A question for hams/radio heads: radio astronomy

In summary, the conversation is about using a cheap shortwave radio, the Kaito KA-2100, for radio astronomy and whether it is possible to disable the AGC (automatic gain control) for this purpose. The conversation also touches on different controls and settings on the radio that could potentially affect the AGC, as well as the type of antenna that would be best for radio astronomy. There is a disagreement about whether the AGC should be disabled for radio astronomy, with some saying it is necessary for linear amplification and others saying it can introduce unwanted noise. Ultimately, the conversation ends with a suggestion for a specific antenna design that could work for the radio astronomy project.
  • #1
linear_shift
16
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I am looking at a cheap $100 shortwave radio, the Kaito KA-2100 (google it, there are a bunch of links, too many to mention), and I would like to know if disabling the AGC (automatic gain control), is possible for use as a radio telescope. Now I don't know that much about radio and electronics (a little here and there though), mainly being an optical astronomer, but under my scope of knowledge there are a few things that stand out:

1. The is an RF Gain control. Could using this disable or set the AGC threshold in any way?
2. There is a DX/Local switch. Could setting this in Local kill the AGC (seems to on my cheapo R'shack SW radio)?
3. This is most pressing, there is an IF (intermediate frequency) output. Could this bypass the AGC?

Now I found a schematic on the web, but I can't make any sense of it (too much stuff, poorly labeled/laid out, and even some traces connected nonsensically). It is http://nowsat.mforos.com/132607/6582475-roadstar-tra2350p/?pag=2 (search the page for the phrase "on the interweb" or look for the only post there in English). Anyway, any help is appreciated (even by telling me of another SW radio with an AGC disable under $200 would be helpful). Thanks, ls.
 
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  • #2
I for one am confused about why you are concerned about AGC (automatic gain control).
It reduces gain when the siignal gets too strong and has no action on a weak signal.

For astronomy I would think that the antenna would be a problem.
 
  • #3
I'm with NoTime on this. You probably want max gain for your purposes, which is where the AGC will end up with only weak signals. You want to be in DX (distant communication) mode for the max gain.

What frequencies are you intending to monitor? What antenna configuration are you going to build? What antenna gain are you shooting for? What terrestrial signals are you going to need to avoid, and how do you plan to avoid them?

http://en.wikipedia.org/wiki/Antenna_(radio)
 
  • #4
According to other astronomers, the AGC tries to hold the audio level constant, which is a no no in radio astronomy. Correct me if I am wrong, again, I don't know a whole lot about radio. The antenna... Well there I was planning to use what has been coined as the "phased dipole." In essence I need to use a half-wave dipole with the highest gain I can get on the 20-10m band (about around 13 MHz or 22 MHz, this corresponding to the FCC RAS quiet-zone allocations). Anyway, thanks for replying (at least I'm not going on about toroidal magnets and Gauss guns this time, remember me now? :wink: ), ls.
 
  • #5
linear_shift said:
According to other astronomers, the AGC tries to hold the audio level constant, which is a no no in radio astronomy. Correct me if I am wrong, again, I don't know a whole lot about radio. The antenna... Well there I was planning to use what has been coined as the "phased dipole." In essence I need to use a half-wave dipole with the highest gain I can get on the 20-10m band (about around 13 MHz or 22 MHz, this corresponding to the FCC RAS quiet-zone allocations). Anyway, thanks for replying (at least I'm not going on about toroidal magnets and Gauss guns this time, remember me now? :wink: ), ls.

Could you please provide some web pointers to this advice from the other astronomers? I need to understand what they are saying on a more technically precise level. Thanks.

AGC increases the gain to maximum when there is no signal, and pulls the gain back down when there is a signal, to keep the input amplifier string in the best linear range for the size of the signal coming in. Does that make sense?

And for the antenna, you need to plan to use the highest gain, most directional antenna that you can for your radio astronomy project. At the very least, that would mean a 3 element Yagi (main dipole plus one director pair [slightly shorter] and one reflector pair [slightly longer]). A better antenna would be a many-element Yagi, but the 3-D directional motor swivel for radio astronomy will start to get pretty challenging (and fun!), the bigger you make your antenna.
 
  • #6
I really couldn't find any technical data on the reason for disabling the AGC but is apparently generally accepted that you shouldn't use the AGC for *any* radio astronomy (link here). Also, a *yagi* for the HF band? That thing would be huge, even for the shorter HF wavelengths! I don't know of anyone who has built anything like that, ever (or least mounted it on a amateur equatorial mount). I was going to build a small yagi and mount it on my equatorial mount, however, but I need a rather costly uhf receiver for that (I need it to do 406 MHz, the FCC allocation for UHF continuum scans).
 
  • #7
For a 10-20m antenna you could try something like this.
Nice idea.
Yea they are big.

http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=9351

I looked at your link. It didn't help much.
In general for an AM receiver AGC reduces the gain to the IF circuits to keep them from being overloaded.
Overloaded circuits can produce outputs which have nothing to do with any actual input signal.
Signal compression is usually associated with FM IF where the compression is intentional and beneficial.
 

1. What is radio astronomy?

Radio astronomy is a branch of astronomy that involves the study of celestial objects using radio waves. This includes the use of specialized equipment, such as radio telescopes, to detect and analyze radio emissions from various objects in the universe.

2. How is radio astronomy different from other branches of astronomy?

Radio astronomy is different from other branches of astronomy, such as optical astronomy, because it focuses on the detection and analysis of radio waves rather than visible light. This allows us to study objects and phenomena that may not be visible in the optical spectrum.

3. What can we learn from radio astronomy?

Radio astronomy allows us to study a wide range of objects and phenomena in the universe, including galaxies, stars, planets, and even the cosmic microwave background radiation. By analyzing radio emissions, we can gather information about the composition, structure, and movement of these objects, as well as study processes such as star formation and the evolution of the universe.

4. How do radio telescopes work?

Radio telescopes work by collecting and focusing radio waves from outer space onto a receiver. The receiver then amplifies and converts the radio waves into electrical signals, which can be analyzed and interpreted by scientists. Modern radio telescopes also use advanced technology, such as interferometry, to improve their sensitivity and resolution.

5. Why is radio astronomy important?

Radio astronomy is important because it allows us to expand our understanding of the universe and its origins. By studying the radio emissions from celestial objects, we can gain insights into the physical processes that shape our universe and potentially discover new objects and phenomena that may have been previously unknown. Additionally, radio astronomy has practical applications, such as in communication and navigation systems, that rely on the use of radio waves.

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