How feasible is home radio astronomy?

[parac-EU]

We used no dish and no low noise amplifier; just a bucket as shown in the picture.
The project description can be found on
http://parac.eu/projectmk9.htm

 

[Baluncore]

We used no dish and no low noise amplifier;

Was the antenna followed by three satellite TV amplifiers in series ?
What was the RTL dongle plugged into, and controlled by ?
Where was your mobile phone at the time ?

 

[Paul Colby]

We used no dish and no low noise amplifier; just a bucket as shown in the picture.
The project description can be found on

So, the plot shown in post #55 shows a peak. How many dB above the background is it? I ask because RTL dongles have pretty high noise figures. In this link

https://www.idc-online.com/technica...se_Figure_Measurements_of_RTL_SDR_Dongles.pdf

The author measures a noise figure of 17dB at 700 MHz for a garden variety SDR. This means the radios noise temperature is 17dB hotter than room temperature. That’s pretty hot. Is the peak you’re seeing consistent with the noise temperature of the hydrogen you’re looking at?

Did some digging. This paper

https://web.mit.edu/lululiu/Public/8.14/21cm/21cm.pdf

Is an interesting read and quite basic. If I understand Figure 8, the gas you’re observing is only 45 or 50 K which is colder than 300K room temperature by a good amount. Makes me question the results in #55.

 

[Baluncore]

Is an interesting read and quite basic. If I understand Figure 8, the gas you’re observing is only 45 or 50 K which is colder than 300K room temperature by a good amount.

That estimate for galactic source temperature is about right. When the Sun is quiet, it has lower flux than the galactic centre.

The observations from the SDR only cover 13 hours. If it was an astronomical emission, galactic or solar, then we would expect to observe a daily cycle with sidereal or solar periodicity. That is not evident in the short record presented. Even with a 13-hour observation of astronomical signals, I would expect to see some artefact of the antenna pattern, swept across the source by the rotating Earth. A two element interferometer, arranged E-W, with only a couple of hours of observation, would be conclusive. Maybe they use one broad-beam bucket, rather than two, to avoid disappointment.

Until we see a longer record, something that shows a repeating daily periodicity over at least two days, we have to assume the signal being presented is local RF interference, thermal noise in the TV amplifier chain, or in the front-end of the SDR.

At this stage, all the evidence says the project is not radio astronomy.

 

[Paul Colby]

There are much simpler tests I would definitely recommend first. Trying 0 buckets would top my list. These SDRs have a wealth of artifacts, noise sources, gain drift and other limitations that need to be understood first. They could well find this peak shows up anyway. It's quite typical that not all signals detected come in through the SDR RF input.

Long term averaging wouldn't be done this way even with the best of receivers. In the simplest cases people use RF switches to continually cycle the receiver input between very stable known source and the antenna feed. I've actually attempted this using resistive loads and mechanical coaxial switches. I'm not convinced I ever got this working.

 

[parac-EU]

You said that you were not successful with your method.
So, just try our method.
If still sceptical, read att paper written by Jon Wallace (SARA).

 

[Paul Colby]

Thanks. This will get a good read. Nice antenna design. I've dusted off my coaxial switch box which allows me to pragmatically switch between antenna and a 50ohm load. This should make differencing quite a bit easier. This should help reduce the effects of receiver drift. I also have boatloads of RF amplifiers laying about.

 

[parac-EU]

You do not need an RF switch.
The plot indicates that frequency shifting is used. You compare "on" the bump with "off" the bump.
For stability; just enable the Automatic Gain Control on the SDR# gui.

Later you can experiment with all other settings.

 

[parac-EU]

Ps; That horn is not my design.
I am not the author of that paper.
I am the person who is mentioned (about) 10 times.

 

[Paul Colby]

The plot indicates that frequency shifting is used. You compare "on" the bump with "off" the bump.

I'd prefer to have some assurances that any bumps measured actually came in through the antenna feed and not some random interference or drift introduced by moving things like antennas and cables about.

This paper definitely used 2 LNAs and a bandpass filter. LNAs can significantly reduce the effects of system noise temperature, which are considerable with RTL-SDR dongles. Given that any out of band saturation of the amplifiers will add considerable noise in band, the bandpass would be better placed before and not after the LNAs.

 

[AlexB23]

Hey guys. I understand 0% of this article, but the guy made a home radio telescope using an old TV receiver.

https://www.arrl.org/files/file/ETP/Radio Astronomy/Build a Homebrew Radio Telescope-QST-0609.pdf

 

[Baluncore]

In the paper written by Jon Wallace (SARA), it appears, from the 24-hour records, that radio astronomy is being done. That system employs LNAs, based on ultra-low-noise HEMT FETs. Each stage has a gain of about 19.5 dB at 1420 MHz, at a cost of US$150 each. Those LNAs lift the RA signal to just above the thermal noise of the SDR mixer front-end. The low-cost signal amplifiers available for TV, do not have that expensive ultra-low-noise advantage.

The SDR software will also have a transform gain from the use of an FFT, which is then followed by noise reduction through Power Spectrum Accumulation, PSA. At some point in the software process, the narrowband local interference is being removed, the resulting holes in the spectrum are then being smoothed over.

 

[Paul Colby]

Here’s a reference to some SDR evaluations.

https://www.klofas.com/blog/2020/sa...odifications/Evaluation_of_SDR_Boards-1.0.pdf

They look at noise figures. I post this because the SDRs evaluated preform much better than I was expecting at 21cm. I also happen to own several of these devices. The Sdrplay radios come with internal LNAs which typically provide 4.4dB ish NF at 21 cm. This isn’t anywhere near as bad as I was expecting. I spend most of my radio time in the HF and VHF bands where the NF is quite a bit worse. Some of the newer RTL-SDR dongles are also not so bad.

 

[Paul Colby]

Before I get out a hacksaws and take multiple trips to the hardware store building an antenna, I thought I would first see if my Sdrplay RSPdx is even capable of detecting the small temperature changes needed to make observations of the 21cm hydrogen line. The radio has a LNA built in making life simple. I'm using a coaxial switch box to alternate between a 50 ohm load at room temperature and one I've placed in ice water. The temperature difference, $\Delta T = 22.2$ degrees kelvin. The change in power output expected is,

$\Delta P = 4 k_B \Delta T R$

where $R=50$ ohms. The x axis is in MHz while the y axis is mW. The blue curve is the difference in the measured power in mW with both terminators at 72 degrees F. The displacement from 0 is due to small differences in the cables, connectors, switch positions etc. Ultimately I hope to calibrate these out. The yellow curve is the same with one terminator is immersed in ice water. A ballon is used to keep it dry.

This is still very much a work in progress. It's unclear I have all my factors of 2 in place. The observed change seems smaller than expected. Anyway, I'm convinced this test shows enough life that astronomical observations using this setup are in fact possible.

The power spectra shown are 2048 bins wide. Data is collected 1000 times for each switch position and 100 such cycles were taken and averaged. Each curve is about 6 min of data collecting. Between each switch change a dwell of 10 spectra are taken and discarded to ensure switching noise has died out between data sets. The IQ sampling rate is 2MHz.

 

[Baluncore]

If you get serious about doing home RA, you will build your own two channel receiver, from FET based LNAs, with matched band-pass filters to remove the nearby and adjacent channel interference. Once sufficient gain has been applied, you can down convert the signal to an IF or baseband. By using two channels with crossed dipole antenna elements, you can select polarisation, linear or circular. Your two front-end mixers will need to use the same LO, so pick your SDR wisely.

Depending on how serious you are, those LNAs will be cooled by a cheap bar fridge, a stack of Peltier junctions, LN₂, or a helium pump refrigerator, if you should be so lucky.

For a 2MHz wide observation band, you will digitise the two signals at about a 10MHz conversion rate. That could be done by an A-D converter, or an SDR module.

Having gathered two channels at 10 megasamples each, in let us say 0.1 second, you have two records of 10⁶ samples each. You then take the FFT of the 10⁶ samples, (using a PC to compute the FFT in real time), and see a reduction in the noise floor of; √10⁶ = 10³, about +60 dB of transform gain.

Over 5 minutes, you Power Spectrum Accumulate 5 * 600 = 3000 of those spectra. That lowers the noise floor again by √3000 = 54 times, yielding another +34dB of gain, making a total of 60+34 = 94 dB in the processing after the LNAs. That is why you need multiple external LNAs in the receiver at the focus, it will get you the cool gain you need later to make deeper observations, like looking for chemical MASERS.

With a dish, bigger than a bucket, you will then be able to observe more than just our galactic centre once each day.

 

[Paul Colby]

Well, being serious about doing science and being interested in learning how things work by doing them really aren’t the same. I had serious doubts I could even detect the thermal noise from a 50 ohm terminator let alone a 10% change in said noise with the equipment on hand.

Having gathered two channels at 10 megasamples each, in let us say 0.1 second, you have two records of 106 samples each. You then take the FFT of the 106 samples, (using a PC to compute the FFT in real time), and see a reduction in the noise floor of; √106 = 103, about +60 dB of transform gain

So, this is an elementary point I’d really like to understand. I chose averaging 2048 wide FFTs rather arbitrarily. Each is scaled by 2048 to obtain mW/Hz as a vertical axis. Wider FFTs would just yield finer frequency bins. If the phenomena one’s interested in isn’t narrow band, why select finer grain bins?

 

[Baluncore]

If the phenomena one’s interested in isn’t narrow band, why select finer grain bins?

You are free to select your transform size.
For one project, I was monitoring the background noise floor across the HF band from 1 MHz to 30 MHz. I needed to look between the transmitters, which required narrow bins. There was some pulse noise, so I also kept score in a counterintuitive way, by following the minimum of every bin.

A noise pulse that falls in the time record, will raise the noise floor across the spectrum, unless you can overwrite it with zero in the time record, before the transform.

Any interfering carrier or spur that falls in your data needs to be removed by zeroing those bins in the frequency domain.

If you look at the pass-band of an SDR you will see the in-band ripple of the digital BPF, and alias spurs. That digital BPF is a source of noise that is added to the signal. For RA, it is better to build a matched pair of LC filters for the duplicated front-end LNAs.

 

[parac-EU]

The very first time I could detect hydrogen with an antenna I was amazed and overjoyed.
That is why I offer,whoever want to do likewise, my support.
Dont build complicated systems; just start simple and have success.

I have repeated the project
http://parac.eu/projectmk9.htm
I am now in Portugal and now the bucket is replaced by a tube on the ground and an ali amplifier.
The tube was positioned first to the polar star, so the milky way is circumpolar and always visible.
The beamwidth of the tube is 57*lambda/tube diameter, so about 80 degrees.

Next the tube was placed to the zenith.
I have put a flare onto it, and again according to Huygens principle with 320mm diam outer rimm, it should become 40 degrees.
This narrower beam will give more details of the milky way arms.

Directing the tube with flare more south will give a transit twice through the milky way plane, giving separate bumps on a different frequency.

The signal is strong enough; if you wat more detail, then make the beam more narrow, for instance by adding a flare or dish.


I want to add that the s/n ratio of the sdr is not so important; the first amplifier is.
See the excellent explanation by Marcus in this thread.

Further it is important that fast sample and FFT software is chosen; I use free CFRAD2.exe
This software will sample and add 100000 spectra in 5 minutes.
Also use a multicore PC, because one core will be fully occupied.
see www.parac.eu

Concerning masers; see for instance the project 'masers with a 1m dish'
http://parac.eu/projectem01.htm

 

[parac-EU]

forgot to add

 

[parac-EU]

Other good examples of home radio astronomy projects were done by the late Job Geheniau.
His website is available on

https://jgeheniau.wixsite.com/radio-astronomy

He steered his 1.5m mesch dish remotely via internet.

Here are a few project examples:
16 High Velocity Clouds
Hydroxyl maser W3(OH) on 1665 MHz
Pulsar B0329+54
Andromeda Hydrogen line
Rotation Curve of the Milky Way
drift scans of milky way
drift scans of Cygnus complex
and a lot more

So my conclusion is; home radio astronomy is feasible.

 

[Baluncore]

So my conclusion is; home radio astronomy is feasible.

By virtue of the cosmic noise it receives, every receiver that is connected to an antenna, is doing some radio astronomy.

The band from 1400 MHz to 1427 MHz is reserved for passive hydrogen line radio astronomy, above that are 4G and 5G mobile phones, but across everything is RFI as sub-harmonics from home computers and from handheld devices with processors, all now clocked at multi-GHz frequencies.

Home RA involves buckets, a coaxial cable and an SDR. There is usually insufficient signal-to-noise ratio, to actually confirm that what you are observing is astronomical.

Amateur RA is different. The antenna will be mounted away from the home, to avoid RFI. The receiver will be at the focus of a significant aperture, and will most likely have cooled LNAs, followed by a down converting mixer to the first IF. Only then will the IF be sent through a coaxial cable to the signal processing and recording equipment in a nearby structure. Ham radio EME is entry level radio astronomy.

 

[sophiecentaur]

Get creative. Find an unusual RA project, evaluate the cost of the engineering and building approvals needed for the antennas. Then think again. Rinse and repeat.

That (and the rest of your post) is very pessimistic. No one should expect a high resolution image of the RF environment out there, any more than they shouldn't expect to see beautiful coloured nebulae through their (even very expensive) telescope until they venture into astrophotography and have at least £1k to spend.

A very few hundreds of quid will get you a small reflector antenna which you can stick on an 'adjustable, not controllable' mount. The Earth will scan the universe at a given altitude once a day (and that's all day) for you. SDR is available and cheap and so is the processing power and advice to deal with your received signals. Raspberry Pie will do pretty well everything you need if you choose appropriate projects.

Compared with just better than entry level optical astronomy, RA is very good value and there is much less temptation to buy, buy, buy, to increase your astronomy equipment stable. Anyone who thinks it's easy to run out of things to do at RF, has never done 'boring research', only to find it's actually very absorbing and satisfying. And everyone has a chance to find something new to impress the world with.

"Rinse and repeat" can apply to every hobby.

 

[sophiecentaur]

There is usually insufficient signal-to-noise ratio, to actually confirm that what you are observing is astronomical.

You really don't want home RA to work, do you? Signal to noise ratio involved receiver bandwidth and, just as with deep sky imaging, you take your time and choose your target. You also need to define your wanted performance, but you do the sums first, unlike the deep pocketed imagers who's emotional decision is always that spending another £1k will solve all their present problem.

 

[parac-EU]

By virtue of the cosmic noise it receives, every receiver that is connected to an antenna, is doing some radio astronomy.

The band from 1400 MHz to 1427 MHz is reserved for passive hydrogen line radio astronomy, above that are 4G and 5G mobile phones, but across everything is RFI as sub-harmonics from home computers and from handheld devices with processors, all now clocked at multi-GHz frequencies.

Home RA involves buckets, a coaxial cable and an SDR. There is usually insufficient signal-to-noise ratio, to actually confirm that what you are observing is astronomical.

Amateur RA is different. The antenna will be mounted away from the home, to avoid RFI. The receiver will be at the focus of a significant aperture, and will most likely have cooled LNAs, followed by a down converting mixer to the first IF. Only then will the IF be sent through a coaxial cable to the signal processing and recording equipment in a nearby structure. Ham radio EME is entry level radio astronomy.

@Baluncore
I am sad to see that you keep hammering this subject so no one will even try a radio astronomy project.
Please stop that.
cryo needed? realy; do some more googling.
From your username I presume that you are a 27MHz Radio HAM; (there is no core above 1000MHz); [Insult deleted by the Mentors]

 

[Baluncore]

From your username I presume that you are a 27MHz Radio HAM; (there is no core above 1000MHz); [Insult deleted by the Mentors]

I maintained and operated a radio astronomy observatory before I retired.

 

[sophiecentaur]

@Baluncore No fond memories?
Why would you want to discourage newbies? Maybe the equipment was not off the shelf. No seriously soldering needed these days.

 

[Baluncore]

Why would you want to discourage newbies?

I warn the neophytes of how steep the learning curve really is. It may seem easy to engineer an amateur RA observatory, for some particular project. Actually implementing the design is a bigger, and somewhat more difficult task.
It is rare to find one person with all the skills needed, and sufficient time. For that reason, it takes a team to produce RA results. I have seen teams of amateurs try, but they have always fallen apart due to lack of team management, and because most of them were only dreaming of doing RA.

Maybe the equipment was not off the shelf. No seriously soldering needed these days.

Ham EME equipment is getting close to RA. You can buy the modules and plug them together, but you will still need a tracking antenna.

Serious RA uses two channels to receive orientation independent circular polarisation. Those two channels must share the same first LO, so RA can not use a cheap SDR with one input port.

The use of a coaxial cable between the antenna and the receiver makes a good attenuator, and demonstrates that the constructor had no idea of what is most critical in the RA signal budget.

 

[Baluncore]

cryo needed? realy; do some more googling.

It does not have to use cryogenic cooling, a stack of Peltier refrigeration modules will do the job at the focus of an amateur RA observatory.

The aim is to maximise signal to noise. There is little available S/N gap without expending effort. In effect, you must wedge open the S/N, to make room for the RA, by using more gain with colder amplifiers, and bigger antennas. Cooling the front end amplifiers is more feasible than building and steering a bigger dish. Peltier modules cooling the LNA is the first step.

 

[berkeman]

Thread is closed for Moderation.

 

[berkeman]

After removing some insults by @parac-EU this thread is reopened provisionally and parac-EU is now thread banned from this thread.