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Go Back   IceInSpace > General Astronomy > Radio Astronomy and Spectroscopy

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  #1  
Old 20-09-2016, 02:07 PM
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My Small Radio Telescope (SRT), version 2.0

Yesterday I received the Nooelec RTL module (with TCXO, 0.5ppm) from ebay. It works with rtlsdr_scan (PC Spectrum Analyser, also records results in scv format).

Also, I started building the new interdigital BP filter (3 stages, design is here), hopefully this time the performance will be better in terms of flatness and losses, so there will be less interference (last time I didn't use filter).


I will post again about the progress when there is one... one of the most significant improvements to the system will be motorization of the antenna, and coupling to CdC.

Last edited by bojan; 20-09-2016 at 05:26 PM.
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  #2  
Old 22-09-2016, 08:17 PM
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Keep us updated progress, even I am starting to get curious about RA due to 'visual factors' lately!
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  #3  
Old 26-09-2016, 07:42 AM
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Construction of BPF.
The sides were cut from Aluminium bar 20x6mm (Aluminium Trade Centre).
Rods are from ebay, brass hex 50x5mm spacers, cut to required length (47.5mm)
The internal surface of the resonator cavity will be covered by copper sticky tape, to ensure the best possible conductivity (according to design, losses should be no more than 0.4dB)
The filter on the image is not tuned yet, it is only test assembly.



**************
Interdigital Bandpass Filter, based on work of Jerry Hinshaw,
Shahrokh Monemzadeh (1985) and Dale Heatherington (1996).
http://www.changpuak.ch/electronics/...r_designer.php
Javascript Version : 09. Jan 2014
-------------------------------------------------------------------------
Design data for a 3 section interdigital bandpass filter.
Center Frequency : 1420 MHz
Passband Ripple : 0 dB
System Impedance : 50 Ohm
Cutoff Frequency : 1410 MHz and 1430 MHz
Bandwidth (3dB) : 20 MHz
Fractional Bandwidth : 14.1 MHz
Filter Q : 71
Estimated Qu : 2064.25
Loss, based on this Qu : 0.597 dB
Passband Delay : 31.831 ns
-------------------------------------------------------------------------
Quarter Wavelength : 52.78 mm or 2.078 inch
Length interior Element : 47.59 mm or 1.873 inch
Length of end Element : 47.71 mm or 1.878 inch
Ground plane space : 20 mm or 0.787 inch
Rod Diameter : 5.5 mm or 0.217 inch
End plate to center of Rod : 13 mm or 0.512 inch
Tap to shorted End : 2.64 mm or 0.104 inch
Impedance end Rod : 89.870 Ohm
Impedance inner Rod : 91.889 Ohm
Impedance ext. line : 50.000 Ohm
-------------------------------------------------------------------------
**** Dimensions, mm (inch) ****
# End to Center Center-Center G[k] Q/Coup
0 0.00 (0.000)
1 13.00 (0.512) 32.44 (1.277) 1.000 0.707
2 45.44 (1.789) 32.44 (1.277) 2.000 0.707
3 77.88 (3.066) 0.00 (0.000) 1.000 1.000
4 90.88 (3.578)

**** Box inside dimensions ****
Height : 52.78 mm or 2.078 inch
Length : 90.88 mm or 3.578 inch
Depth : 20.00 mm or 0.787 inch
Attached Thumbnails
Click for full-size image (1.42GHz BPF filter.jpg)
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Last edited by bojan; 26-09-2016 at 12:14 PM.
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  #4  
Old 26-09-2016, 01:59 PM
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Neat flat ali bar. good to see how well it works
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  #5  
Old 27-09-2016, 01:02 PM
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Question

Bojan, did you ever consider making the box from double-sided PCB and using brass rod for the lines?
Robert
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  #6  
Old 27-09-2016, 01:26 PM
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Yes, I have... and I would have done it if I had thicker PCB copper cladded fiberglass plate at hand (even single-sided would be OK).
I think the approach with aluminium should be mechanically more stable than standard 1.6mm PCB. Also, I can open the aluminium box much easier, in case there is a need for minor adjustment (like length of the rods are not adequate, or tap needs to be moved a bit to get better matching...).
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Old 22-12-2016, 02:05 PM
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There was an recent update of rtlsdr_scan (PC Spectrum Analyser) software.
I asked the author couple of weeks ago for continuous scan recording on the hard disk, and apparently it is happening just about now or very soon (release should be dated 20/12/2016... with version from September, only last 500 scans were available for saving in cvs format).
With this new feature, the application will become really useful for radio astronomy.
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  #8  
Old 17-08-2017, 07:47 AM
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Still no 100% joy..
This filter seems to have too low coupling between resonators. Maybe a thicker rods would improve that? I will try on first occasion.
Copper sheets (RS components) did not help to reduce the losses either ...
However, this filter is better than the previous one, loss is ~2.5dB, ripple is 0.5dB in 2MHz band... good enough to be placed between LNA and amplifier.
Attached Thumbnails
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Click for full-size image (DSC_0434.jpg)
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Click for full-size image (DSC_0436.jpg)
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  #9  
Old 17-08-2017, 03:06 PM
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I checked Dishal method on Johanson webage..
I think I remember now using it back in 80's...
Anyway, it seems it is not necessary to monitor a phase, return loss may be sufficient. I will try....
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