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