Evaluation : 4 Small Radio Telescope Systems for Hydrogen Line Acquisition

Evaluation  : 4 Small Radio Telescope Systems for Hydrogen Line Acquisition 

                                                                                                            Alex Pettit   Oct 2023

A Qualitative Comparison of four radio telescope systems was performed

using parabolic reflectors from  1.2 meters to 3.7 meters in diameter .

Two systems used Spectra Cyber 1420 MHz Spectrometers,

 Two used Software Defined Radio ( SDR ) ( Receiver / Analog to Digital Converter ) Systems.

 

 The 1.2m  SDR and the Spectra Cyber 3.7m system used  the same RAS (WD5AGO) Low Noise Amplifier .

The  SDR based 3.7m radio telescope used the nooelec SAWbird+ H1  LNA

 

Data Acquisition

The data sets were from 24 hour drift scans at a Declination of approximately + 40 degrees .

    (  1.2m +40⁰,  2.7m  +40⁰,  3.7m_SDR +36⁰ , 3.7m_SpecCy +37⁰ ) . 

Each scan ( file ) was acquired over a 4 to 5 minute time frame ( 1⁰ to 1.25⁰ drift in RA )

 

Comparisons

 

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1.2m Dish with SDR  Rx/ADC

 

The 1.2 m dish shows the greatest difference between Cold_Sky and Peak H1 brightness .  This probably  the result of the deep f/D = 0.27 reflector reducing the 290K ground noise pickup.

 

The wide beam-width of the 1.2m reflector captures a wide area of the sky but lowers its ability to resolve details.

 

With a software defined radio (SDR) , all frequency channels are simultaneously acquired .  A 300 second scan allows 300 seconds of data to be acquired and averaged for each frequency channel thus providing good statistical convergence (smoothing)  of the random data.

 

Software :  Air Spy SDR# Studio with IF_Ave plug-in for acquisition, averaging and file saving.

The dB scale  is  referenced to  Cold_Sky  = 0 dB.

Cold_Sky vs Peak H1 brightness ( 2.9 dB ) =  1.95:1 Power Ratio

The plot amplitude calibration was verified using  a series of attenuators.

The data file  frequency resolution :  2.93 kHz  ( 512 ch FFT )

Antenna Beam-Width FWHM 1.2meter dish  = 21.1   degrees

 

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2.7m and 3.7m Dish with Spectra Cyber Spectrometer Analyzer

 

These systems show the result of using a  Spectra Cyber  "stepped frequency scanner " Spectrometer.

 

By setting a Sample and Integration Time of 1 second, each frequency channel takes one second to be acquired. 

A full frequency set of 280 channels thus requires 280 seconds, but each frequency channel is averaged over only one second .

 

The Spectra Cyber displays and saves data scaled in "Volts".  It is related to signal power, but this has

not as yet been quantified .   The plot amplitude , "Volts" ( power) ;  the frequency scale is ' freq channels'

 

There is more spatial detail being resolved in the 2.7m and 3.7m dishes over  the 1.2m, but the lower signal from one second duration per channel acquisition and lessened statistical averaging masks much of that improvement.

The 2.7m  and 3.7m systems have similar H1 Peak signal levels relative to Cold_Sky.

There is slightly more spatial resolution shown in the larger 3.7m  reflector  data.

 

Software :   Spectra Cyber Software for acquisition,  averaging and file saving.

The ( uncalibrated)  power  amplitude is referenced to   Cold_Sky = ' 1.0 '

Uncalibrated  Cold_Sky vs Peak H1 brightness for each  ~  5.2:1 (Power  Ratio  ?)

The data file  frequency resolution :  5.0  kHz

Antenna Beam-Width FWHM   2.7meter dish =  9.40  degrees

Antenna Beam-Width FWHM   3.7meter dish =  6.85 degrees

 

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3.7m Dish with SDR Rx/ADC

 

The 3.7 m dish shows up to 2.1 dB difference between Cold_Sky and Peak H1 brightness  .

The more narrow beam width allows this antenna to resolve  greater spatial details.

 

With a software defined radio (SDR) , all frequency channels are simultaneously acquired.  A 300 second scan allows 300 seconds of data to be acquired for each frequency channel thus providing  good statistical convergence (smoothing) of the random data.

 

Software :  Air Spy SDR# Studio with IF_Ave plug-in for acquisition,  averaging and file saving.

The dB scale  is  referenced to  Cold_Sky = 0 dB.

Cold_Sky vs Peak H1 brightness ( 2.1 dB ) =  1.62:1 Power Ratio

The data file  frequency resolution :  6.24 kHz ( 512 ch FFT )

Antenna Beam-Width FWHM = 3.7meter dish =  6.85 degrees

 

 

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Hydrogen Line Resolution : 1.2m Parabolic Dish vs 3.7m Parabolic Dish

 

An interesting performance comparison of two parabolic reflectors

Both Spatial Brightness and Doppler Velocity Resolution is correlated to the beam size.

 

The 1.2m dish captures a larger area of the sky but at a reduced resolution . It creates an overall average in Brightness and Doppler shifted Velocity from its wide 'field of view' of Milky Way neutral hydrogen clouds.

 

The more narrow beam-width of the 3.7m dish captures a smaller  'slice' of the neutral hydrogen clouds.

This enhances its ability to resolve details in Brightness and Doppler shifted Velocity within its  'field of view'.

 

Milky Way Drift Scans  @  Dec +40 Degrees, RA  00:00 to 08:00 Hrs

Alex KK4VB