The ItalianSouth African combined effort Pietro Bolli INAF Astrophysical observatory of arcetri Mariet venter SOUTH Africa radio astronomy observatory Italy South Africa Radio Astronomy Workshop ID: 813124
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Slide1
Analysis of the optics of the antenna in Ghana: The Italian-South African combined effort
Pietro Bolli (INAF – Astrophysical observatory of arcetri)Mariet venter (SOUTH Africa radio astronomy observatory)
Italy - South Africa Radio Astronomy Workshop 24-25 October 2018
Slide2Radio telescope
P. Bolli, “Analysis of the optics of the antenna in Ghana: The Italian-South African combined effort”, Italy - South Africa Radio Astronomy Workshop, 2018 2
Antenna
Large collecting area
Large bandwidth (
f
max
=1000*
f
min
)
Front-end
Gain > 100 dB with low
added noise
Frequency down-conversion
Back-end
Power level of order of few
mW
Frequency few GHz
Slide3EM analysis of reflector antenna
Simulate the antenna patterns (also including specific features of the antenna design, like shaped reflector): directivity, aperture efficiency, side lobes levels, half-power beam width, cross-polarization levels …Reverse engineering of unexpected behaviour from measurements
Analysis of the mechanical tolerances and a-priori verification of possible new optical configurationsApplications of the active optics
3P. Bolli, “Analysis of the optics of the antenna in Ghana: The Italian-South African combined effort”, Italy - South Africa Radio Astronomy Workshop, 2018
I.
Prandoni
, The
Sardinia
Radio
Telescope
,
A&A, 2017
Variation of the SRT beam pattern
at 7.24 GHz as a function of elevation
Slide4Agenda
Expertise at INAF in the EM analysis of reflector antennasINAF radio telescopesSome examples of optical analysis applied a the INAF radio telescopes Electromagnetic modelling of the 32-m Ghana Radio TelescopeTechnical information available and discovered features
Antenna beam pattern Leakage from SKA1-Mid dishEM modelPreliminary results
4
P. Bolli, “Analysis of the optics of the antenna in
Ghana: The Italian-South African combined effort”, Italy - South Africa Radio Astronomy
Workshop, 2018
Slide5The INAF radio telescopes
San
Basilio (CA)
Medicina (BO)Noto (SR)Instrument
64-m
SRT
32-m MRT
Northern Cross
32-m NRT
Optics
Gregorian (shaped) antenna
Cassegrain
antenna
Two arms of
cylindric
-parabolic antennas
Cassegrain
antenna
Active surface
Yes
No
No
Yes
Primary mirror size (m)
Ø 64
Ø 32
564 x
35
64 x 23 x 8
Ø 32
Year
of inauguration
2013
1983
1967
1988Frequency range0.3 – 116 GHz 1 – 26.5 GHz400 – 416 MHz 0.3 – 86 GHz
5
P. Bolli, “Analysis of the optics of the antenna in
Ghana: The Italian-South African combined effort”, Italy - South Africa Radio Astronomy
Workshop, 2018
Slide6Prime focus operations for SRT
Shaped
profile
6
Elliptical
profile
Aperture
efficiency
due to the RMS
P. Bolli, “Analysis of the optics of the antenna in
Ghana: The Italian-South African combined effort”, Italy - South Africa Radio Astronomy
Workshop, 2018
Slide7P. Bolli, “Analysis of the optics of the antenna in Ghana: The Italian-South African combined effort”, Italy - South Africa Radio Astronomy Workshop, 2018
Active surface in Noto
Active surface installed on the primary mirror of the Noto antenna compensates deformation effects of the antenna backup
structure due to gravity.
7
Electronics
Letters
, 2003
ON
OFF
Slide8EM software tools
GRASP (used for the Ghana antenna)FEKO (used
for the SKA dishes)
Software package which provides a set of tools for analyzing reflector antennas and delivering EM radiation performance. GRASP is based on the Optical Physics method.
FEKO
is
a
comprehensive
computational
electromagnetics
software. FEKO
combines
several
methods
,
among
them the full
-wave
Method of Moment.
8
P. Bolli, “Analysis of the optics of the antenna in
Ghana: The Italian-South African combined effort”, Italy - South Africa Radio Astronomy
Workshop, 2018
Slide9EM analysis of the Ghana antenna
32-m telecoms dish in Ghana constructed in 1980, previously owned by Vodafone Ghana and recently given to radio astronomers for joining the AVN.Inherited minimal documents on optics and performance of antenna.
Research centered around modelling and understanding of optics by using GRASP.Able to provide ongoing support to engineering commissioning (performed in 2017 in
C-band).
9
P. Bolli, “Analysis of the optics of the antenna in
Ghana: The Italian-South African combined effort”, Italy - South Africa Radio Astronomy
Workshop, 2018
Slide10Ghana antenna modelling
Known features:Cassegrain dual-reflector system with 4 mirrors in slanted beam-waveguide32 m primary reflector, 2.9 m sub-reflector.Primary reflector f/d = 0.32.Discovered features by modelling the antenna:Established
Cassegrain focus and sub-reflector optics (eccentricity, foci distance) using the information on the BWG mirrors.Shaped main and sub-reflectors (prompted by early results & investigation into similar class dishes constructed in the same era).The approximate shaping geometry was computed by Christophe Granet (Lyrebird Antenna Research) with a best-fit spline through reflector panel joints.
10
P. Bolli, “Analysis of the optics of the antenna in
Ghana: The Italian-South African combined effort”, Italy - South Africa Radio Astronomy
Workshop, 2018
Slide11Antenna beam pattern
11Unshaped
@ 6.7 GHzP. Bolli, “Analysis of the optics of the antenna in Ghana: The Italian-South African combined effort”, Italy - South Africa Radio Astronomy Workshop, 2018
Shaped
@ 6.7 GHz
Slide12Mechanical tolerances
12
10 cm (2l)
P. Bolli, “Analysis of the optics of the antenna in
Ghana: The Italian-South African combined effort”, Italy - South Africa Radio Astronomy
Workshop, 2018
Slide13Leakage in SKA1-Mid dish
Understanding the amount of leakage from the main reflector. Requirement: maximum 2 Kelvin noise temperature from transmission through panel gaps.The SKA dish is a 15 m offset shaped Gregorian system with 5.16 m sub-reflector. Primary mirror composed by 66 triangular panels with gaps 3 mm wide. The physical area of all the gaps is 0.3% the total physical area of the primary mirror.
P. Bolli, “Analysis of the optics of the antenna in Ghana: The Italian-South African combined effort”, Italy - South Africa Radio Astronomy
Workshop, 2018
Slide14Aperture efficiency
14The aperture efficiency in band 2 is very high above 85%, while it reduces to >70% for band 1 (likely due to a rough model of the feed). The reduction in the aperture efficiency due to the leakage from the panels ranges between: 95% and 97% in band 1
96% and 99% in band 2P. Bolli, “Analysis of the optics of the antenna in Ghana: The Italian-South African combined effort”, Italy - South Africa Radio Astronomy Workshop, 2018
Slide15Measurement method
15Simulate the main reflector leakage by using a broad band radiator (Log Periodic Dipole Antenna) behind the dish.The coupling between the transmitting antenna and the real band 1 and 2 horn is simulated for two cases: solid reflector and reflector with gaps.
The resulting intensity pattern represents: only diffraction from the solid antenna diffraction + leakage through the main reflector for the reflector with gaps
P. Bolli, “Analysis of the optics of the antenna in Ghana: The Italian-South African combined effort”, Italy - South Africa Radio Astronomy Workshop, 2018
Slide16Preliminary
result of the EM coupling16P. Bolli, “Analysis of the optics of the antenna in Ghana: The Italian-South African combined effort”, Italy - South Africa Radio Astronomy Workshop, 2018
The EM coupling for the case “with gaps” was expected to be always higher than “w/o gaps”.
Band 1
Band 2
Slide17Conclusions
Established a productive collaboration in the EM analysis of reflector antennas between INAF and SARAO.During two different stays of Mariet Venter at the Astrophysical Observatory of Arcetri, it has been possible to:
Model the 32-m Ghana Radio Telescope (February, 2017) Analyse the leakage from SKA1-Mid dish (September, 2018)
Collaborations in similar areas (microwave holography and EM design of feed-horn) have been initiated.17
P. Bolli, “Analysis of the optics of the antenna in
Ghana: The Italian-South African combined effort”, Italy - South Africa Radio Astronomy
Workshop, 2018
Slide18Acknowledgments
18With support of Italy's Ministry of Foreign Affairs andInternational Cooperation, Directorate General for the CountryPromotion. Bilateral Grant Agreements:Mapping the Universe
on the Pathway to SKA.RADIO SKY 2020 - Fostering cooperation between Italy and South Africa through radio astronomyWith support from the National Research Foundation (NRF) and the Department of Science and Technology (DST) of South Africa.
P. Bolli, “Analysis of the optics of the antenna in
Ghana: The Italian-South African combined effort”, Italy - South Africa Radio Astronomy
Workshop, 2018
Slide19New vertex room for the Medicina RT
19
Case
123
4
5
Tx
Nominal
No tilt
Tilted
No tilt
Tilted
M2
Nominal
No tilt
No tilt
Tilted
Tilted
Gain (
dBi
)63.562.2
63.462.263.5
Beam In-axisOff-axis
Off-axis
In-
axis
In-
axis
Feed
translated
in the
focal
plane
of 12
l
at 5 GHzP. Bolli, “Analysis of the optics of the antenna in Ghana: The Italian-South African combined effort”, Italy - South Africa Radio Astronomy Workshop, 2018