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Extra-Galactic Hydrogen Line Observations

A System for Observing Neutral Hydrogen Line Emissions in Nearby Galaxies


System Design

The system design will be described in order starting from the antenna right through to the results display.

Antenna

The antenna used is a re-purposed 3 metre diameter TVRO mesh dish...

The gain @ 1420 MHz is calculated to be about 30 dBi with a 3 dB beamwidth of about 60. The dish is attached to an azimuth-elevation style mount - but is motorised in elevation only.  The azimuth direction is set manually by hand.  The elevation movement is adjusted semi-manually via a TVRO actuator and is set by applying power to the actuator while monitoring elevation via a digital inclinometer...

The dish, therefore, has no real-time tracking ability and is operated in drift scan mode only.

The azimuth-elevation mount is the standard polar-type mount supplied with the dish, but adjusted such that the polar rotation axis is exactly horizontal - and so becomes the elevation axis...

The azimuth axis is the mounting pole and azimuth settings are done by loosening the bolts securing the mount to the pole and rotating the whole mount.

To get the elevation axis exactly horizontal required a slight modification to the mount.   The travel of the polar axis setting is limited by the minimum length possible of the adjustment arm and so the lower mounting bolt was removed to allow greater movement.   The adjustment to exactly horizontal can now be done...

...with the weight of the dish keeping the adjustment arm en-trapped in its normal u-bracket cup mount location.

Feed

The feed at the focus of the dish is a DL4MEA-type loop tuned for 1420 MHz...

Adjustment can be made to position the feed at the focus position.

First Low Noise Amplifier (LNA)

The first LNA is mounted right behind the feed groundplane...

...and is a 0.8 dB NF 20dB gain unit bought on eBay...

This LNA model has been tested (by another party) against a Mini-Circuits ZX60-33LN and found to have similar characteristics with slightly better NF.

A 5 metre length of LMR-195 brings the signal down to the base of the dish mount where filtering and further amplification takes place.

Bandpass Filter

The bandpass filter placed after the first LNA is a unit bought from M. Leech (SBRAC)...

...which gives a passband from about 1395 MHz to 1430 MHz...

This filter reduces spurii from intermodulation products caused by strong signals at frequencies outside the range of interest.

Line Drivers

After the bandpass filter two amplifiers are run in series to raise the level of the signal to overcome the loss in the 25 metre run of RG-213/U coaxial cable from the antenna base into the observatory station...

The first one is a LNA-1800 from RF Bay (NF=2.2 dB, gain=30 dB).   The second one is a ZKL-2R7 from Mini-Circuits (NF=5 dB, gain=25 dB).

The two amplifiers and the bandpass filter are housed in an enclosure made from 90 mm diameter plastic plumbing tube stock and appropriate end caps...

Second Low Noise Amplifier

The second LNA is located inside the observatory and is the same model 0.8 dB NF 20dB gain unit bought on eBay.  This LNA is primarily used to amplify the signal to a level where gain settings of around 30 dB can be used on the following RTLSDR dongle to ensure enough bits of digitisation occur.

At this point in the RF chain an LNA is not needed (as a higher noise figure general purpose amplifier will not degrade the noise figure significantly), but it was available and so was used.  The LNA could be replaced by an MMIC amplifier (ERA-5) without significant NF penalty.

NOTE: this 2nd LNA was removed in later activities as it was found that the dongle seemed more stable running at maximum gain (49.6 dB) with Digital AGC enabled and input levels which exercised only about 5 bits of the 8-bit ADC range.  This may be entirely a local effect and no statement is made other than what was observed.

RTLSDR Dongle

The ubiquitous RTLSDR dongle is actually a DVBT USB-based receiver re-purposed by some clever people.  They have made available a range of software which allows access to the 8-bit IQ RF data stream.  This system uses a modified (by the author) version of the provided 'rtl_sdr.exe' console application.

The dongle itself has been modified to accept an external TCXO 28.8 MHz input - improving the stability of the sampling rate...

...and a tinplate (from the lid of a salmon tin) fin has been soldered to the pad below the RTL2832U chip onto which a small 5V fan blows air.  The provision of this heatsink has been shown to improve the stability of the data stream - especially at the higher sampling rates.

An unmodified unit shown in the foreground.

Installation instructions can be found hereThis is left as an exercise for the reader to achieve. That is, please do not ask the author for help with this - a non-response may offend...   Remember - Google is your friend...

If you can get your dongle to work with SDRSharp then it should work with the console applications which can be used to capture data from the dongle and save to a file.

Details about RTLSDR and available applications can be viewed here and here.

Data Stream Interface

The IQ data is acquired via a USB interface connected to the RTLSDR dongle.  General wisdom says that the dongle should not be plugged in directly into the PC USB port, but connected by a good-quality cable of a few metres in length.  This is to reduce the level of RFI induced from the PC.   The dongle is a USB 2.0 device, so any port capable of supporting that speed should suffice.  If an external USB hub is used it is strongly recommended that it be a self-powered hub.  Ideally a dedicated internal USB card would be used, with only the dongle plugged in to ensure unfettered access to bandwidth and power.

Data Acquisition Software

The data acquisition consists of a special version (produced by the author) of the standard console application 'rtl_sdr.exe' and a Windows C# GUI wrapper.   The author has modified the source code and recompiled to produce a version of 'rtl_sdr.exe' which uses 64-bit variables - necessary to handle files > 4 Gb in size.  At a nominal sampling rate of 2.4 Msps the data size produced is about 17 Gbytes/hour.

The modified 'rtl_sdr.exe' console application is called from within the Windows GUI application with parameters set via the GUI.  The IQ data is saved in a binary unsigned 8-bit file identified by a number of parameters in the filename.  These filename parameters are used by the data analysis software to identify the conditions under which the data was acquired - e.g., centre frequency, sampling rate, doppler correction, time of acquisition, hardware configuration, target object...

The GUI calculates the time when the target passes through the antenna beam and initiates a scheduled data run of a duration corresponding to the passage of the target through the beam.

Note: the GUIs shown here are for illustration purposes only.  Please don't request copies - they are bespoke tools for the particular system setup here and I have neither the time nor the inclination to provide support.  Sorry.

Data Analysis Software

The data analysis software is accessed through a separate GUI window.   Here various analysis parameters can be set and the result displayed...

The data analysis GUI allows various graphical result display options including display limits, tilt correction, offset, running average.  A typical result display is shown below...

Results can be exported to other programs via the clipboard or to a graphic file in various formats.

Details about the analysis process can be viewed by clicking on the 'Data Analysis' tab.