New to radio astronomy

[Fong Zi Hao]

Hello all,

I am a A levels student currently studying in Malaysia and recently got interested in radio astronomy, particularly hydrogen line detection. I started by first looking at suitable antennas, used solid dishes are difficult to find here but the wifi parabolic kind is availalble online, costing about 60 dollars for a 90cm*60cm dish. I also sent emails to the local satellite company and amateur radio society for used dishes, still waiting for replies... 

Then I came upon the horn antenna assembly by DSIPRA https://drive.google.com/file/d/1qdc5lhKErFyIsc8b52ZIkCPJLi-XykSb/view

How does that horn antenna design compare with dishes like the wifi type and a solid 1m dish in terms of resolution and gain? One of my objectives is to complete a map of milky way, not expecting details from the optical, which I usually do with my 6 inch newtonian and equatorial mount ( which I believe can be used for the horn antenna in the future, so no need for drift scanning)

I also have picked up the pdf about Basics of Radio Astronomy for GAVRT, and looking for a book about antennas paramaters, like concepts about impedance which is so new to me. Would appreciate it very much if anyone can share websites/books regarding those concepts.

Thanks in advance.

[Dave Typinski]

Hello Zi Hao,

This whitepaper from Rohde & Schwarz covers the basics and some vocabulary:
https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/premiumdownloads/premium_dl_brochures_and_datasheets/premium_dl_whitepaper/Antenna_Basics_8GE01_1e.pdf

The gold standard is the textbook Antennas, by Kraus.

Hope that helps!
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[B & MR Randall]

Hello Dave, Zi Hao,
Dave, thanks for link to Rhode & Swartz paper. It is a good beginner reference. It does not mention Horn antenna???

Zi Hao, The horn antenna you mention is good. It will give much more consistent results than a modified Wi-Fi dish.
Advantages of Horn:
Clean pattern. It receives almost no signal in directions where it is not pointed.
The effective area of signal capture is well defined and can be calculated.
It was designed for 1420MHz and will match the preamp properly.
Disadvantages of horn:
It is physically larger than a dish that is of the same effective area.
It has a lot of wind load and is harder to mount securely.
It will collect rain water if it is left in the weather. It is a good funnel for rain as well as radio signals.

The radio telescope can be thought of as a one pixel camera with the pixel being several degrees across. It needs to be moved around to get an idea of an area of sky. The earth's rotation does a very good job of that. I use only drift scans. I will move the antenna by loosening bolts, position it to a new point on a north/south line, tighten bolts and, wait another day for object to go by the antenna again. The data must be time stamped accurately. It takes a few days drift scan and a bit of work with Excel sheet or Python code to produce results. The modern world uses Python or Octave for this. Because I am almost a fossil, Excel fits me.

Most people with lot of optical astronomy experience do not like the idea of a drift scan. It really does work well.

An accurate North-South line and an accurate clock are needed for drift scan to work. Any other mount for radio telescope has the same problem. Most radio objects are not visible optically so accurate telescope position and accurate time are needed.

Bruce Randall

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[Fong Zi Hao]

Hi Dave and Bruce,

Thanks for the link. The antenna bible by Kraus, kind of overwhelming for me, to be honest hehe

Yea, Bruce, after some consideration, I think that the diameter of horn antenna doesn't really come along with the increasing physical size, also the difficulty of mounting it on the mount saddle. 

Too bad, I couldn't find any used dishes still, but I found a new one selling 40 dollars, fair price I believe for a 1m dish. But I will try to dismantle the 65cm Ku band dish with the LNB I have at home, to start with simple sun detection, checking for the offset in altitude of the dish as well. Similar to the Itty-Bitty concept, but instead I will buy a rtl sdr to replace the signal meter so I can learn the software side first before diving into hydrogen line. No affiliation, https://shopee.com.my/RTL-SDR-Blog-V3-RTL2832U-1PPM-TCXO-HF-BiasT-SMA-Software-Defined-Radio-i.126241109.2505380545 . Do these work well for hydrogen line? Because I want to optimize most components for the HI line, so no need to spend extra cash. To conclude, the components I'll need now is a sdr and some coax cable, please correct me if I'm missing something.

I think with my mount that has sidereal tracking, although not useful at all for much bigger dishes, but for the ones I start with, I believe the pointing accuracy don't matter too much. With the 65cm, I am curious to see whether it is able to distinguish the HI peak or not, with long integration time, but that is for later.

Happy observing

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[David Fields]

Hello Zi Hao,

Building a horn antenna is a nice project.  My first horn was useless, though, since the reflective layer turned out to be non-conductive.  After I checked with an ohmmeter, I coated it with Al foil, and the horn worked very well.  This was the start of my design:

http://www.setileague.org/articles/horn.htm

I think you'll enjoy solar studies with an SDR, but that you will need to add a low-noise preamp to detect the H1 signals.  Modern preamps are much lower noise than what I used.  Here is a recent reference:

https://www.rtl-sdr.com/cheap-and-easy-hydrogen-line-radio-astronomy-with-a-rtl-sdr-wifi-parabolic-grid-dish-lna-and-sdrsharp/

If you're in an urban area then interfering signals may be a problem.  The horn may have lower gain than a 1m parabolic dish, but it will likely be better at rejecting interference.  If the spurious signals are still a challenge, then you might add a band-pass filter at one the other end of your preamp.  By the way, preamp power can either be through separate power wires, or with care, through the signal cable.

I usually mounted my horn on a tripod, but it was fun to tape it to my SCT telescope (equatorial mount) and use the setting circles.  That would have been even better with a GoTo mount.  Actually, my horn was almost too heavy for this approach.  A better approach would be to use the thinnest foam board and thin internal bracing.

Wishing you success -- let us know your progress!

David Fields

[Paul Oxley]

Zi Hao

If you build a horn, it can be optimized on size by following the design parameters from the web. Just search for "Optimal Horn". The following is an example from Wiki:

Horn antenna - Wikipedia

As it states, the optimization occurs when a balance is achieved between two reflections in the horn. It is changed by the slant angle of the sides.

Hope this helps.

Paul Oxley

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[Marcus D. Leech]

I'll point out that the original Green Bank 300ft radio telescope was a
"transit" type instrument. Carefully aligned on the N-S
meridian line, and driven only in elevation (effectively in declination).

Our (www.ccera.ca) 21cm dual-polarization spectrometer telescope is also
a transit-mode ("drift scan") instrument.

Such instruments are very good for doing sky-mapping and broad surveys.
Not quite so good for doing studies that require
"following" a specific object for a period of time to allow long
integration times for that specific object. Most pulsars require
tracking, since their pulse brightness is very low, so you have to
integrate for a long time--usually requiring tracking.

[Fong Zi Hao]

Some updates:

I have a Ku Band dish with me now and I am seeking for some advice on the budget setup I plan to use for solar studies. 

LNB --> RG6 ---> Bias T ---> RTLSDR

-Does cable length matter for solar detection with sun being a strong radio source? I have an old 1.2m RG6 cable with one end connecting to a F male connector and not sure whether I should I get a longer one to avoid interference from the laptop as the LNB converts the frequency down to 950-2150Mhz.

-The bias tee uses SMA K connector, I believe it should be the same as the usual SMA connectors? And will be it okay to directly connect the bias tee to the RTLSDR using SMA Male to SMA Male connector, so I don't have to buy another cable. The bias tee doesn't have a metal cover so I wonder whether it is susceptible to interference. I will be powering the bias tee with a 15V 1A DC adapter which should work for the LNB as I couldn't find the specs of the identical model.

-On the LNB, it says SCR and standard, which one do I use?

Thanks in advance.