=-2.25 m
50 55 60 65 70 75 80 85 904
4.2
4.4
4.6
4.8
5
5.2
5.4
5.6
5.8
6
Freq (MHz)
SE
FD
(lo
g10 J
y)
Existing EVLA 4-m System Model
Double Ring at z=-1.0 m & z=-2.25 m
with Optimized Beamforming Coeffs
Double Ring at z=-1.0 m & z=-2.25 m,
Combined with Fixed Length (Non-Equal) Cables
![Page 19: Design and Analysis of Low Frequency Strut Straddling Feed ... · Wavelength Array (LWA), Low Frequency Array for Radio Astronomy (LOFAR). All of these telescopes use dipole like](https://reader034.fdocuments.net/reader034/viewer/2022043007/5f946039ec94bd01ed11c8d9/html5/thumbnails/19.jpg)
Summary• Strut Straddling Feed Array
– Single Ring
– Double Ring
• Comparable performance to the existing EVLA 4-m system
• Advantage: Presumably reduced blockage
Future Work:
• Implementation
• Obtain 2 orthogonal polarizations
Acknowledgements:
Jim Ruff (NRAO)
Frazer Owen (NRAO)
Rick Perley (NRAO)