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Weak Signal Detection of Virgo A
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Exploring some Limitations in Amateur Radio Astronomy
Amateur Radio Astronomy
Dr David Morgan
Exploring some Limitations in Amateur Radio Astronomy
17/5/2014
Weak Signal Detection of Virgo AContents
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3m Dish L Band Horn408MHz Quagis
Antennas & Receiver properties
Radio source strength & spectra
Limitations with small antennas
Weak Signal Detection of Virgo ATelescope Characteristics
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Antenna properties
Weak Signal Detection of Virgo AAntenna Fundamentals
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
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80 HPBW
Need Large antenna aperture
A 3m dia. dish polar diagram
Weak Signal Detection of Virgo AAntenna Equations
Antenna Gain
• G = η (4 π / λ2) A η= Aperture efficiency
A= Antenna aperture m2
λ = wavelength
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Antenna Beamwidth
For a reflector antenna, the area is simply the projected area.
For a circular reflector of diameter D, the area is A = π D2/4 and the gain is
G = η (π D / λ )2
HPBW = α = κ λ / Dκ= a factor that depends on the
shape of the reflector and the
method of illumination
For a typical antenna
G = 27,000/ (α )2
Antenna diameter drives performance
Weak Signal Detection of Virgo AAt UHF things get big
Rare to find an amateur with a 9m antenna
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John Smith (1924 -1998) with 9m dish
Weak Signal Detection of Virgo AYagis vs Dishes
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
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DL7APV Array (used for EME) Part of my 408MHz Quagi Array
Weak Signal Detection of Virgo AConsequences
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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Weak Signal Detection of Virgo AResult of Antenna limitations
L band is probably most practical,
useful and affordable option
for amateurs
(L Band (IEEE) = 1-2GHz)
Weak Signal Detection of Virgo ANoise and Gain Stability
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Receiver Requirements
Band coverage
Available bandwidths
Detector functions
Sensitivity
Noise & gain stability
Discuss the last two items
Weak Signal Detection of Virgo ALNAs and Receivers
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.
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ICOM IC-R7000 Receiver FunCube Dongle SDRRealtek RTL2832U DVB-TV dongle
Weak Signal Detection of Virgo ASDR Dongle Receivers
The common SDR Dongles have gaps in frequency coverage
FCDPro
RTL
FCDPro+
1MHz 10MHz 100MHz 1GHz 2GHz
Frequency
60 1.1 1.27 1.7
240 420 1.9
26 1.1 1.25 2.2
Device
Frequency Coverage for FCD & RTL Dongle Devices
408MHz 1.4GHz
Coverage Gaps
Weak Signal Detection of Virgo ANoise /Gain Stability
FunCube Dongle Pro + :
Stability is better than 0.05dB over 3 hours
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Cheap stable SDR receivers widely available
No important receiver limitations
Weak Signal Detection of Virgo AObjects to Observe
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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 Angular Size
Weak Signal Detection of Virgo ASource Signal Strengths
Look at typical source signal strength
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Po
we
r F
lux
De
ns
ity (
W m
-2 H
z-1
)
10-15
Broadcast signals 100µµµµV/m
10-17
1µµµµV/m
10-19
10-21
10-23
10-25
1Jy=10-26 W m-2 Hz-1
Assuming 6kHz bandwidth
10
10
31
05
10
71
09
1
011
Po
we
r F
lux
De
ns
ity J
y
(10
-26W
m-2
Hz
-1)
Communication receiver
Sun storms
Supernova remnants
Radio galaxies
Jupiter
Pulsars
Weak Signal Detection of Virgo ARadio Sources – signal strengths
In more detail
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1 1
0 1
02
10
310
410
510
610
710 8
10
9
Po
wer
Den
sit
y
Jy
(10
-26
W m
-2H
z-1
)
Solar storms& plages
Quiet sun
Solar bursts
Su
n
Ta
uru
s A
Vir
go
A
Pu
lsa
rs
Ju
pit
er
Ca
ss
iop
eia
A
Cyg
nu
s A
Ori
on
Ne
b.
These are ‘continuous’ sources –not dealing with radio transients
Weak Signal Detection of Virgo ASource Spectrum drives receiver frequency
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
SUN : Thermal Source SNR : Synchrotron Source Galactic Hydrogen : Line Source
quiet sunstorms
bursts
1cm 3cm 10cm 1m 3m 10mWavelength
10
410
10
610
1
0
10
10
10
Po
wer
flu
x d
en
sit
y (
10
-26W
m-2
Hz
-1)
Hercules
Virgo
Cygnus
Cassiopeia
1cm 10cm 1m 10mWavelength λλλλ
1
1
02
10
310
4
Po
wer
flu
x d
en
sit
y
(10
-26
W m
-1H
z-1
) P λλλλx (λλλλ=wavelength, x=spectral index) 1420MHz 1421MHz
H Line Spectrum
Weak Signal Detection of Virgo ARadiation Mechanisms
Source Spectra : Three mechanisms – Three spectra
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1cm 10cm 1m 10mWavelength λλλλ
1
1
0
1
02
10
3
Rad
iate
d P
ow
er
P
xy
z
P=εεεε kT/λλλλ2
K = Boltzmann's constT= temperature
ε λε λε λε λ2222 ε=1ε=1ε=1ε=1
Semi-transparent opaque
Thermal Spectrum
Ion
moving electron
electromagneticemission
Synchrotron radiation(non thermal)
Ionised gas(thermal)
1cm 10cm 1m 10mWavelength λλλλ
1
1
0
1
02
10
3
Rela
tive p
ow
er
flu
x
Synchrotron Spectrum
magnetic field
charged particle
vr
circularpolarization linear
polarization
1cm 10cm 1m 10mWavelength λλλλ
1
1
0
10
2
Rela
tive p
ow
er
flu
x
Hyd
rog
en
21cm
OH
18cm
Meth
an
ol 4
cm
Line spectrum
electron reverses spin & radiates a photon
protonproton
electron
Weak Signal Detection of Virgo AWhat can amateurs measure? – 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)
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11GHz image of SUN
~20
Using L band for H line
• Measuring Doppler shifts & mapping galaxy
• Reasonable spatial resolution achievableGalactic 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 detectGalactic Synchrotron
Try using UHF for Synchrotron emissions
• Higher signal but worse antenna gain – no improvement
• HPBW rather poor – limited spatial resolution
Extra galactic interferometry
Weak Signal Detection of Virgo A11 GHz Radiometer image
School Radiometer project – show some principles of Radio Astronomy
SatellitesSatellites
SUN was
behind building
hereUnknown signal
sources Can see radio emission
from chimneys
South20 0 East of South West North West Azimuth
Ele
vation 0 to 4
50
0 0
0 0
45 0
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Weak Signal Detection of Virgo A
Galactic H line
De
cli
na
tio
n (
de
gre
es
)
22 21 20 19 18 17
Right Ascension (hrs)
Galactic Centre
Cygnus Arm
-10
0
1
0
2
0
3
0
4
0
5
0
6
0
Hydrogen Line Velocity Distribution
As amateurs we can measure the intensity, spatial distribution and velocities of Hydrogen in the galactic plane.
Galactic Longitude
Velo
cit
y
Fre
qu
en
cy
Hydrogen Velocity v Galactic Longitude
Plane
Weak Signal Detection of Virgo A
0 20 40 60 80 100 120 140 160 180
Azimuth Bearing (degrees)
Ele
vation (
de
gre
es)
0
65
60
55
50
45
40
35
30
25
20
15
10
5
1420.4MHz Image of Milky Way 13.08 GMT 28/11/2013
North SouthEast
HPBW
Galactic Centre
Cygnus arm
Ground Noise
Weak Signal Detection of Virgo AWhat is difficult for amateurs?
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 ?
Weak Signal Detection of Virgo AWide beams & point sources
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
ΩA = Antenna beam solid angle, Pn = polar responseMB
TB = source brightness Temp, ΩS = source solid angle
(10 K=1.38x10−23 W Hz−1)
http://www.cv.nrao.edu/course/astr534/AntennaTheory.html
Weak Signal Detection of Virgo ATwo examples
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.
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Using an antenna main beam HPBW = 80
Angular diameter = 5 arc min, TB= 37920K
Background galactic plane Temp = 860K
80 beam
Cass A
Example: Cass A
Example: SUN
Weak Signal Detection of Virgo ACass A example
For Cass A set in the galactic plane background
Contribution from Cass A Temp = 0.4110 K
Background GP temp = 860 K
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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5 arc min
Cass A
Weak Signal Detection of Virgo ADiscrete Source is lost in background
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.
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Without access to a large antenna theonly practical way for amateurs to observe
point sources is with Interferometry
Weak Signal Detection of Virgo A
This table summarises the issues when restricted to small antennas
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Frequency Performance Possible Sources Remarks
KU Band 11GHz High gain, narrow
beams (<20)
Few of interest ‘High performance’
system but little of
interest to detect
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)
H Line is best target for amateurs
Weak Signal Detection of Virgo AHydrogen 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
Hydrogen Emission distribution80 HPBW
Weak Signal Detection of Virgo ABig Aspirations ?
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
What are the chances of a similar UK project ?
What a challenge that would be ……..
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Dwingeloo
Stockert
Weak Signal Detection of Virgo APossible amateur radio astronomy 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)
Weak Signal Detection of Virgo AGetting Involved at Goonhilly
Thank You
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Would you like to consider participating in Amateur Radio Astronomy at
Goonhilly ?
Put your contact details in the book
Weak Signal Detection of Virgo A
Weak Signal Detection of Virgo ACass A example
For Cass A set in the galactic plane background
Discrete source lost in the background with an 80 beam
Cass A Temp = 0.4110 K
Background GP temp = 860 K
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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Cass A Temp = 3792 x 0.0153 / 0.00000166
TCassA ΩΩΩΩΑΑΑΑ ΩΩΩΩS
5 arc min
Cass A
Look at a simple spreadsheet model of the situation
Weak Signal Detection of Virgo ASimple spread sheet model
Use Excel to model point source in a wide beam
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Create a beam profile
Generate a ‘slightly noisy’ background level
80
80
Background Galactic Noise
Add in Cass A ‘ point source’
80
80
Cass A = 44x Background
5 arc min
Sum the background noise powerSum the noise power + ‘point’ sourceCalculate % change with point source
Weak Signal Detection of Virgo ADiscrete extra galactic objects - interferometry
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
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Synchrotron emission from Milky Way GP
Virgo A expected 03:12hrs
Total Power
Interferometer
Convolution
Weak Signal Detection of Virgo AH Line – Sky View
So 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
I produced H Line maps in celestial coordinates (2006)
Wanted to know what Galaxy looked line in Az and El
How we would see it – if we had Radio Eyes
0 20 40 60 80 100 120 140 160 180
Azimuth Bearing (degrees)
Ele
vation (
de
gre
es)
0
65
60
55
50
45
40
35
30
25
20
15
10
5
1420.4MHz Image of Milky Way 13.08 GMT 28/11/2013
North SouthEast
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