a question for hams/radio heads: radio astronomy

[linear_shift]

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 here (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.

[NoTime]

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.

[berkeman]

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_%28radio%29

[linear_shift]

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.

[berkeman]

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.

[linear_shift]

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 (http://ufro1.astro.ufl.edu/dec3.htm)). 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).

[NoTime]

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.