seeking radio hacking advice

Hans Fugal hans at fugal.net
Sat Feb 3 16:29:33 MST 2007


Hi Steve et al,

I tried this with the not-so-old alarm clock radio and although I found
4 screws none of them seemed to change the tuning range. So I fished on
eBay until a radio that receives air band came up within my budget.

I can hear the airport quite well, and I'm overall quite happy with my
deal. However it is obvious that they are using the overdrive FM filter
trick, because to listen to air you put it on the FM/air switch and just
tune up above FM. Thought that would be an interesting thing to throw
in. The radio is a Steepletone SAB9 MK3.

When someone is talking on the radio, there's no noise to speak of. If
the pilot pauses to think for a second while transmitting, it's silent.
But when there's no signal at all, the static is unbearably loud (well,
you can bear it for awhile, but you can't just listen continuously).  I
think I can simulate some kind of squelch in postprocessing (hooking its
headphone jack up to my sound card), but I'm very curious.

Why is the static (noise) where there's no signal (e.g. between
channels) so loud? It seems to be the same effect on TV, AM, FM, etc. as
long as there's nothing squelching. Why, if the noise floor is so far
below the signal floor, is the noise so loud? Is the receiver just
amplifying/limiting to a certain power output, so disproportionately
amplifying the noise? 

On Mon, 15 Jan 2007 at 10:53 -0700, Steve wrote:
> No, you'll not be able to mess with an FM radio and tune AM traffic.
> However AM radios especially if you can find an AM older AM radio, at
> like DI or savers or something, are a deffinite candidate.
> 
> Here's what you do.
> The "tuner" in an AM radio is really just a variable capacitor.
> That capacitor is set to be variable within range x to y.
> This setting is or at least used to be, set using little tiny screws
> inside the "tuner"
> These are typically easily accessible.
> Just turn the radio on,
> 
> Tune it to the lowest range it will reach.
> Then insert a screwdriver into one of (note this will be trial and
> error) the little holes in the tuner and turn very, very slowly, until
> you start hearing traffic from the highest end of the spectrum.
> Then turn the dial all the way to the max, and do the same thing
> (probably going to be a different screw this time), until you hear the
> out of band traffic you are looking for.
> 
> I recommend looking for an older AM radio, possibly from the 70s or
> 80s before "digital tuning" was common.  For ease of adjustment I've
> found the older analog clock radios to be the best bet, and those with
> multiband (am/fm/TV) are a sure fire candidate since Air Traffic
> Control operates (or at least it used to operate) just out side the TV
> spectrum and uses AM.
> 
> I hope thats helpful,
> 
> Regards,
> 
> On 1/15/07, Hans Fugal <hans at fugal.net> wrote:
> >I'd like to listen to "airband" traffic. That's 108-137 MHz amplitude
> >modulated.
> >
> >They sell scanners that cover airband, but they start at $80 minumum and
> >I'm much too cheap for that at the moment.
> >
> >I've found a few options.
> >
> >First, I see a couple of people vaguely recommending that if you take a
> >radio with an analog tuning wheel, and do some funny stuff to the
> >inductance coils or something or other then it will magically tune
> >airband instead of FM. Frankly I don't understand this but it sounds
> >promising.
> >
> >Second is to build something like this:
> >http://hans.fugal.net/tmp/ajp.html (his site annoyingly circumvents deep
> >linking with silly javascript, so I mirrored it) I can go to Radio Shack
> >and buy parts, and I know my way around a soldering gun, so I think I
> >could do this. It looks like fun and not too big a project. But are
> >there special techniques to winding or anything else that will make any
> >attempt by novice me a likely failure? If Radio Shack doesn't have the
> >TBA820 can I use some other amplifier? etc.
> >
> >Third is to get lucky on eBay.
> >
> >Basically I'm looking for someone to answer my silly questions as they
> >come up so I can have a fun learning experience. I know there's a few
> >HAM operators in here and others with experience in this sort of thing.
> >
> >And if anyone has a scanner that does the VHF frequencies listed above
> >and just wants to give it to me, I wouldn't balk at that either. :-)
> >
> >Just to keep it on topic, I'd like to hook up my gadget to the computer
> >running linux and broadcast what I pick up for KLRU at LiveATC.net.
> >
> >--
> >Hans Fugal ; http://hans.fugal.net
> >
> >There's nothing remarkable about it. All one has to do is hit the
> >right keys at the right time and the instrument plays itself.
> >    -- Johann Sebastian Bach
> >
> >
> >-----BEGIN PGP SIGNATURE-----
> >Version: GnuPG v1.4.5 (GNU/Linux)
> >
> >iD8DBQFFq7qw/JjozeCq1GARAhIVAKCQCHPBpHdRZDvdesJqnhV0LfilagCg6qZt
> >FN30lg2360OGVVKvcsQG1wE=
> >=s3bx
> >-----END PGP SIGNATURE-----
> >
> >
> >
> >/*
> >PLUG: http://plug.org, #utah on irc.freenode.net
> >Unsubscribe: http://plug.org/mailman/options/plug
> >Don't fear the penguin.
> >*/
> >
> >
> 
> /*
> PLUG: http://plug.org, #utah on irc.freenode.net
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> 

-- 
Hans Fugal ; http://hans.fugal.net
 
There's nothing remarkable about it. All one has to do is hit the 
right keys at the right time and the instrument plays itself.
    -- Johann Sebastian Bach
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