Choosing antennas (dish, maybe not). Re: [SARA] U.H.F. Interferometry using a Phase Sensitive Detector & Lock-in Summing Amplifier and Radio Skypipe.

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David Fields (fieldsde@gmail.com)

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Jul 24, 2018, 9:54:27 AM7/24/18
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Hi Mayf,
I think that interferometry has a lot of potential.  It's a big topic, and there are tricks.  Sometimes one can use just one antenna.  And there is actually a variation that doesn't require keeping the phase of the RF.

You mentioned wanting to erect a dish.  Someone needs to mention that you aren't obligated to erect a dish for radio astronomy.  I chose dish antennas, because I wanted to see cold hydrogen clouds emitting at 1420MHz (21 cm).  I erected 3 (old satellite TV dishes) at the local observatory, and two at home, one of them a 15' dish.  The dishes all provided good signals, but my little horn antenna that I built with (foam) wall insulation and aluminum foil was fun, and the H1 (21 cm) signal was detectable.  The antenna was a variation on Paul Shuch's SETI horn:
Here's a more impressive horn antenna (the 'Little Big Horn'):

Dishes and horns cover a wide range of frequencies, and sometimes a receiver that covers this range is desirable.  If you like computers, this might mean an SDR receiver -- perhaps an RTL dongle for about $15 from Amazon, or even one of our RASDR receivers (more pricey and experimental, built and supported by SARA members!). The SDR receivers won't be so useful for radio astronomy unless you add a preamp and perhaps a filter.  Some of our RASDR documentation, is a free download.

If you want to enjoy astronomy at lower frequencies, then dishes are uncomfortably large. For example, at 70cm wavelength, a 70cm diameter dish resolves about 60 degrees.  You'd want a 20' diameter dish to resolve about 20 deg at 70cm wavelength, whereas a nice yagi would be lighter and very useful.

If you want to listen to Jupiter, then dipole wire antennas are useful.  My first (for 20.1 MHz) was a single dipole, oriented N-S, that covered the entire ecliptic.  The better approach (better signal and resolution) is a double dipole antenna.

Voyager showed us that Jupiter is a strong RF emitter from 5khz-35MHz, but our ionosphere blocks signals below typically, 5 MHz or10 MHz.  I'd like for us radio astronomers to put a VLF antenna on a small satellite, above the ionosphere, to listen to Jupiter and other astronomical sources.  That's another project -- see attached note.  Our local astronomy group (Tamke-Allan Observatory -- TAO) has launched 8 balloons with telemetry packages (altitude, temperature, ionizing radiation, transverter relays) and are building a VLF receiver package to fly with antenna/receiver/analysis/telemetry.
        www.roanestate.edu/obs

Cheers,
David Fields
 



-----Original Message-----
From: Larry Mayfield <drm...@mayfco.com>
To: sara-list <sara...@googlegroups.com>
Sent: Sun, Jul 22, 2018 1:09 pm
Subject: RE: [SARA] U.H.F. Interferometry using a Phase Sensitive Detector & Lock-in Summing Amplifier and Radio Skypipe.

David, great explanation!  Many thanks and this noe is in my keeper file!
 
mayf
From: 'David Fields (fiel...@gmail.com)' via Society of Amateur Radio Astronomers [mailto:sara...@googlegroups.com]
Sent: Saturday, July 21, 2018 10:26 AM
To: sara-li...@googlegroups.com
Subject: Re: [SARA] U.H.F. Interferometry using a Phase Sensitive Detector & Lock-in Summing Amplifier and Radio Skypipe.
 
Hi Mayf,
 
Nice discussion.  My way of understanding the interference patterns from 2 antennas is simple, I think -- will see if I can explain:
 
If you were to cut a couple of little slits, close together, in a piece of Al foil and direct a LASER beam at both slits, and put a screen behind the slits, you'd see wiggles of intensity.  These are fringes of what is a 'double slit interference pattern', where light through the 2 slits combines (interferes).  You can take a photo and see the fringes.  Or you can move a little photodiode across the pattern, then the fringes are captured as voltage vs. time.  Or you could move the LASER and the fringes would move across the photodiode (voltage vs. time, again).
 
You'd find a similar 'double slit interference pattern' with similar fringes, when you combine the signal from two antennas that are both observing the same distant transmitter.  You must not lose the 'phase' of the signals, since combination of the phases (constructively or destructively) is what forms the fringes.  You cannot take a photo because these are radio fringes.  You measure output voltage vs. time as the earth rotates and the source passes through the beam.
 
Intensity:  The amplitude peaks of the fringes are 2x as high as the amplitude of a single antenna.  The peak intensity is 4x that of a single antenna.   Not to mention that you have 2x the antenna area, so the intensity is even higher than that.  So the interferometer signal is much more easily seen above the noise.
 
Directional resolution:  A single dipole 'sees' (resolves) from horizon to horizon across the beam (180 deg).  If the frequency is 440MHz (0.7m wavelength) then a 0.7m diameter dish resolves about 60 deg (nicer for mapping).  A nice multielement yagi antenna might resolve 15-20 deg along one beam axis.   Take 2 yagis, spaced 7 m apart and tuned to 0.7m, and you'd resolve about 6 deg across (along one axis).   Mapping is better.
 
More elements:  Adding a 3rd element can narrow the beam or shape the beam or multiply the intensity (3x) but you might want to use a computer to interpret the information in the fringes.
 
Noise:  Local noise (QRM) may effect the antennas differently, and provide a means of identifying or rejecting the noise.  With a little work, if you use N antennas then the signal amplitude can be increased by N times, the signal intensity might be up by N*N times, and the noise amplitude for certain types of noise might be up by sqrt(N) times.
 
Signal variability:  If the source (transmitter or pulsar, etc.) has a time-varying signal amplitude then the fringes will bounce up and down, but the shape of the pattern will stay the same (except for noise problems)  If the source has a time-varying frequency then the fringes will keep somewhat the same amplitude, but the spacing will bounce around. 
 
Cheers,
David Fields                                             
 
 
 
-----Original Message-----
From: Marcus D. Leech <patchv...@gmail.com>
To: sara-list <sara...@googlegroups.com>
Sent: Fri, Jul 20, 2018 10:38 pm
Subject: Re: [SARA] U.H.F. Interferometry using a Phase Sensitive Detector & Lock-in Summing Amplifier and Radio Skypipe.
On 07/20/2018 05:23 PM, Larry Mayfield wrote:
But doesn’t that make the trace artificial in that it is an artifact of the manner that it was obtained? That only an interferometer based system can see it this because of the way it is constructed and implemented in order to add the signals together? If I used my single antenna and recorded lots of data and integrated (ie summed) them over time wouldn’t I see a different trace? Would a single yagi recording the data see it this way?  I suspect not but then as I keep saying I am really, really, ignorant of this stuff.   What exactly do the ‘fringes’ as processed  convey to us? Maybe that is the part I am missing.  Heaven Knows, I miss a lot these days!
 
In THAT sense, any measurement made by man is an "artifice".

The NRAO page I linked earlier really does elucidate interferometry quite well....



Keep trying! I am bound to get some of this by osmosis.
 
Oh, and for goodness sake, if I am using up bandwidth with my questions, feel free to tell me so, lol.  Sometimes I do not know when to cease..
 
And thank all ya’ll for listening
 
mayf
 
 
From: sara...@googlegroups.com [mailto:sara...@googlegroups.com] On Behalf Of Marcus D. Leech
Sent: Friday, July 20, 2018 11:47 AM
To: sara...@googlegroups.com
Subject: Re: [SARA] U.H.F. Interferometry using a Phase Sensitive Detector & Lock-in Summing Amplifier and Radio Skypipe.
 
On 07/20/2018 12:30 PM, Larry Mayfield wrote:
Yup, the link one is the jpg I was referring to.  What does a time snippet of the one below you saw is raw?  Can you look at time slices of this? Say the big peak any short span, say 1 second?  Are the filters time constants in play? Or maybe what is it that causes that beautiful sine wave on the jpg screen shot in the link below? That just looks artificial to me.  But more than likely, just shows my ignorance, lol…    Are you in drift scan mode?  Or tracking? 
Larry, those "beautiful sine ways" *ARE* the fringes.   As the source moves through the sky, the phase-length to each antenna varies, thus producing
  a continuous pattern of constructive and destructive interference when coherently combined.  The larger the distance between antenna,
  the higher the fringe rate.

Again, the NRAO tutorials on this are pretty good:

https://www.cv.nrao.edu/course/astr534/Interferometers1.html


 
May I ask what part of the world do you live in? And do you know a  Jean Marie Polard?
 
mayf
 
 
From: sara...@googlegroups.com [mailto:sara...@googlegroups.com] On Behalf Of jan Lustrup
Sent: Friday, July 20, 2018 8:54 AM
To: sara...@googlegroups.com
Subject: RE: [SARA] U.H.F. Interferometry using a Phase Sensitive Detector & Lock-in Summing Amplifier and Radio Skypipe.
 
Hi mayf,
 
Did you mean this one?
 
If this is the one well it is recorded on RadioSkypipe a lot of averaging ( I think X20 or more). The “locked-in” output goes into a R.C. (resistor and capacitor) low pass filter that “cleans up “ the signal a bit.
I will post below an image from last night done without averaging and hardly any low pass filtering. That’s about as “raw” as it gets.
 
raw data.jpg
 
This is meridian transit of the GP. (the Milkyway, Galatic Plane) passing thru (transiting) my antenna bean, pointing due south .
 
The blue recoding is galatic noise picked up from my antenna while working as an  “adder”( two antenna signals added together at a coaxial “T” connector or combining transformer), working here as a total power detector and not as a interferometer. The milkyway is producing a lot of noise as you can see. Can you make out the  point source noise from Cygnus A. It is buried deep in the noise at midnight UTC (top of  blue peak). Guess not!
 
The green recording was done simultaneously,  the unfiltered output from my locked-in phase sensitive detector. The rising of lowering of noise from the milkyway passage is not seen anymore. But Cygnus A fringes is clearly seen about the same time as  milkyway maximum noise (blue). Cygnus A is a point source the phase sensitive detector records easy.
 
Here you see some averaging that is done in RadioSkypipe. A little less fuzzier now isn’t it?
raw with a little filter.jpg
 
You can go on and on averaging more and more, making the fringes look realy nice, but you loose more and more detail and resoltion  by doing too much of this process.
 
 
Cheers,
Jan Lustrup (LA3EQ- Norway)
 
Ps Maybe I saw your speed car on Discovery channal some time ago….breaking speed records on a saltlake bed.
 
 
From: Larry Mayfield [mailto:drm...@mayfco.com]
Sent: fredag 20. juli 2018 16.33
To: sara-li...@googlegroups.com
Subject: RE: [SARA] U.H.F. Interferometry using a Phase Sensitive Detector & Lock-in Summing Amplifier and Radio Skypipe.
 
Jan, and all of the others who replied to my question of “why?” on taking and resolving fringe data.  I am glad you took the time to respond and as I had hoped, the reasons were perfect!  I think I can pretty much sum I tup by saying that the “journey” is the reason. Doing something we haven’t done before or which simply interest us.  That is my sole driving purpose as well for just about anything I do these days.  It was the reason I went racing on the salt flats: I just wanted to see if I could do it, lol. I designed and rebuilt a car to run there, I did everything myself and when I was finished I had a car that was and is recognized as the fastest on planet earth. Even broke an outstanding record that had stood for more than 80 years. That is my reason for entry into RA now, I want to see if I can do this! I have a goal that drives me and that is to find ET, lol. Probability of that is virtually zero and is recognized by me as being that.  But the journey to getting there is what I am after!  The experience of doing something totally out of my realm of experience and academics.  I salute all of you! 
 
And again thanks for reminding me of the real “why”.
 
Now, I do have to ask some really dumb questions regarding the very first jpg Jan sent on his results. That beautiful sine wave with the “hairy” signal trace on it.   I keep looking at that and asking myself , why does it look that way?  Raw data doesn’t look like that does it? The plot looks artificial to me but because I am so new at this, I do not know if real or not, lol. I look at the time line and it seems like the trace is periodic with about a 7 cycle per hour rate.  That looks artificial to me and as I have handled a lot of engineering data in my lifetime, part of the data reduction methods used? Some aliasing going on?  Isn’t the hairy fringes of the trace the real data?   See, I am out of my element here. So be gentle…
 
I do, as mentioned, understand the “just because” and “the journey” of this and it is the reason I continue and hopefully get my two sons involved (one is 38 and the other is 36).   I have my eye on a larger dish here in Pahrump that I may see if I can acquire and convert. Looks to be around 10 or 12 feet in diameter!
 
From: sara-li...@googlegroups.com [mailto:sara...@googlegroups.com] On Behalf Of jan Lustrup
Sent: Thursday, July 19, 2018 10:16 AM
To: sara-li...@googlegroups.com
Subject: RE: [SARA] U.H.F. Interferometry using a Phase Sensitive Detector & Lock-in Summing Amplifier and Radio Skypipe.
 
Hi mayf (Larry),
 
Glad you enjoy this mail thread….
 
You ask about why is taking  “fringes” important ?
Well, one might ask why do people climb mountains etc. Well, I guess because it’s challenging. Even though it’s been done already by others, the sheer delight and pleasure of knowing you did it yourself is priceless. And there is  the learning process along the way. Thinking out something fascinating  to try out (usually while trying to fall asleep). Then designing the schematics yourself, making something “scientific” out of bits, pieces form a junkbox, and soldering parts together. Trail & error. Making it work.  All this is partly what amateur radio astronomy is about for many people.
 
What about usefulness of fringes….well they are useful in many ways. Many others can explain the uses much better then me. Here are just a few things: confirming the declination of radio source in the sky by reading the fringe rate. Detecting point radio sources buried in the noise of the broad galactic plane noise that is usually not seen on normal meridian drift scan observations.  The fringe rate when directly south will tell you how high in the sky the object is etc.
 
Speaking for myself, the building & learning process is my main goal, not long term observations. After a project is finished I take it apart and use the parts for a new project!
Just my two cents worth.
 
Cheers,
Jan Lustrup (LA3EQ, Norway)
 
 
 
From: Larry Mayfield [mailto:drm...@mayfco.com]
Sent: torsdag 19. juli 2018 14.33
To: sara-li...@googlegroups.com
Subject: RE: [SARA] U.H.F. Interferometry using a Phase Sensitive Detector & Lock-in Summing Amplifier and Radio Skypipe.
 
Good morning, all.  I am watching this thread with great interest and fascination! I am new to RA and am  trying to grasp not only the hardware and software of it all, but the “Why” of it.  I’d like to request that the active and obviously very technical people, add a small snippet of information to their postings to tell us the Why things are being done. Take fringes. I can see the results of  this system for acquiring and recording and inspecting the signals. It is actually very impressive. But,  the why of it escapes me. Why are “fringes”  important?  What information is gleaned from it? As yet I cannot fathom what the data is to be used for other than to see if it can be done. I am not picking  on this but rather want to be able to participate one day and it is important for me to have a why I do this reason. Are we increasing the knowledge of the stuff we observe? If so where is that repository of data kept,  I mean, who gets the  results and why?  And certainly “just because “ is a pretty good answer if it is for simple pleasure.  I did that when I was racing on the salt flats at Bonneville. That was a ‘just because” and I enjoyed it immensely.  So, a little help, and I mean just a little,  on the why of things is in itself important to us very , very, new wannabe’s. 
 
Thanks for listening and it looks really fun!
 
mayf
 
From: sara-li...@googlegroups.com [mailto:sara...@googlegroups.com] On Behalf Of jan Lustrup
Sent: Thursday, July 19, 2018 2:55 AM
To: sara-li...@googlegroups.com
Subject: RE: [SARA] U.H.F. Interferometry using a Phase Sensitive Detector & Lock-in Summing Amplifier and Radio Skypipe.
 
Hi Jim Abhsier and all,
 
Thanks for all the good advice and kind remarks. I have been doing some experimenting and would like to share with you the results.  
 
 
 
 
Experiment 1: Observing the phase switching frequency (tag along) on FFT screen.
I was looking  at the FFT audio spectrum from the receiver audio output while receiving fringes with the locked-in, and saw a 800Hz signal from eminating from the phase switcher voltage. This “tag along” signal changes strength along with the fringe amplitude. Positive and negative peaks show the strongest “tag along” signal and at zero crossing it vanished. So I set up a CW 20Hz wide audio filter in “Spectran V2” spectrogram and routed it’s audio output via “Virtual Cable” to “Radio Skypipe” channel “2” in blue color, while channel “1” was recording the “locked-in” fringes in green color. Now I could see the “tag along” on the “Radioskypipe” chart screen, it was following the sun fringe positive and negative peeks, while at zero crossing fringes the “tag along” vanished altogether. I also noticed the “tag along”  peaks did not have the same max value. Positive fringes produced the larger “tag along”. Why was this?  So I figured that maybe the signal strength from each antenna leg was not the same. I added 6 dB attenuator to the east leg and the opposite happened. Now only negative fringes resulted in maximum “tag alongs”. Hmmm, so I replaced the 6dB pad with a 3dB pad on that eastern leg, and now there was perfect balance on “tag alongs”. I saw no difference on locked-in fringes (See image below.)
 
 
 
Experiment2: Use the spectrum analyzer instead of a receiver.
I connected the antenna to my H.P. 8565A spectrum analyzer’s RF input and it’s “vertical” BNC output the my “locked-in” input port.
Well, there was not enough RF gain it seems (this old boy’s deaf below -70dBm), so I added a 30 dB RF amplifier and put 2 MHz wide cavity bandpass filters in before and after the amplifier. Now things were looking up. Fringes were easy to detect. Over night Cygnus A even can out nice with 30 kHz BW setting before the VCO drifted off frequency.
 
 
 
Experiment3: The effect of IF bandwidth on Sun fringes.
I tested the effect of 10 kHz, 30 kHz, 100kHz, 300 kHz, and 1MHz bandwidth settings. As suspected, the fringe amplitude increased with increasing bandwidth. (See image 3 below)
 
 
 
 
That’s all for now….
73’s Jan Lustrup (LA3EQ , Norway)
 
 
 
 
Image 1 (800Hz tag along seen on FFT display)
800hz tag along.jpg
 
 
Image 2  (Cygnus A on the spectrum analyzer)
Cygnus 100kHz BW_l.jpg
 
 
Image3 (testing sun fringes with different bandwidth filters on my spectrum analyzer)
10 to 1mhz.jpg
 
From: sara-li...@googlegroups.com [mailto:sara...@googlegroups.com] On Behalf Of Jim Abshier
Sent: mandag 16. juli 2018 18.36
To: sara-li...@googlegroups.com
Subject: Re: [SARA] U.H.F. Interferometry using a Phase Sensitive Detector & Lock-in Summing Amplifier and Radio Skypipe.
 
Hi Jan,
First, congratulations on the fine results from your interferometer.  I view your results as a further demonstration of the efficacy of the phase switched interferometer in amateur radio astronomy.  With regard to your mystery fringes, I seem to recall getting strange fringes like that when I used to have a 400 MHz interferometer.  I concluded that the fringes were being generated by energy from the sun reflecting off some local object.  In my case, it seemed to be reflections from the roof of my house.  I was using horizontal polarization at the time.  I rotated the antennas to vertical polarization and the strange fringes disappeared. 
I am intrigued by how well your system is working with only 5 KHz bandwidth.  Of course, the objects you have detected so far are rather strong sources, but you could detect weaker sources by averaging multiple fringe records.  Perhaps you are already aware of that.  There are also other things you can do with the fringe data.  You can take an FFT to get some SNR improvement and use the frequency information to measure declination.  You can also produce an image of sorts by generating a spectrogram.  A more ambitious undertaking would be to collect fringes with multiple baselines and produce images by 2D aperture synthesis.   An interferometer system such as yours can provide data for lots of post-processing fun.
Jim Abshier
 
On 07/16/2018 11:42 AM, jan Lustrup wrote:
Hi again,
I am trying to find out what is making the fringes showing  around 06:00 to 08:00 UTC?
“RadioEyes” does not show any strong radio sources at that time.
The fringes are too long (12 min) to be “Taurus A”.  “Cass A” at the meridian passing would fit the fringe rate, but the time does not fit.
Any suggestions guys?
 
Jan Lustrup  (LA3EQ , Norway)
 
 
 
Fringes in question.
 
what at 0700a.jpg
 
 
 
 
 
The southern sky at 07:00 UTC
 
aaaa0700.jpg
 
 
 
From: sara-li...@googlegroups.com [mailto:sara...@googlegroups.com] On Behalf Of jan Lustrup
Sent: mandag 16. juli 2018 12.28
To: sara-li...@googlegroups.com
Subject: RE: [SARA] U.H.F. Interferometry using a Phase Sensitive Detector & Lock-in Summing Amplifier and Radio Skypipe.
 
 
Hi all,
 
Recording from last night,
From left to right: Fringes of Cygnus A , (+ Cygnus X ?), Cassiopeia A and the Sun starting to come in on the antenna side lobes , all in one take.
Using a single 19 element yagi on each end of the new baseline of 28.3 meters and elevation of 65 degrees (my max).
This is a phased switched, locked-in amplifier recording. Tonight I will try a simple “Adder” setup  to compare.
 
Jan Lustrup (LA3EQ, Norway)
 
 
 
all in one.jpg.
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Satellite Research Initiative Supporting Interstellar Missions.pdf

radioastron...@gmail.com

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Jul 24, 2018, 2:53:20 PM7/24/18
to Society of Amateur Radio Astronomers
Hi David,

Horns do work very well, here is some photos. Carls RAS horn at Green Bank and plot plus some other. (Not a sales pitch!) I'm using our 1420 EP horn (1st section) with our Spectracyber and the new control Spectracyber Rpi software. Great alternative especially in a small area our HO community.

Jeff

On Tuesday, July 24, 2018 at 8:54:27 AM UTC-5, David Fields wrote:
Hi Mayf,
I think that interferometry has a lot of potential.  It's a big topic, and there are tricks.  Sometimes one can use just one antenna.  And there is actually a variation that doesn't require keeping the phase of the RF.

You mentioned wanting to erect a dish.  Someone needs to mention that you aren't obligated to erect a dish for radio astronomy.  I chose dish antennas, because I wanted to see cold hydrogen clouds emitting at 1420MHz (21 cm).  I erected 3 (old satellite TV dishes) at the local observatory, and two at home, one of them a 15' dish.  The dishes all provided good signals, but my little horn antenna that I built with (foam) wall insulation and aluminum foil was fun, and the H1 (21 cm) signal was detectable.  The antenna was a variation on Paul Shuch's SETI horn:
Here's a more impressive horn antenna (the 'Little Big Horn'):

Dishes and horns cover a wide range of frequencies, and sometimes a receiver that covers this range is desirable.  If you like computers, this might mean an SDR receiver -- perhaps an RTL dongle for about $15 from Amazon, or even one of our RASDR receivers (more pricey and experimental, built and supported by SARA members!). The SDR receivers won't be so useful for radio astronomy unless you add a preamp and perhaps a filter.  Some of our RASDR documentation, is a free download.

If you want to enjoy astronomy at lower frequencies, then dishes are uncomfortably large. For example, at 70cm wavelength, a 70cm diameter dish resolves about 60 degrees.  You'd want a 20' diameter dish to resolve about 20 deg at 70cm wavelength, whereas a nice yagi would be lighter and very useful.

If you want to listen to Jupiter, then dipole wire antennas are useful.  My first (for 20.1 MHz) was a single dipole, oriented N-S, that covered the entire ecliptic.  The better approach (better signal and resolution) is a double dipole antenna.

Voyager showed us that Jupiter is a strong RF emitter from 5khz-35MHz, but our ionosphere blocks signals below typically, 5 MHz or10 MHz.  I'd like for us radio astronomers to put a VLF antenna on a small satellite, above the ionosphere, to listen to Jupiter and other astronomical sources.  That's another project -- see attached note.  Our local astronomy group (Tamke-Allan Observatory -- TAO) has launched 8 balloons with telemetry packages (altitude, temperature, ionizing radiation, transverter relays) and are building a VLF receiver package to fly with antenna/receiver/analysis/telemetry.
        www.roanestate.edu/obs

Cheers,
David Fields
 



-----Original Message-----
From: Larry Mayfield <drm...@mayfco.com>
To: sara-list <sara...@googlegroups.com>
Sent: Sun, Jul 22, 2018 1:09 pm
Subject: RE: [SARA] U.H.F. Interferometry using a Phase Sensitive Detector & Lock-in Summing Amplifier and Radio Skypipe.

David, great explanation!  Many thanks and this noe is in my keeper file!
 
mayf
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