Re: AstroSpy #radioastronomy #21cm #1420mhz
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David Eckhardt
1) What is the aperture of your horn? 2) How much gain do you have after the two LNA4ALL preamps? 3) What is the loss of your BPF for 1420? 4) What is you detection bandwidth? 5) What is the loss of your coaxial cable(s)? The reason for all the questions is based on our experience with a 14-foot off-boresite fed dish reflector as an antenna, a full-wave loop antenna element placed 1/4-wavelength in front of a splash plate. That is fed to (all Minicircuits) a 0.5 dB NF 23 dB gain preamp. That feeds a 1 GHz HPF to eliminate (mostly, but not completely) a near-by cell tower installation (grrrr....). The HPF feeds another preamp of the same ilk. Then some 70-feet of low loss rigid coaxial cable into the 'warm room' where we have installed roughly another 40 dB of gain. Inside, we have also installed 3 dB attenuators between active stages for impedance stabilization and gain vs. SWR stabilization. That finally feeds an AirSpy SDR. The system operates under SDR#. Our noise temperature is nominally 100 to 120 K, established (somewhat painstakingly) by several different and independent methods (not all 'calibrated' noise sources exhibit consistency in the third decimal place!!). Installation of the 1 GHz filter to get rid of a very dirty cell tower installation is responsible for half the noise temperature...... As you are aware, the H1 emission can be up to 500 to 750 kHz wide. You are detecting only a small amount of the power of the H1 emission with the SDR#-limited available max bandwidth. With this system, we are easily able to detect the H1 line with the antenna drift scanned across SDR-A* or the center of the galaxy on the SDR# RF display with no additional processing. We are also able to detect some structure in the target rather than just a 'blob'. However, until recent installation of a hack from Russian SW, we were limited to the bandwidths allowable with SDR#. That had severely limited our detection threshold of the H1 emission as most of the power from the target emission was not being gathered for analysis. With 15 or 30 kHz bandwidths under SDR#, it took post-detection processing to bring out the H1 line. With the hack which allows roughly 0.5 MHz of bandwidth the H1 line is easily visible on both the spectrum and waterfall display under SDR#. Another check of your system is that you should 'see' the LO roughly 10 MHz below (in our case) the H1 emission. We do not use AstroSpy, but, rather, the full 10 MHz (to view the LO), and 2 MHz to yield display of the H1 line on the spectrum and waterfall. The reason for that is AstroSpy does not allow for AirSpy parameter adjustments as does the 'normal' operation of the SDR under SDR#. Your trace looks much like you have detected the H1 emission, but many variables need to be considered before one can nail it. I've given you our experiences gained over some three years operation of our H1 'scope. Hopefully, it bounds you detection threshold. A good 1420 MHz low-level signal source is highly usable. I'll attach a write-up of a source I did and is published in the SARA Journal (Society of Amateur Radio Astronomers) a couple of years ago. The comb generator output in the attachment should not be connected to any efficient antenna it is highly likely to cause interference, but is a highly useful tool in setting up an H1 radio telescope. See the attachment. This installation is part of the offerings at the Little Thompson Observatory on the high school grounds in Berthoud, Colorado. It is offered as an educational tool for the school, the community along the Front Range, and physics department and is totally supported technically by us volunteers and is a completely volunteer organization. You can access us at <starkids.org>. We also annually work with a small group of STEM students on projects associated with radio astronomy. The observatory also offers 18" and 24" optical telescopes for the school and once a month to the public on "Public Star Nights". We also offer at no charge 'special events' of scientific interest to the community on a request basis. Dave - WØLEV Volunteer: Little Thompson Observatory
On Sun, Jun 9, 2019 at 10:34 AM <erbe.joshua@...> wrote: I have the Airspy Mini and I have built a horn antenna, I'm using 2 LNA4ALLs and a 1420-1470 MHz bandpass filter in between them. I think I have picked up the hydrogen emission signal. I would like to know more about Astro Spy software. Specifically, what is it doing? What does the the integration time do? Does the program average the signal strength over the time indicated by the Integration time box? -- Dave - WØLEV Just Let Darwin WorkJust Think
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