Transcript Slide 1

Weak
Signal
Detection
of Virgo A
Amateur
Radio
Astronomy
Exploring some Limitations in
Amateur Radio Astronomy
Dr David Morgan
17/5/2014
www.dmradas.co.uk
Contents
Weak Signal Detection of Virgo A
 Antennas & Receiver properties
 Radio source strength & spectra
 Limitations with small antennas
3m Dish
L Band Horn
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408MHz Quagis
Telescope
Characteristics
Weak Signal
Detection of Virgo A

Antenna properties
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Antenna
Fundamentals
Weak Signal
Detection of Virgo A
 Two fundamental properties of an antenna of concern to
amateur radio astronomers
• Gain
• Beamwidth
 These are related – the higher the gain the smaller the
beamwidth
 We want both high gain and narrow beamwidth
• Gain = sensitivity
• Beamwidth = spatial selectivity
A 3m dia. dish polar diagram
Need Large antenna aperture
80 HPBW
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Antenna
Equations
Weak Signal
Detection of Virgo A
 Antenna Gain
• G = h (4 p / l2) A
h= Aperture efficiency
A= Antenna aperture m2
l = wavelength
For a reflector antenna, the area is simply the projected area.
For a circular reflector of diameter D, the area is A = p D2/4 and the gain is
G = h (p D / l )2
 Antenna Beamwidth
HPBW = a
= k l/D
k= a factor that depends on the
shape of the reflector and the
method of illumination
For a typical antenna
G = 27,000/ (a )2
Antenna diameter drives performance
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At
UHFSignal
thingsDetection
get big of Virgo A
Weak
 Rare to find an amateur with a 9m antenna
John Smith (1924 -1998) with 9m dish
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Yagis
Dishes
Weak vs
Signal
Detection of Virgo A
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Would not tend to use a small dish at UHF
Yagi arrays probably cheaper and easier to build
But – effective aperture must be similar to dish area
So arrays will be a few metres square
Complicated to construct and phase together
DL7APV
Array (used for EME)
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Part of my 408MHz Quagi Array
Consequences
Weak Signal Detection of Virgo A
 Can have high sensitivity and good spatial resolution at
11GHz with1m antenna
• ~40dB gain and few degrees HPBW
 Reasonable gain and resolution with 2.4m dish at C band
•
~ 35dB gain and <5 degrees HPBW
 Workable sensitivity and resolution with 3m dish at 1.4GHz
• eg 30dB gain and >5 degrees HPBW
 But low gain and poor spatial resolution with 3m dish at
408MHz
• eg 19dB gain and 18 degrees HPBW
 Impractical at VHF (space & cost)
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Result
of Antenna
limitations
Weak Signal
Detection
of Virgo A
L band is probably most practical,
useful and affordable option
for amateurs
(L Band (IEEE) = 1-2GHz)
Noise
and Gain
Stabilityof Virgo A
Weak Signal
Detection

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
Receiver Requirements
Band coverage
Available bandwidths
Detector functions
Sensitivity
Noise & gain stability
 Discuss the last two items
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LNAs
Receivers
Weak and
Signal
Detection of Virgo A
 Receiver must have high gain and low noise
 System Noise figure (NF ) determined by first amplifier
 Low Noise Amplifiers (LNA) now capable of 0.2dB at 1.4GHz
 Conventional Coms receiver or SDR sensitivities are
adequate when used with LNA ( gains of 20 – 40dB)
 Noise & gain stability are crucial:
• Maintain common parameters from hour to hour and day to
day – to enable radio maps etc to be made.
ICOM IC-R7000 Receiver
Realtek RTL2832U DVB-TV dongle
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FunCube Dongle SDR
Weak
SDR Signal
DongleDetection
Receiversof Virgo A
 The common SDR Dongles have gaps in frequency
coverage
Frequency
Device
1MHz
10MHz
100MHz
1GHz
2GHz
FCDPro
1.1 1.27
60
1.7
FCDPro+
240
420
1.9
RTL
1.1 1.25
26
408MHz
1.4GHz
Frequency Coverage for FCD & RTL Dongle Devices
Coverage Gaps
2.2
Noise
/Gain Stability
Weak Signal
Detection of Virgo A
 FunCube Dongle Pro + :
 Stability is better than 0.05dB over 3 hours
 Cheap stable SDR receivers widely available
 No important receiver limitations
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Objects
to Observe
Weak Signal
Detection of Virgo A

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Astronomical Radio Sources
What sources are in the Northern Hemisphere ?
How strong are they ? – are they detectable by Amateurs ?
What spectrum do they have ?
Are they discrete or spatially distributed ?
Key parameters
Source Flux
Spectrum
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Angular Size
Source
SignalDetection
Strengthsof Virgo A
Weak Signal
100mV/m
109
Communication receiver
Sun storms
107
10-17
10-19
105
Jupiter
10-21
10-23
10-25
Radio galaxies
Pulsars
Assuming 6kHz bandwidth
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103
Supernova remnants
10
Power Flux Density (W m-2 Hz-1)
1mV/m
Power Flux Density Jy (10-26W m-2 Hz-1)
Broadcast signals
10-15
1011
 Look at typical source signal strength
1Jy=10-26 W m-2 Hz-1
Radio
Sources
– signal of
strengths
Weak Signal
Detection
Virgo A
109
 In more detail
106
107
These are ‘continuous’ sources –
not dealing with radio transients
Quiet sun
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Pulsars
Virgo A
Orion Neb.
Taurus A
Cygnus A
Jupiter
Sun
Cassiopeia A
102
103
104
105
Solar storms
& plages
10
1
Power Density Jy (10-26 W m-2 Hz-1)
10 8
Solar bursts
Source Spectrum drives receiver frequency
Weak Signal Detection of Virgo A
 Each source has its own predominant radiation mechanism
 This determines the emission spectrum
 The source spectrum drives the telescope configuration
• eg Frequency of operation, Gain & Antenna size
1cm
3cm
10cm 1m
Wavelength
storms
3m
10m
1420MHz
104
103
102
quiet sun
Cassiopeia
Cygnus
Virgo
Hercules
1cm
10cm
1m
Wavelength l
Galactic Hydrogen : Line Source
H Line Spectrum
P l x (l=wavelength,
x=spectral index)
1
bursts
SNR : Synchrotron Source
Power flux density (10-26 W m-1 Hz-1)
Power flux density (10-26W m-2 Hz-1)
104 10 106 10 10 10 1010
SUN : Thermal Source
10m
1421MHz
Radiation
Mechanisms
Weak Signal
Detection of Virgo A
 Source Spectra : Three mechanisms – Three spectra
electromagnetic
emission
charged particle
magnetic
field
r
v
electron
linear
polarization
P=e kT/l 2
K = Boltzmann's const
T= temperature
Ionised gas
(thermal)
OH 18cm
y
Methanol 4cm
Synchrotron radiation
(non thermal)
Relative power flux
102
10
z
1
1
102
e=1
opaque
10
x
1
Radiated Power P
103
e
l2
Semi-transparent
102
103
Ion
electron reverses spin
& radiates a photon
Hydrogen 21cm
circular
polarization
Relative power flux
10
moving
electron
proton
proton
1cm
10cm
1m
Wavelength l
Thermal Spectrum
10m
1cm
10cm
1m
Wavelength l
10m
Synchrotron Spectrum
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1cm
10cm
1m
Wavelength l
Line spectrum
10m
What
amateurs
measure?
Weak can
Signal
Detection
of Virgo A– some examples
 Use microwave receiver for thermal sources
• Few interesting objects to detect
• Small objects < 0.50 diameter - eg SUN & Moon
• Measurements will be HPBW limited (~20)
~20
11GHz image of SUN
 Using L band for H line
• Measuring Doppler shifts & mapping galaxy
• Reasonable spatial resolution achievable
Galactic H line
 Use L Band for Synchrotron emission
•
•
•
•
Galactic emission can be mapped
Reasonable spatial resolution achievable
SNRs are discrete sources – smeared out by large HPBW
This makes SNRs difficult to detect
Galactic Synchrotron
 Try using UHF for Synchrotron emissions
• Higher signal but worse antenna gain – no improvement
• HPBW rather poor – limited spatial resolution
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Extra galactic interferometry
11 GHz
Radiometer
imageof Virgo A
Weak
Signal
Detection
 School Radiometer project – show some principles of Radio Astronomy
45 0
Satellites
0 to 450
Satellites
Unknown signal
sources
Elevation
00
00
SUN was
behind building
here
20 0 East of South
South
Azimuth
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Can see radio emission
from chimneys
West
North West
Hydrogen
VelocityofDistribution
Weak
SignalLine
Detection
Virgo A
 As amateurs we can measure the intensity, spatial distribution and
velocities of Hydrogen in the galactic plane.
60
Hydrogen Velocity v Galactic Longitude
Velocity
(degrees)
Declination
0 10 20 30 40 50
Galactic H line
Frequency
Cygnus Arm
-10
Galactic Centre
22
21
20
19
18
Galactic
Longitude(hrs)
Right Ascension
Plane
17
Weak Signal Detection of Virgo A
1420.4MHz Image of Milky Way
13.08 GMT 28/11/2013
65
Cygnus arm
60
HPBW
55
50
Elevation (degrees)
45
40
35
30
25
20
Ground Noise
Galactic Centre
15
10
5
0
0
North
20
40
60
80
100
East
Azimuth Bearing (degrees)
120
140
160
180
South
WhatSignal
is difficult
for amateurs?
Weak
Detection
of Virgo A
 Difficult to detect discrete sources at UHF / VHF
•
•
•
•
•
We are limited by using small antennas
Only moderate gain
Relatively wide beamwidths
Discrete sources << beamwidth
Leads to source intensity loss & spatial smearing
 How significant is the effect for discrete sources ?
Wide
point sources
Weakbeams
Signal &
Detection
of Virgo A
 Evaluating loss of signal and point source smearing
• Antenna temperature relationship with source flux density
TA = ‘Antenna Temperature’ , S = Source flux
Ae = Effective Area , k = 1.38x10-23 J K-1
(10 K=1.38x10−23 W Hz−1)
WA = Antenna beam solid angle, Pn = polar response
MB
TB = source brightness Temp, WS = source solid angle
http://www.cv.nrao.edu/course/astr534/AntennaTheory.html
Two
Weakexamples
Signal Detection of Virgo A
Example: SUN
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For a hot source like the SUN, TB~104 K
Angular diameter = 0.50
With a 50 HPBW antenna beam
Source will only add 100K to the antenna temperature.
Example: Cass A
Using an antenna main beam HPBW = 80
Angular diameter = 5 arc min, TB= 37920K
Background galactic plane Temp = 860K
Cass A
80 beam
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Cass
example
WeakASignal
Detection of Virgo A
 For Cass A set in the galactic plane background
Cass A
 Contribution from Cass A Temp = 0.4110 K
 Background GP temp = 860 K
5 arc min
 So Cass A is hardly detectable against 860K background with a
‘Total Power’ system
 Detection of ‘point’ sources requires very narrow beams
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Discrete
Source
is lost in
Weak Signal
Detection
of background
Virgo A
 Must have a larger antenna with a narrower beam to detect
SNRs or extra galactic objects when using a Total Power
System
 Requires Antenna HPBW of < 10 at UHF (Synchrotron Sources)
 Better than 20m diameter required.
Without access to a large antenna the
only practical way for amateurs to observe
point sources is with Interferometry
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H
LineSignal
is bestDetection
target for of
amateurs
Weak
Virgo A
 This table summarises the issues when restricted to small antennas
Frequency
KU Band 11GHz
Performance
Possible Sources
Remarks
‘High performance’
system but little of
interest to detect
High gain, narrow
beams (<20)
Few of interest
C Band 4 GHz
Medium gain,
reasonable beam
As above
No ‘available’
sources
L Band 1.4 -1.6GHz
Satisfactory gain,
rather wide beam
(50 -80)
Galactic H Line
Low spatial
resolution but OK for
Galactic Hydrogen
work
UHF 408MHz
Low gain, poor
beamwidth
Many synchrotron
SNRs, galaxies etc
Many sources, but
poor sensitivity and
resolution
VHF 150MHz
Need very large
antenna
Many SNRs and
Pulsars
Not really practical
for amateurs
(thermal only)
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Weak Signal
Detection
of Virgo A
Hydrogen
Line
measurements
 So as amateurs with modest antennas we can do a good
job of measuring H Line emission - as Galactic features
fill the beam
 (TA = TB) with only a little spatial smearing
80 HPBW
Hydrogen Emission distribution
Big
Aspirations
?
Weak
Signal Detection
of Virgo A
 Where does this leave UK amateur radio astronomers?
• Each of us is working with small antennas
 >10m dia antenna too expensive for an individual
 Clubs or groups unlikely to have funds, commitment &
discipline to collaborate on large scale project
 However – it has been done !
• Dwingeloo telescope in Holland
• Stockert telescope in Germany
• Now in service for Amateur Radio
Astronomers & EME
Dwingeloo
 What are the chances of a similar UK project ?
 What a challenge that would be ……..
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Stockert
Possible
amateur
radioofastronomy
Weak
Signal
Detection
Virgo A at Goonhilly (Cornwall)
 Two large dishes at Goonhilly will soon be used for
professional Radio Astronomy – amateurs may be able to
play a part ??
Goonhilly 1 (L band)
Goonhilly 3 (C band)
Getting
Involved
at Goonhilly
Weak Signal
Detection
of Virgo A
Would you like to consider participating in
Amateur Radio Astronomy at
Goonhilly ?
Put your contact details in the book
Thank You
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Weak Signal Detection of Virgo A
Cass
example
WeakASignal
Detection of Virgo A
 For Cass A set in the galactic plane background
 Discrete source lost in the background with an 80 beam
Cass A
Cass A Temp = 3792 x 0.0153 / 0.00000166
TCassA
WA
WS
 Cass A Temp = 0.4110 K
 Background GP temp = 860 K
5 arc min
 So Cass A is hardly detectable against 860K background with a
‘Total Power’ system
 Detection of ‘point’ sources requires very narrow beams
Look at a simple spreadsheet model of the situation
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Simple
spread
sheet model
Weak Signal
Detection
of Virgo A
 Use Excel to model point source in a wide beam
Create a beam profile
Background Galactic Noise
Generate a ‘slightly noisy’
background level
80
80
Add in Cass A ‘ point source’
Cass A = 44x Background
5 arc min
Sum the background noise power
Sum the noise power + ‘point’ source
Calculate % change with point source
80
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80
Discrete
extraDetection
galactic objects
interferometry
Weak Signal
of Virgo- A
 Observing extragalactic synchrotron objects at UHF with small
antennas results in poor spatial resolution (HPBW ~180 - 3m AE )
•
•
•
•
Example: M87 / Virgo A galaxy
Only ~ 100Jy at 408MHz
Fortunately it is out of GP – less obscured
Still difficult to determine as a point source with Total Power receiver
Synchrotron emission from Milky Way GP
Total Power
Interferometer
Virgo A expected 03:12hrs
Convolution
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Weak
Signal
of Virgo A
H Line
– SkyDetection
View
1420.4MHz Image of Milky Way
13.08 GMT 28/11/2013
65
60
55
50
Elevation (degrees)
 45So as amateurs we can do a good job of measuring H Line emission
as Galactic features fill the beam (TA = TB) & only a little smearing
40
35
 30I produced H Line maps in celestial coordinates (2006)
25
 20Wanted to know what Galaxy looked line in Az and El
15
 How we would see it – if we had Radio Eyes
10
5
0
0
North
20
40
60
80
100
East
Azimuth Bearing (degrees)
120
140
160
180
South