Well, as I written before here and here, I am currently working on radio astronomy. I had been putting off working on it for almost 2 years until last summer when things finally picked up some pace. Well, atleast in one sense.
The first thing any amateur astronomer interested in pursuing radio astronomy and building a radio telescope should know about is The Radio Jove project. It is an amateur radio astronomy project backed by NASA based on a dipole antenna capable enough to observe radio emissions from jupiter and the sun. As for why this particular frequency was chosen instead of others, i have no answer. The sun is a black body and emits a continuum radio spectrum and jupiter has an emission peak at 20.1MHz. You can read up more about the emissions processes on wiki or google for the, that is not the topic for this blog post.
Now, looking at the material from the project site, I estimate that the original team stopped updating the manuals after version 1.2, the update which involves interferometry. So, when I started pursuing the project seriously, I started hitting some road blocks.
Before anything more, let me brief you on my project.
is pretty much what my project is.
So, now let's look at the different components of the experiment. The antenna is the part of the setup which receives the signal. for now let's assume that we have an antenna and we are getting some kind of signal from it. It is to be noted that while there are different kinds of antenna, the Radio Jove experiment uses a Dipole Antenna. And dipole antenna are designed to pick up specific wavelengths (n*lambda). So, the dipole antenna itself acts as a Low Frequency Noise Block or an out of range noise block in a sense. But then, as the circuit mentions, we have a additional bandpass filter (usually just an RC tank circuit). And what a band pass filter circuit does is amplify the signal of the frequency band selected and attenuate the out of band frequency signals.
And as you will understand later on, i intend to observe the radio continuum and not just the specific 20.1MHz frequency, which is what the radio jove project is designed to observe.
Moving further, the Low Noise Amplifier (LNA) is necessary as the input voltages are of the order microVolts @ 20MHz and we (students and amateurs) usually dont have the equipment to measure microVolt variations in a signal (yes, the radio signals from outer space are very very weak). And the twist here is the frequency of observation. There are commercially available LNAs for the frequency ranges of 500MHz to 4GHz as this is the commercial telecommunication band. So, for the sake of the project, either construct an LNA yourself or scour the internet for others who have done similar radio astronomy projects and ask them for advice.
Now, let me introduce you to the circuit mentioned in the Radio Jove project and show you a couple of holes (no, not the electrical meaning for holes, the metaphorical meaning).
IC1 - SA602AN (modulator)
IC2, IC3 - LM 387 (audio pre-amp) - as the circuit mentions
Loosely talking, the LM387 part of the circuit, after the SA602AN is the audio pre-ampifier part of the circuit which is used for data acquisition and analysis. I will elaborate on that later on but for now let's look at the part of the circuit before that i.e IC1. Well, you see, SA602AN has been obsolete for over an year now and finding a perfect replacement for it is almost impossible, from what my search results tell me. The website still says that the circuit works and that we can order kits and make the circuit ourselves. Maybe they stock piled or maybe they update the circuit without making corrections in the manual, i dont know but the basic point is i cannot build this circuit right now.
Which is the reason why i need to make my own LNAs.
Modulation and low pass filters make sense because we want to selectively tune to and study the frequency around 20.1 MHz (which is what the radio jove is designed for) and attenuate the rest of the frequencies. This part of the circuit, i still haven't been able to figure out and also dont have the need for currently. My current method for data retrieval doesn't depend on the frequency of the signal i.e it does not need to be modulated to the ~10KHz range (as the laptop's sound card can only acquire signals till ~100KHz and the laptop's sound card is what we use to acquire the signal from the antenna. wipe that expression off your face and read on)
Then comes the other interesting part, data retrieval. Using an audio pre amp, we push this data into a computer's audio jack and as the signal has already been modulated from MHz to KHz, the sound card of any PC will be able to decipher it. And using relevant software on the PC, you should be able to process the signal, check for noise, harmonic distortion and what not. But I was doubtful as to how scientifically accurate this method was. I did not know how much noise the audio pre-amps (LM387) added, how much of a noise the PC sound card added. So, I came up a different method for data retrieval, albeit a bit more complicated and intensive, a more scientifically one I feel. And i've tried it with simulated data and the system works perfectly fine. Now, I need to compare this method of acquisition to the method mentioned in the Radio Jove Project because i also need to look for a universal method, not just one which is scientifically accurate.
Well, obviously the guys from NASA had a good intention in mind when they started this project, put up all of the material online. If you are an amateur radio astronomer, the radio jove -project is still one of the best way to start and understand the basics of Radio Astronomy.
But alas, without regular updates, nothing will work forever...
There is one small question i would like to address now. The question of the dipole antenna used to acquire the signal, which we've been trying to process all this while. So, as i mentioned, a dipole antenna is designed for specific wavelengths and can only pickup those frequencies. So, it does make sense to design the circuit around this fact. But as i mentioned, i intend to look at the radio continuum in the 1-50MHz range. So, i can't use the dipole antenna!
Then comes the more puzzling question regarding the antenna mentioned in the project. It is mentioned in the manual that the height at which the antenna should be mounted is dependent on the latitude of the place and on the season of the year, which is quite baffling to start with. Well, given, im not exactly an electical engineer, but even I know that this statement sounds absurd at the first time (I still havent seen a meaningful reason behind this yet). And well, just to be sure, I inquired with a professor from the electrical department here and he gave me the strangest look ever, sent me back with material to read up and asked me to do more ground work before coming up with such questions again :d...
I have mailed the relevant people at Radio Jove (the mailing list) hoping for a reply about these and quite a few other doubts I have. No reply as of yet. I'll try one last time and move on to bothering someone who'd reply…
So, as of now, im working on a couple of things -
- In search of good LNAs that work in the 1-50MHz range. And just to get things rolling, Im studying the frequency dependent response of regular Op-amp amplifier circuits, a circuit based on a FET. The experiment is to check the response of circuits which have gain in the range of ~40 dB and study the circuit's frequency response. perform noise analysis and harmonic distortion studies. I'll tell you guys how that works out...
- Study the signal retrieved using the LM387s (audio pre-amps) and compare this signal to that retrieved using my own method, to see if the audio pre-amp method is any good to perform scientific studies.
- Construct the receiver and hope for a signal
I'll keep you guys updated.
Until next time ...
PS - if you guys know anything about radio astronomy, comment here or mail me or ping me or anything! I want to get in touch with you!