Helping my 13 and 15 yr old daughters with home-built 21-cm telescope first-light spectra of the sun (below): is our spectra correct?

[Eric Ross]

I am helping my 13 and 15 year old daughters build a hydrogen-line 21-cm telescope for a Milky Way study (training the next generation of scientists: they are doing all the work and I am an advisor). We had our first complete system test yesterday (image below) but question the results. We'd appreciate any advice you might be able to provide us.

We first tested our SDR/system with an FM antenna at FM frequencies to make sure the software/collection/SDR worked (which it did). We then connected our home-built 1.7m Yagi-Uda antenna and as a test yesterday obtained spectra of the sun (pointed directly at the sun). And it appears we obtained some kind of signal at roughly 1.4024 GHz when pointed directly at the sun which is roughly what we expected. We also checked to make sure by pointing the antenna 90 degrees away from the sun in multiple directions (virtually zero signal, but a tiny amount), disconnecting power to the LNA (no signal we could discern from the noise) and disconnecting the antenna completely (just noise). So it would seem we are obtaining some sort of real signal of the sun.

However, in my naivete I had expected at best we might obtain a wide blob spectra at 1.420405 GHz. But instead we obtained multiple distinct spectral lines around 1.420405 GHz but none exactly on the expected midpoint. Also, they were strong but "intermittent" (i.e. they were mostly present, but then disappeared for short times) -- you can see the waterfall spectra in blue/yellow. These aren't averaged signals: the chart is just raw in from the SDR (I assume if we average the signals we'd get very nice signal/noise but very sharp peaks at the same frequencies not exactly at 1.420405 GHz).

So are these actual signals? (I hope they are because I can't explain them otherwise.) Are the separate lines simply some sort of ringing or harmonics around a midpoint or issue with our electronics or antenna? -- something we can/should fix? Any advice you might have for us? -- we're excited to move to the next step (night observations ideally of the Milky Way) but want to understand/fine-tune our equipment and results first.

Thank you for your expertise.

(if the below chart is difficult to read, the spectra starts at 1,420.0 MHz on the far left and goes by 0.2 MHz increments, ending at 1,421.8 MHz on the far right.)

[Eric Ross]

Sorry, my pasted-in spectra didn't come through. It's attached here.

[superkuh]

To me this looks like the third harmonic of 470 MHz DMR mobile radio basestation/handsets. So they're not real signals at 1420 MHz but they are real at ~470 Mhz. Try looking around down there to see if you see the same thing. If it is then a high pass filter >1GHz would help.

[Eric Ross]

superkuh,

Thanks so much for your reply. We'll take a look over the next few days.

We have a SAW filter with 65MHz width centered at 1.42 GHz surrounded by two LNA's (from Nooelec). We'll check the signal at 470 MHz to see if this is the case but we had hoped that this filter in-line would clean up the signal. 

We can add another bandpass filter to the setup as you suggest, but should we filter out the signal directly from the antenna before heading into the first LNA?

so instead of 

<ant> -- <LNA> -- <SAW> -- <LNA> -- <coax> -- <SDR> 

we should have:

<ant> -- <filter> -- <LNA> -- <SAW> -- <LNA> -- <coax> -- <SDR> 

I didn't think this signal was interference with human produced radio signals because when we turned the antenna away from the sun by 90 degrees (in both dimensions) the signal disappeared, and when the antenna was pointed straight up at the sun the signal was strongest. So does this imply that instead of making a proper Yagi antenna, we instead made a Yagi antenna that has stronger absorption at 90 degrees off axis?

I also saw your discussion on http://superkuh.com/ and realized that we didn't shield our components. I know in the "final" version we would, but just trying to get first light I thought we might skip it. Is this where the noise is coming in from?

We'll give this a try and let you know.

Thank you,

Eric

[Marcus D. Leech]

On 06/30/2020 12:06 AM, Eric Ross wrote:

superkuh,

Thanks so much for your reply. We'll take a look over the next few days.

We have a SAW filter with 65MHz width centered at 1.42 GHz surrounded by two LNA's (from Nooelec). We'll check the signal at 470 MHz to see if this is the case but we had hoped that this filter in-line would clean up the signal. 

We can add another bandpass filter to the setup as you suggest, but should we filter out the signal directly from the antenna before heading into the first LNA?

so instead of 

<ant> -- <LNA> -- <SAW> -- <LNA> -- <coax> -- <SDR> 

we should have:

<ant> -- <filter> -- <LNA> -- <SAW> -- <LNA> -- <coax> -- <SDR> 

I didn't think this signal was interference with human produced radio signals because when we turned the antenna away from the sun by 90 degrees (in both dimensions) the signal disappeared, and when the antenna was pointed straight up at the sun the signal was strongest. So does this imply that instead of making a proper Yagi antenna, we instead made a Yagi antenna that has stronger absorption at 90 degrees off axis?

I also saw your discussion on http://superkuh.com/ and realized that we didn't shield our components. I know in the "final" version we would, but just trying to get first light I thought we might skip it. Is this where the noise is coming in from?

We'll give this a try and let you know.

Thank you,

Eric

Putting a filter IN FRONT of the first LNA stage is very likely to kill any sensitivity you might have.

Your problem is likely self interference.  How far away is your antenna/LNA/filter assembly from your computer and receiver?  Also, YAGI antennae
  are fairly sensitive to measurement errors in construction, making them sensitive in directions other than what you might expect.

You might find that a large pyramidal horn would be more successful.  Or even a simple 1m parabolic dish with a circular-waveguide feed.

[Michiel Klaassen]

Hi Eric and Daughters,

Indeed, most of us here have good results with a Can type horn.

see also  http://parac.eu/projectmk9.htm

Most important of all is that you do the same as in optical astronomy; stacking.

When you have a small telescope, you have to stack a lot of pictures on top of each other to see a result. 

In radio astronomy the pictures are spectra; you must use software to stack some 100000 spectra together.

see also the article of Kenneth W. Kornstett in the last SARA journal; april 2020.

""Hydrogen Line (H1) Antenna Made from Coffee Cans and a Wood Duck Box Pole Snake

Guard""

Regards,

Michiel  

parac.eu

Op di 30 jun. 2020 om 05:25 schreef Marcus D. Leech <patchv...@gmail.com>:

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[Jon Wallace]

Eric,

I saw Michiel just responded with most of what I was going to say - integration/stacking is critical. When I made my first SDR I detected a bunch of junk because I was using SDR# and reading quick scans. When I increased my integration time to 5 minutes then I really saw what I was searching for. I also have an article on SARA if interested - here is the link -

http://www.radio-astronomy.org/pdf/JonWallace-Article-SARAJournal-Feb-2019.pdf

it is a step-by-step of my experience with a small horn antenna SDR. There are many way to do this and many people on this list who are willing to help.

Best of luck!

Take care and stay safe!

Jon

[Raydel CM2ESP]

Hi Eric,

Those are terrestrial signals. Under certain conditions if the transmitters are very powerful and nearby they can get into the system, even using SAW, and in many other cases, they are just entering the system if the LNA is not shielded of even through the coax or some connector.

On Hydrogen profiling, you need a massive amount of Hydrogen to get the spectral line, so using such kind of antenna and pointing to the Sun, there will be more Solar Continuum (Solar RF noise) than actual Solar Hydrogen. You need a lot of Hydrogen, like the whole Galaxy arm to be able to get spectral monitoring.

I bet others, which are really experts can explain better.

Regards,

Raydel

On 30/06/2020 04:06, Eric Ross wrote:
> superkuh, > > Thanks so much for your reply. We'll take a look over the next few > days. > > We have a SAW filter with 65MHz width centered at 1.42 GHz surrounded > by two LNA's (from Nooelec). We'll check the signal at 470 MHz to see > if this is the case but we had hoped that this filter in-line would > clean up the signal. > > We can add another bandpass filter to the setup as you suggest, but > should we filter out the signal directly from the antenna before > heading into the first LNA? so instead of <ant> -- <LNA> -- <SAW> -- > <LNA> -- <coax> -- <SDR> we should have: <ant> -- <filter> -- <LNA> > -- <SAW> -- <LNA> -- <coax> -- <SDR> > > I didn't think this signal was interference with human produced radio > signals because when we turned the antenna away from the sun by 90 > degrees (in both dimensions) the signal disappeared, and when the > antenna was pointed straight up at the sun the signal was strongest. > So does this imply that instead of making a proper Yagi antenna, we > instead made a Yagi antenna that has stronger absorption at 90 > degrees off axis? > > I also saw your discussion on http://superkuh.com/ > <http://superkuh.com/rtlsdr.html> and realized that we didn't shield > our components. I know in the "final" version we would, but just > trying to get first light I thought we might skip it. Is this where > the noise is coming in from? > > We'll give this a try and let you know. > > Thank you, > > Eric > > On Monday, June 29, 2020 at 7:11:18 PM UTC-4, superkuh wrote: > > To me this looks like the third harmonic of 470 MHz DMR mobile radio > basestation/handsets. So they're not real signals at 1420 MHz but > they are real at ~470 Mhz. Try looking around down there to see if > you see the same thing. If it is then a high pass filter >1GHz would > help. > > -- -- You received this message because you are subscribed to the > Google Groups "Society of Amateur Radio Astronomers" group. To post > to this group, send email to sara...@googlegroups.com To > unsubscribe from this group, send email to > sara-list-...@googlegroups.com For more options, visit this > group at http://groups.google.com/group/sara-list?hl=en --- You > received this message because you are subscribed to the Google Groups > "Society of Amateur Radio Astronomers" group. To unsubscribe from > this group and stop receiving emails from it, send an email to > sara-list+...@googlegroups.com > <mailto:sara-list+...@googlegroups.com>. To view this > discussion on the web visit > https://groups.google.com/d/msgid/sara-list/7ea5fd14-32b0-4de1-888e-9260a0943101o%40googlegroups.com > <https://groups.google.com/d/msgid/sara-list/7ea5fd14-32b0-4de1-888e-9260a0943101o%40googlegroups.com?utm_medium=email&utm_source=footer>.

[Jon Wallace]

Eric,

Raydel made a good point - the Sun is a 'small' object (0.5 degrees) so if your beamwidth is large you can't detect the Sun (I discuss it more in my paper - for my horn antenna it was something like 0.001 of the area of the beam!). If you can figure out where the galaxy is going to be, set your alt and az to catch it and try running a few hours before and let it drift into your antenna beam and collect data for a few hours after - that way you 'should' see a curve within your graph.

Take care!

Jon

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[Eric Ross]

I want to thank everyone here for the huge number of very insightful and helpful replies. They are very much appreciated. From the replies I think there are a clear number of steps:

1) I am a bad advisor for my daughters because I thought testing our hydrogen-line detector on the closest massive collection of hydrogen (the sun) made logical sense. I see from a number of comments that this was a bad assumption (thanks Raydel -- I didn't know this about the solar continuum but given the immense heat this probably makes sense) (and thanks Jon -- I had assumed the roughly 23 degree sensitivity of our Yagi-Uda antenna would easily capture the 0.5 degrees of the sun, but that, too, was a bad assumption). So we'll try for the Milky Way. Incidentally, we plan on fully characterizing the antenna we built, but the beam width should be roughly 23 degrees (in the direction of the antenna).

2) We'll check for the 470 MHz signal to see if what we are seeing is DMS radio third harmonics (thanks superkuh). Question on harmonics: the antenna we built is supposed to collect our desired signal (that's supposedly the point of most of the waveguide rods in the Yagi-Uda antenna design) and attenuate signals meaningfully outside our desired wavelength/freq, then is the problem that a stray (strong) 470 MHz signal is entering the antenna and creating harmonics in our electronics? Put another way: if the DMS radio third harmonics (roughly 1410-1440 MHz depending on the base signal) are spawned/created by 470 MHz photons/waves interacting in the air to create 1420 MHz harmonics, and then they enter the antenna/our electronics, wouldn't a filter which cleans most signals outside of 65MHz from 1420 MHz still let these third harmonics in? Or is the problem that a strong (roughly) 470MHz signal is getting into the antenna and we need to filter this from the antenna?

3) We should immediately do a better job of cleaning up external RFI as well (thanks Raydel and  superkuh): we'll put our LNAs and filters in foil (a box with ventilation probably best). We should probably do this for our SDR, and perhaps for the laptop as well. And then also power our LNAs with battery rather than the USB to Arduino Uno to LNA which we currently have set up (using what we have available sounds practical but may not be the best setup).

4) Obviously we'll need to sample for a longer amount of time to collect a viable signal -- several minutes at least (thank you, Michiel and Jon -- 5 minutes sounds smarter). And that viable signal is NOT the sun (again, since I had misunderstood that the sun could be a strong signal test case, that was my bad guidance -- corrected!). But I would have expected the waterfall to show the bump a bit as a slight color change (apparantly not). (and thanks for the article reference, Michiel)

5) We'll eventually need to space the  "telescope" (antenna and signal processing pre laptop) from the laptop with a longer cable: we're looking at a longer USB extension to get the SDR off the laptop, and then a longer coax (it's now < 2 meters) to get it away.

6) We definitely WON'T put a filter as the first element between the antenna and the next signal processor (thanks Marcus). But we might need another filter and LNA to clean the signal more if it comes to that (we live in a VERY noisy radio environment).

7) Regarding the Yagi-Uda versus Can-Horn antenna design: we plan to make a horn antenna (to test the different antenna configurations as a good project) but wanted to try this first. We actually took the Yagi-Uda design from a 2013 articles from the SARA journal. We wondered why so few people tried this first, but perhaps we just proved why! (thanks, Marcus and Michiel, and thanks for the horn antenna article, Jon -- I've seen that one and plan to try this as well). We also though about other configurations to see if they are better (including longer Yagi-Uda, four connected Yagi-Uda, offset dish with different feedhorns, larger horn, differing LNA/filter configurations, etc.)

Thanks again for everyone's help. Our next step is to TRY IT AGAIN. We'll definitely let you know how it works out.

Eric

On Monday, June 29, 2020 at 10:41:09 AM UTC-4, Eric Ross wrote:

[Marcus D. Leech]

On 06/30/2020 12:58 PM, Eric Ross wrote:
> I want to thank everyone here for the huge number of very insightful
> and helpful replies. They are very much appreciated. From the replies
> I think there are a clear number of steps:
>
>
> 1) I am a bad advisor for my daughters because I thought testing our
> hydrogen-line detector on the closest massive collection of hydrogen
> (the sun) made logical sense. I see from a number of comments that
> this was a bad assumption (thanks Raydel -- I didn't know this about
> the solar continuum but given the immense heat this probably makes
> sense) (and thanks Jon -- I had assumed the roughly 23 degree
> sensitivity of our Yagi-Uda antenna would easily capture the 0.5
> degrees of the sun, but that, too, was a bad assumption). So we'll try
> for the Milky Way. Incidentally, we plan on fully characterizing the
> antenna we built, but the beam width should be roughly 23 degrees (in
> the direction of the antenna).
>

The "quiet Sun" at 21cm is very approximately 100,000K in black-body
equivalent noise temperature. Your beam-width is 23deg, which gives
you a beam *area* ratio of about 2000:1. So, divide that 100,000K by
2000, giving a rise of about 50K noise temperature when the Sun is
in the center of your beam. We'll assume that Tsys (overall system
noise temperature) is about 120K (typical for a first attempt an an
L-band weak-signal observing system). We also set the sky-averaged
noise temperature at 21cm to be about 20K, so the two cases:

(A) Pointing at "cold" sky -- the noise temperature referred to the
input is 20K+120K
(B) Pointing at Sun -- the noise temperature referred to the input is
50K + 20K+120K

That gives you a ratio at the output of about (linear) 1.35, or about 1.3dB.

So, assuming your system is working, and completely RFI free, you should
see a rise in your noise floor of over 1dB pointing at the Sun with your
antenna, and then it should fall as you move off-Sun by 1 to 1.5dB.

Now, that assumes that your LNAs and filters are working properly, that
you have the right gain set on your SDR, that your SDR is even
sensitive at 21cm, etc, etc.

>
> 2) We'll check for the 470 MHz signal to see if what we are seeing is
> DMS radio third harmonics (thanks superkuh). Question on harmonics:
> the antenna we built is supposed to collect our desired signal (that's
> supposedly the point of most of the waveguide rods in the Yagi-Uda
> antenna design) and attenuate signals meaningfully outside our desired
> wavelength/freq, then is the problem that a stray (strong) 470 MHz
> signal is entering the antenna and creating harmonics in our
> electronics? Put another way: if the DMS radio third harmonics
> (roughly 1410-1440 MHz depending on the base signal) are
> spawned/created by 470 MHz photons/waves interacting in the air to
> create 1420 MHz harmonics, and then they enter the antenna/our
> electronics, wouldn't a filter which cleans most signals outside of
> 65MHz from 1420 MHz still let these third harmonics in? Or is the
> problem that a strong (roughly) 470MHz signal is getting into the
> antenna and we need to filter this from the antenna?

Strong signals present at the antenna terminals can drive the very-first
gain stage into non-linear operating conditions, creating what are
called "intermodulation products"--the LNA is acting as a mixer,
producing "ghost" signals. Modern LNAs are both fairly low noise, and ALSO
have reasonable linearity, so unless the unwanted signals are quite
"loud", this should not be happening at the first stage. More filtering
between
stages MAY help to eliminate intermodulation possibilities further
down the line.

>
>
> 3) We should immediately do a better job of cleaning up external RFI
> as well (thanks Raydel and superkuh): we'll put our LNAs and filters
> in foil (a box with ventilation probably best). We should probably do
> this for our SDR, and perhaps for the laptop as well. And then also
> power our LNAs with battery rather than the USB to Arduino Uno to LNA
> which we currently have set up (using what we have available sounds
> practical but may not be the best setup).
>
>
>
> 4) Obviously we'll need to sample for a longer amount of time to
> collect a viable signal -- several minutes at least (thank you,
> Michiel and Jon -- 5 minutes sounds smarter). And that viable signal
> is NOT the sun (again, since I had misunderstood that the sun could be
> a strong signal test case, that was my bad guidance -- corrected!).
> But I would have expected the waterfall to show the bump a bit as a
> slight color change (apparantly not). (and thanks for the article
> reference, Michiel)
>
>
>
> 5) We'll eventually need to space the "telescope" (antenna and signal
> processing pre laptop) from the laptop with a longer cable: we're
> looking at a longer USB extension to get the SDR off the laptop, and
> then a longer coax (it's now < 2 meters) to get it away.
>

You're likely going to want your laptop/SDR far away from the
antenna+LNA--like 10s of meters. This is particularly important for
antenna with fairly "sloppy" side and back-lobes, like a YAGI.

>
>
> 6) We definitely WON'T put a filter as the first element between the
> antenna and the next signal processor (thanks Marcus). But we might
> need another filter and LNA to clean the signal more if it comes to
> that (we live in a VERY noisy radio environment).

This may be the limiting factor in your set-up. A very noise-prone RF
environment is anathema to weak-signal RF science, even for a relatively
"bright" cosmic "signal" like the 21cm line.

>
>
>
> 7) Regarding the Yagi-Uda versus Can-Horn antenna design: we plan to
> make a horn antenna (to test the different antenna configurations as a
> good project) but wanted to try this first. We actually took the
> Yagi-Uda design from a 2013 articles from the SARA journal. We
> wondered why so few people tried this first, but perhaps we just
> proved why! (thanks, Marcus and Michiel, and thanks for the horn
> antenna article, Jon -- I've seen that one and plan to try this as
> well). We also though about other configurations to see if they are
> better (including longer Yagi-Uda, four connected Yagi-Uda, offset
> dish with different feedhorns, larger horn, differing LNA/filter
> configurations, etc.)
>
>
> Thanks again for everyone's help. Our next step is to TRY IT AGAIN.
> We'll definitely let you know how it works out.

Horn antennae have the very-nice advantage of having very low side and
back lobes, making it easier to "run away" from terrestrial noise sources.

[Dave Typinski]

On 6/30/20 12:58, Eric Ross wrote:
>
> 1) I am a bad advisor for my daughters because I thought testing our
> hydrogen-line detector on the closest massive collection of hydrogen (the sun)
> made logical sense.

I /does/ make logical sense, almost. The key factor is that the spin-flip
signal is from neutral (i.e., atomic) hydrogen. There's not much neutral
hydrogen inside the Sun, but there is some outside -- although the mass of the
outer components (photosphere, chromosphere, corona) is 12 orders of magnitude
smaller than the mass of the Sun itself.

A thought experiment, in very ballpark terms:

1) How strong would the spin-flip signal be from the Sun /if/ the Sun were made
of one solar mass of neutral hydrogen?

2) Looking at it the other way, how many solar masses of /neutral/ hydrogen
exist within a 30-arcminute HPBW (diameter of the Sun) out to 50 kly looking at
galactic longitude 90 or 270?

3) How do the answers to 1 and 2 compare in terms of 1/r^2?
--
Dave

[Wolfgang Herrmann]

Hi Eric,

There has been a lot of good advice given already, so there is no need to get into that again. Just as something in addition, you may want to have a look at a series of articles dealing with the observation of hydrogen with small antennas and simple setups at our website:

https://astropeiler.de/beobachtungen-der-21-cm-linie-mit-einfachen-mitteln

Best regards

Wolfgang

 

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[B & MR Randall]

Does the sun have any emission at 1420MHz as a spectral line? The 1420 is from transition between the 2 lowest energy states of Hydrogen. To me that implies that very cold (few K) parts of space are producing this line. Comments??
Bruce Randall

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[Marcus D. Leech]

On 06/30/2020 08:43 PM, B & MR Randall wrote:
> Does the sun have any emission at 1420MHz as a spectral line? The 1420 is from transition between the 2 lowest energy states of Hydrogen. To me that implies that very cold (few K) parts of space are producing this line. Comments??
> Bruce Randall

I would expect there to be essentially no 21cm line emission from the
Sun. You are correct in that the line emerges from the hyperfine
transition in
atomic hydrogen--in interstellar space.

There IS hydrogen emission from the Sun, but it is the Hydrogen-Alpha
emission at approximately 656nm wavelength, and I think others in
the Balmer series.

[Marcus D. Leech]

On 06/30/2020 08:43 PM, B & MR Randall wrote:

> Does the sun have any emission at 1420MHz as a spectral line? The 1420 is from transition between the 2 lowest energy states of Hydrogen. To me that implies that very cold (few K) parts of space are producing this line. Comments??
> Bruce Randall

It's a ground-state transition, so the photon-energy difference is quite
small--around 5.9ueV. The Sun is such a roiling monster, it seems unlikely
to see much in the way of this ground-state transition...

[Tom Crowley]

If you are asking about HI, than the answer is little to none.  For an HI emission the Hydrogen atom must be at ground state for 10,000,000 years (on average).  To be at ground state for that period, the temperature is typically under 20K.

 

Tom Crowley

 

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[Hamish Barker]

There is however plenty of continuum ( not line emission) energy being emitted by the sun at 1420Mhz simply because it is hot. 

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[Eric Ross]

I want to again thank everyone for all your advice and help. I apologize that we took so long to get a result but please be assured that we absorbed all of it, discussed/argued about what we can/should do in what order (planned for a time when the girls could stay up late enough to obtain readings, and worked through to a result.

Attached is an image from last night's multiple runs on the milky way at roughly 40 degrees from horizon. We have a tighter peak centering at roughly 1420.46 to 1420.48, and a smaller broader peak at roughly 1420.01 to 1420.07.  Next to it is the chart from Australian Journal of Astronomy, vol 5 issue 4 (1994), pp 121-133 for comparison.  

We hope that this looks like a real result -- the girls are pretty excited about the chart. And this is from a rooftop in one of the most radio-loud locations in the world. 

The girls are documenting their progress in a "thesis / lab report" so the girls get all their ideas down in a clear explanation and record, and so the concepts/calculations are clear to them.

There's still some weird ringing we need to remove (and signal shape issues), and continue shielding and isolating our LNA power supply. But we'll have a chance to run our telescope at some national parks over the next month so hopefully the terrestrial noise will lessen (at least so we can see the noise difference). And then we can gather enough data to map the milky way more accurately (better line of sight to the horizon, many more runs, more averaging, more shielding, more isolation, etc.). And then in the Fall we'll try some other antenna types/modifications to see if we can improve results.

I'd love to know if we are finally seeing the hydrogen line data?

Thank you for your help.

[Eric Ross]

Wolfgang,

You are correct: there has been some terrific advice by many respondents, and we have done our best to incorprate as much as we could (while keeping the attention of a 3 year old and 15 year old).

However, I wanted to specifically thank you for your website and series of articles. This has given us a lot of potential direction and refinements for our work in the coming year.

Eric

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[Eric Ross]

My initial assumption (that the sun would be a good initial target)  was a fatal mistake. Very helpful that everyone corrected me so we could move on to simply measuring (I hope successfully) the milky way. Thank you.

On Tuesday, June 30, 2020 at 9:06:18 AM UTC-4, Raydel CM2ESP wrote:

Hi Eric,

Those are terrestrial signals. Under certain conditions if the transmitters are very powerful and nearby they can get into the system, even using SAW, and in many other cases, they are just entering the system if the LNA is not shielded of even through the coax or some connector.

On Hydrogen profiling, you need a massive amount of Hydrogen to get the spectral line, so using such kind of antenna and pointing to the Sun, there will be more Solar Continuum (Solar RF noise) than actual Solar Hydrogen. You need a lot of Hydrogen, like the whole Galaxy arm to be able to get spectral monitoring.

I bet others, which are really experts can explain better.

Regards,

Raydel

On 30/06/2020 04:06, Eric Ross wrote:

> superkuh, > > Thanks so much for your reply. We'll take a look over the next few > days. > > We have a SAW filter with 65MHz width centered at 1.42 GHz surrounded > by two LNA's (from Nooelec). We'll check the signal at 470 MHz to see > if this is the case but we had hoped that this filter in-line would > clean up the signal. > > We can add another bandpass filter to the setup as you suggest, but > should we filter out the signal directly from the antenna before > heading into the first LNA? so instead of <ant> -- <LNA> -- <SAW> -- > <LNA> -- <coax> -- <SDR> we should have: <ant> -- <filter> -- <LNA> > -- <SAW> -- <LNA> -- <coax> -- <SDR> > > I didn't think this signal was interference with human produced radio > signals because when we turned the antenna away from the sun by 90 > degrees (in both dimensions) the signal disappeared, and when the > antenna was pointed straight up at the sun the signal was strongest. > So does this imply that instead of making a proper Yagi antenna, we > instead made a Yagi antenna that has stronger absorption at 90 > degrees off axis? > > I also saw your discussion on http://superkuh.com/ > <http://superkuh.com/rtlsdr.html> and realized that we didn't shield > our components. I know in the "final" version we would, but just > trying to get first light I thought we might skip it. Is this where > the noise is coming in from? > > We'll give this a try and let you know. > > Thank you, > > Eric > > On Monday, June 29, 2020 at 7:11:18 PM UTC-4, superkuh wrote: > > To me this looks like the third harmonic of 470 MHz DMR mobile radio > basestation/handsets. So they're not real signals at 1420 MHz but > they are real at ~470 Mhz. Try looking around down there to see if > you see the same thing. If it is then a high pass filter >1GHz would > help. > > -- -- You received this message because you are subscribed to the > Google Groups "Society of Amateur Radio Astronomers" group. To post > to this group, send email to sara...@googlegroups.com To > unsubscribe from this group, send email to > sara-list-...@googlegroups.com For more options, visit this > group at http://groups.google.com/group/sara-list?hl=en --- You > received this message because you are subscribed to the Google Groups > "Society of Amateur Radio Astronomers" group. To unsubscribe from > this group and stop receiving emails from it, send an email to > sara...@googlegroups.com > <mailto:...@googlegroups.com>. To view this > discussion on the web visit > https://groups.google.com/d/msgid/sara-list/7ea5fd14-32b0-4de1-888e-9260a0943101o%40googlegroups.com > <https://groups.google.com/d/msgid/sara-list/7ea5fd14-32b0-4de1-888e-9260a0943101o%40googlegroups.com?utm_medium=email&utm_source=footer>.

[Larry Mayfield]

Eric, just a comment… the real experience is that of spending the time doing things with your daughters as involved parent and children. That is absolutely priceless!  Way to go!

 

Larry

Pahrump, nv

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[Eric Ross]

Thanks, Larry -- very much appreciated. This type of project is us city-dwellers' version of building a car in the garage with your teenage kids. The only issue is if we don't show some kind of results after a time, just like actual researcher they'll get bored!

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[Larry Mayfield]

Yeah, I raised 2 boys… it takes even less to get them bored, lol. But, perhaps there is an intermediate objective that can be more easily accomplished? Ask the on list  experts what would your system and the kids really do pretty easily.  Anything to show some real success?  I bet there is. Then to keep them interested, move up to harder stuff. Have them doing some of the math and figuring out things as well. Plotting data, STEM kinda things… But at their pace..

 

Me, I am at the place where your children are, lol, and I am nearly 79 years old. Still fun!

 

Larry

Pahrump, NV

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[Eric Ross]

I wanted to re-attach the spectra we collected (in case, as with my last post, it didn't attach).

[Eric Ross]

Jon, thank you: we ended up taking your advice and using 5-minute scans. Actually the spectra attached below is 4 x 5-minute scans (minus  4 x 5-minute noise floor). Curious to hear your thoughts as to if what we see is a real signal?

[Eric Ross]

Thanks for this advice. We did see a massive clear signal at roughly 475 MHz as you suggested. We changed our location slightly, insulated the components, and re-ran multiple 5-minute scans and subtracted out a noise floor. I attached the spectra we obtained below.

On Monday, June 29, 2020 at 7:11:18 PM UTC-4, superkuh wrote:

[Eric Ross]

Definitely keeping them interested, thanks Larry. We just went through how the red-shift works (and why) and how we can then calculate the galaxy spin speed -- blew their minds. They're doing everything: they've built the antenna; they're programming in python to automate it, etc. We're trying to finish our hardware so we can take measurements while on a national park tour (including potentially near you) so we can get less terrestrial noise (than NYC). They're enjoying it, and I'd like to continue having them motivated and enjoying it.

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[B & MR Randall]

Eric,

The 475MHz DMR signals ARE ALLOWED to transmit a small bit of third harmonic near 1420MHz.  FCC specs are per transmitter service &  classification.  A 100W transmitter may specify 60dB below the carrier.  This would be 100uW at 1420MHz area.   If it is close by, 100uW could be a pretty strong signal, even coming in the side of a your dish .  If the transmitter is legal you are stuck with it. 

 

A cleaner pattern on your antenna could reduce interference a lot, as long as you are looking well above the horizon.  If your dish feed has a choke ring on it, you could move the choke ring towards the dish will narrow the pattern and under illuminate the dish.  This would lose a little gain on the dish but lower interferece by a lot.

Bruce Randall

 

From: sara...@googlegroups.com [mailto:sara...@googlegroups.com] On Behalf Of Eric Ross
Sent: Monday, July 20, 2020 8:30 PM
To: Society of Amateur Radio Astronomers
Subject: [SARA] Re: Helping my 13 and 15 yr old daughters with home-built 21-cm telescope first-light spectra of the sun (below): is our spectra correct?

 

Thanks for this advice. We did see a massive clear signal at roughly 475 MHz as you suggested. We changed our location slightly, insulated the components, and re-ran multiple 5-minute scans and subtracted out a noise floor. I attached the spectra we obtained below.

On Monday, June 29, 2020 at 7:11:18 PM UTC-4, superkuh wrote:

To me this looks like the third harmonic of 470 MHz DMR mobile radio basestation/handsets. So they're not real signals at 1420 MHz but they are real at ~470 Mhz. Try looking around down there to see if you see the same thing. If it is then a high pass filter >1GHz would help.

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[Larry Mayfield]

Good morning, Eric!

 

Glad you and yours are working though the telescope issues together. Solving problems is the fun part, lol.  It is great to hear that your children are enthused about this!  I was a manager at Boeing when I was a working cuss, and had at one time 220 scientist, engineers and technicians working for me. I always implored the different supervisors to consider that when they were interviewing applicants for open positions, to consider women engineers. Why? Well, women have a different perspective on how to solve problems and some solutions are ingenious, at times.  Have your girls Learn all the math and engineering they can.  Working on space programs is FUN.  I barely know anything about RA but I do know about space, lol.   

 

Headed out for a Nat Parks tour? Well, given the covid 19 problems right now finding accommodations is going to be  tough, especially here in the west. Las Vegas is virtually locked down, no hotels no restaurants, etc. Or at least severely reduced in number.  My wife and I are on self-imposed  exile, lol, because we are, ahem, ancient, and both of us have heart issues, diabetes and some others, like me, had pneumonia a long while ago. I ring all the alarm bells. My big daily event is going out to the driveway to snag the newspaper. Otherwise I stay indoors and isolated. Us old folks are truly in a world of hurt with this disease. 80% of us who get it, pass away from it.

 

As to visiting a National Park near us? Well, the only thing actually close is Death Valley and truly I do not recommend that for people not from the area and used to dealing with the heat. Last week or so ago the temp over there probably at, Bad Water, was 128 degrees F. That is only 6 degrees from the highest recorded temperature ever on the entire planet.  And with humidity levels at or near 1% or so, staying hydrated is a true necessity.  When I was racing my Sunbeam I took it to El Mirage dry lake, in Southern CA.  to be inspected and on the way home the truck temp external temp read 127 degrees.  Summers are bad times here unless you are travelling on a tour bus and I would not get on one of those petri dishes with who  knows what is on board with you.  Not trying to discourage you but just be aware that things are and can get difficult during travel.  Not sure where you would be coming from, but consider  everything before heading out.

 

If I can assist in any way, feel free to ask. There is always a chance I might be able to  help.

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[Eric Ross]

Thanks, Bruce.

Can we try the same "choke ring" idea with our Yagi-Uda antenna? -- i.e. put it in a "faraday tube" so we minimize off-angle interference? (and potentially tighten the angular resolution?)

Also, we took multiple readings toward ground of "noise" (same cardinal direction as our readings but at zero degrees / parallel to the earth/city surface instead of pointing straight at the Milky Way) and then subtracted this out. Shouldn't this leave us with (mostly) our desired Milky Way signal?

Importantly: is it your belief that our (messy) chart shows interference from the 475MHz third harmonic and not actual hydrogen line signal? -- after much averaging and subtracting out the noise it looks to me that we've possibly replicated what larger/better telescopes have seen. 

• The 475MHz harmonic we see is a very narrow/specific signal
• It's not at all as wide 
• It's not at the same frequency (although close) as the signal we see in our averaged results that we believe could be the hydrogen line
• Also the terrestrial signal has a VERY clear delineation between the two peaks so we should see this in our data if this is what we are seeing.
  

So I'd argue we've removed it by subtracting it out (I attached both charts below).  And we haven't yet adjusted our data to the shape of our filter/input pattern so it could look cleaner after this.

This is just raw noise without averaging (so you can see the ridiculously strong terrestrial signals):

And this is what we got when averaged 20 minutes of signal and subtracting 20 minutes of noise floor:

And this is the signal after being smoothed (and the chart range tightened):

We plan to eventually try a dish versus a Yagi (and a horn as well). But dishes and horns don't fit well into elevators, making it difficult to get from our apartment. And they don't fit into cars so we can't bring them to the National Parks from which we planned to try collecting data with substantially less terrestrial interference.

I'd love your thoughts.

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[Eric Ross]

Very much appreciate that, Larry. The girls are very excited about space programs and science, and they are very good coders (already published iPhone app games) and now learning how to solve real problems (I've been "accused" of running an in-house innovation lab). I tell them "feel free to pursue anything/career you like, but if you don't know science/tech then someone else will control your environment. And with science/tech, these are the tools you'll use to solve problems in any field."

Regarding the Nat Parks, even though we're from NYC we'll be good: we've done a lot of camping/backpacking in the past and we've mapped a course to open parks with open campgrounds and available backpacking permits. We plan to camp and backpack virtually the entire time so we should be relatively isolated from others (especially versus a NYC subway). We'll probably do a drive-by of Lake Mead since we plan to be at Zion (even just to wade/swim/cool off in Lake Mead). And if there's time for a quick trip to either Death Valley and Area 51 we might decide to make the time.

Regarding the heat: it's always a concern but for city-dwellers we're fairly prepared -- the girls and I backpacked for a week in the Grand Canyon in mid-August. Temps were consistently 120 during the day (and we were hauling packs). We were safe -- plenty of water, spray bottles, creek breaks, moving from water source to water source, etc. We've also spent other time in Tucson and Phoenix including some hiking. We'd not really thinking of hiking in Death Valley (no water sources); just to see it if it isn't too far out of the way (and get a park patch for their collections).

Regarding COVID, you should definitely stay safe -- it's dangerous for pre-existing conditions. We experienced it in NYC, and we all had it in March. We're all mask-wearers (when appropriate) and we luckily have some N-95 masks on hand (a few for each of us) plus other washable-but-effective masks. We're not planning on ever being in a tour bus, and no plans to visit anything outside of a national park or gas station.

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