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  #21  
Old 02-02-2014, 12:05 PM
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mswhin63 (Malcolm)
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Fantastic project Bojan, I have always been interested in building a dish.

As a Radio Technician I wanted to take that step as well, and lo and behold 4 weeks ago found a dish up for auction and bought it for $10.00 but fees.

I have removed the aluminium mesh panel and the frame though is in excellent condition.

Thanks for the information on the LNA, typical Astronomy LNA are the most expensive

One thing as part of the auction I received a heavy duty mounting bracket and the linear motor for adjusting.

I cant really start it yet as the woodwork shop requires my attention along with my rental observatory, only 1 month to go before starting new semester at Uni so I think my time is wearing thin.

I am curious though your feed mount is very low, my calculation have place it fair bit higher. I am using the Parabolic Calculator V2 but I only made rough measurements so far, I hope to make a horn antenna as well.

Good luck with yours not sure if I can offer suggestions, I am at the same point in understanding as you.
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  #22  
Old 02-02-2014, 01:30 PM
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Excellent work indeed, hope it all works for you, I'm impressed.

Leon
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  #23  
Old 02-02-2014, 05:42 PM
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Quote:
Originally Posted by atkinsonr View Post
That's really cool!

I hope it's not a silly question, but what can you do with it once it's operational?
Play cricket in it.
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  #24  
Old 13-04-2014, 11:12 AM
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Signal from Sun at 1420MHz clearly detected

Success.
Finally today, I found some time to try detect the radiation from Sun at 1420MHz.
The proof is here: there are clearly two tracks visible (2dB difference), the top one was obtained with antenna pointed at Sun, the lower one is when antenna was pointed away from the Sun.
The tracks were obtained with Max Hold for 30 seconds, for both cases.
The noise floor of the pre-amplifier is still significantly lower than that of Spectrum analyser (there is no difference in noise level on the screen between preamp ON and OFF), so I can add more gain to the system (so far it is ~36dB).

With SDR (Terratec USB receiver) the signal is clearly visible:
The lowest part of the waterfall display is with Pre-amplifiers OFF, the middle part is Sun, and the top part is away from Sun.
The other screen shot from SDR window is simulated transit (antenna beam was moved across the Sun).

The system works!
This is only the beginning..
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Last edited by bojan; 13-04-2014 at 12:37 PM.
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  #25  
Old 13-04-2014, 12:43 PM
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Good on ya Bojan, great you are having success.

One thing I think might help is a down-converter on the antenna. I think the down-converter could provide better wave-guide transmission down the cable to the receiver. For me it is just a thought, also maybe the RTL SDR will have a lower noise floor at the lower frequencies too.
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  #26  
Old 13-04-2014, 01:03 PM
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Yep, downconverter may be useful for higher frequencies, but the cable I have (RG6) is pretty good for 1.4GHz.. so for now it is OK.
What I need now is a bit more gain after preamplifier, and recorder.

Last edited by bojan; 13-04-2014 at 01:15 PM.
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  #27  
Old 13-04-2014, 04:56 PM
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That is so cool! Great looking radio telescope you've put together there. I remember trying to build this when I was a teenager. I didn't get very far though.
How about Jupiter? It's supposedly easily detectable with shortwave radio at 10-25MHz. There is some good info here: http://www.spaceacademy.net.au/space...rad/jovrad.htm
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  #28  
Old 13-04-2014, 05:55 PM
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Quote:
Originally Posted by bojan View Post
Yep, downconverter may be useful for higher frequencies, but the cable I have (RG6) is pretty good for 1.4GHz.. so for now it is OK.
What I need now is a bit more gain after preamplifier, and recorder.
Yes I understand. This is where the cost can really sky-rocket too. Have you measured with a signal generator of some sort the actual RF input measurement of the RTL. The indicator is -60-70dbm according to the RTL SDR. Curious how accurate this is.
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  #29  
Old 13-04-2014, 06:11 PM
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Yep, the signal is visible at -72dBm (SA), 2dB above noise floor (there is cal factor for this situation.. so my signal will be ~-75dBm). I still have to calibrate the whole thing..
RTL was so-so.. I had to calibrate it using tracking gen, since I do not have SG that can reach 1.4GHz.

The cable has 0.3dB/metre... that is 4.2dB total (14m).

Quote:
Originally Posted by mswhin63 View Post
Yes I understand. This is where the cost can really sky-rocket too. Have you measured with a signal generator of some sort the actual RF input measurement of the RTL. The indicator is -60-70dbm according to the RTL SDR. Curious how accurate this is.

Last edited by bojan; 13-04-2014 at 06:34 PM.
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  #30  
Old 13-04-2014, 06:21 PM
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Quote:
Originally Posted by bojan View Post
Yep, the signal is visible at -72dBm (SA), 2dB above noise floor (there is cal factor for this situation.. so my signal will be ~-75dBm). I still have to calibrate the whole thing..
RTL was so-so.. I had to calibrate it using tracking gen, since I do not have SG that can reach 1.4GHz.
It is what I thought when I first tested my RTL, I think with a good quality SDR the reception should be greatly improved, but that is where it get mighty expensive though. Good luck with it all, looking forward to see how it goes.
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  #31  
Old 13-04-2014, 06:42 PM
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So far it was $15 for RTL, and ~$100 for preamplifier (20dB)..
Line amplifier is my own (12dB gain), but I need more total gain, so there will be another $100 out of pocket..
The filter I made earlier will be placed between them.
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  #32  
Old 21-04-2014, 09:00 AM
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SDR calibration

For SDR reading of -30dB (noise floor is at ~-60dB), I had -61.3dBm CW at input (75ohms).. since I do not have SG that goes that high frequency-wise, I used tracking generator in zero span mode (then I can reach 1.6GHz), with external attenuator to supply the test signal level at SDR input.

On the screen plot it is visible how the recorder (SkyPipe, detector set to peak) has shown the drop in audio level when carrier drifted out of audio frequency range (gray area on spectrum plot), set by sample rate.
So, cal factor for my case is -31.3dB.

Which means the SDR level reading is in dBuW..

With additional 40dB in front od SDR (pre-amplifier) the noise floor will be at ~-130dBm (to be confirmed.. because there will be some noise from preamp as well).. which is pretty good receiver all in all..
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Last edited by bojan; 21-04-2014 at 12:03 PM.
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  #33  
Old 21-04-2014, 02:16 PM
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Yes that is the way to go, using the SDR as a middle amplifier stage.

Still as you mention there could be some additional noise in the earlier amplification but -130dbm will be a great achievement for sure.

Best to reduce the bandwidth as much as possible should get a much better S/N ration as well.

I was going to consider a PCB filter for the higher frequencies not sure if 1.4Ghz is too low for a strip-line (thats what I called it years ago) filtering system. Maybe that is for another frequency detection.

Frequency shifting for spectral analysis should be easier to obtain at radio frequencies.
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  #34  
Old 22-04-2014, 05:46 AM
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Malcolm,
I'm afraid the bandwidth can't be reduced in our case - the received signal is also noise-like and broadband, so the only way to increase S/N is reducing the losses in front of LNA.. that means larger dish, optimized illumination of the feeder, best possible feeder (I am trying with this helix design, however I am not entirely sure how good it is, so this will have to be checked as well at some stage.. and whenever we are dealing with polarized signals, we have that 3dB loss), impedance matching to LNA and shortest possible cabling in between).
And a lots of signal averarging/integration (appropriate software should be providing this..)
The biggest problem here is experimenting - it takes a long time between changing something and seeing the results of the change, so this work requires a lot of experience and confidence in what has been done already..

Last edited by bojan; 22-04-2014 at 10:36 AM.
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  #35  
Old 22-04-2014, 05:38 PM
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I understand, good luck with what you have and will look at some updates. Although I have an RTL SDR with me I may be considering a change in direction with a different SDR. It will be a while before I can get started with so many other things happening.
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  #36  
Old 27-04-2014, 10:36 AM
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First "real" observation of the Sun at 1420MHz

On the graph below I captured half of the Sun's transit through the antenna lobe.
The antenna was pointed at Sun (using the feeder shadow), the Earth rotation did the rest..
The plot is actually the presentation of the the audio signal, coming from audio card.. not ideal method, what I really need is to somehow couple the output from SDR with chart recorder..
The noise bursts visible on the plot are system sounds and random noise, probably from environment.. Couple of magpies trying to nest in my antenna also caused some of them.
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Last edited by bojan; 27-04-2014 at 12:22 PM.
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  #37  
Old 27-04-2014, 06:24 PM
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I can utilise the waterfall graphing better than Skypipe. There is a bit too much noise on the signal to get a reliable reading.

Have you though about auto tracking as part of the project?
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  #38  
Old 27-04-2014, 06:33 PM
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Yes, I have.. It will be fully motorised one day, soon.
Waterfall graph is useless in my opinion because there is no numerical data there, only presentation..
Since the SkyPipe saved the data in file, I will process that eventually and "flatten" the noise.. But even this is not right method, I really need direct output from ADC within USB RTL to SkyPipe..

I was thinking of looking into recorded and saved data file from SDR# (in PCM format).. Still a lot of preparation work to do before the system is really up and running..



Quote:
Originally Posted by mswhin63 View Post
I can utilise the waterfall graphing better than Skypipe. There is a bit too much noise on the signal to get a reliable reading.

Have you though about auto tracking as part of the project?
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  #39  
Old 28-04-2014, 08:15 AM
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I understand there is a difference here, I was considering the aspect of determining the signal for technical purposes. I don't know how to apply the signal as yet for science purposes but that would be another thing to consider for me after making my system.

I was meaning the spectral analysis as well. The waterfall makes it visually easy to see a signal in amongst the background noise, although your signal is higher than previous.
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  #40  
Old 28-04-2014, 09:19 AM
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Yes you are right, waterfall is actually spectrum vs time, and it's very easy to spot features, especially transients.
However, I am not sure this can be recorded with current version of SDR#.. and the data record is essential.
Another problem with recording raw data is the file size..
Since the SDR sample rate is huge, so would be the data file as well..
Maybe SDR# is not the ideal solution after all.. for spectrum analysis that is.
I tried to use my HP 8569B Spectrum analyser for this purpose, but it has limitations as well.. one of them is relatively high internal noise floor, so I need more (variable) gain in front of it (40dB or so).
So for this to do I will have to go for suitable down converter.
Tere are some mixers and VCO's stored in my boxes somewhere..

I am now considering building the AM detector for RF signal, and my own ADC, which can be controlled by SkyPipe.

Last edited by bojan; 28-04-2014 at 10:24 AM.
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