Exploring some Limitations in Amateur Radio Astronomy Exploring some Limitations in Amateur ...

37 Pages·2014·5.02 MB·English

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Dr David Morgan
17/5/2014 www.dmradas.co.uk
Contents
Weak Signal Detection of Virgo A
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Antennas & Receiver properties
(cid:1)
Radio source strength & spectra
(cid:1)
Limitations with small antennas
L Band Horn
3m Dish
408MHz Quagis
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Telescope Characteristics
Weak Signal Detection of Virgo A
(cid:1)
Antenna properties
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Antenna Fundamentals
Weak Signal Detection of Virgo A
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Two fundamental properties of an antenna of concern to
amateur radio astronomers
• Gain
• Beamwidth
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These are related – the higher the gain the smaller the
beamwidth
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We want both high gain and narrow beamwidth
• Gain = sensitivity
A 3m dia. dish polar diagram
• Beamwidth = spatial selectivity
Need Large antenna aperture
80 HPBW
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Antenna Equations
Weak Signal Detection of Virgo A
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Antenna Gain
A
• G = η (4 π / λ2)
η= Aperture efficiency
A= Antenna aperture m2
λ = wavelength
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
(cid:1)
Antenna Beamwidth
κ= a factor that depends on the
α = κλ D
HPBW = /
shape of the reflector and the
method of illumination
For a typical antenna
α
G = 27,000/ ( )2
Antenna diameter drives performance
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At UHF things get big
Weak Signal Detection of Virgo A
(cid:1)
Rare to find an amateur with a 9m antenna
John Smith (1924 -1998) with 9m dish
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Yagis vs Dishes
Weak Signal Detection of Virgo A
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Would not tend to use a small dish at UHF
(cid:1)
Yagi arrays probably cheaper and easier to build
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But – effective aperture must be similar to dish area
(cid:1)
So arrays will be a few metres square
(cid:1)
Complicated to construct and phase together
DL7APV Array (used for EME) Part of my 408MHz Quagi Array
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Consequences
Weak Signal Detection of Virgo A
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Can have high sensitivity and good spatial resolution at
11GHz with1m antenna
• ~40dB gain and few degrees HPBW
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Reasonable gain and resolution with 2.4m dish at C band
•
~ 35dB gain and <5 degrees HPBW
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Workable sensitivity and resolution with 3m dish at 1.4GHz
• eg 30dB gain and >5 degrees HPBW
(cid:1)
But low gain and poor spatial resolution with 3m dish at
408MHz
• eg 19dB gain and 18 degrees HPBW
(cid:1)
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 = 1-2GHz)
(IEEE)
Noise and Gain Stability
Weak Signal Detection of Virgo A
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Receiver Requirements
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Band coverage
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Available bandwidths
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Detector functions
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Sensitivity
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Noise & gain stability
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Discuss the last two items
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