Like Jim's system most of these DF units do not depend on modulation modes
they use doppler effect and electronically rotate the antenna.
Popular Electronics a few years back had a DIY article to built a df unit.
They used a 16 bit counter and used its output to diode switch pairs of
antenna to get the pattern then used 8 LED's to indicate cardinal
direction. They used an external receiver and audio output to drive it so
that you could use on whatever band you like changing only the requirements
for a resonant antenna.
It makes for a nice project though and not overly expensive!
John



Mattsson wrote:

So, this may be an utterly stupid idea, but...

I know I can get bearings from an airport, or more specifically the tower,
by using my com and talking to them when flying and within range. Also,
radio amateurs have these "fox hunts" where they hide a small tx and then
hunt it down.

Now, I have a scanner that covers the civil airbands on AM and I already
have built a few homemade base antennas for it, following some nice
designs made by others. This allows me to listen to nearby airfield
traffic pattern communication.

Now, it would be a lot of fun to be able to use my scanner with a suitable
antenna array, to get bearings to an airplane talking to for instance
tower. I know I know, you report positions most of the time, but sometimes
the small planes give pireps on local weather conditions and the like,
sometimes they just give position reports and so on, not being that
accurate on their position. At other times, spending time at relative´s
summer cottages, having the scanner with me, it would be great to quickly
get info on where to scan the sky for nearby airplanes on the air, then
get the binoculars. And as the chit chat on the airwaves usually is very
brief, there is no time to start pointing that yagi or whatever to get
some idea of where the plane is. You would need some kind of automatic
direction finding, something that would give a direction for you in a
second or two.

Is there anything like this for a DIY project? I googled a bit and found a
lot on automatic FM DF projects, but not for AM. It´s understandable that
most people are interested in searching FM transmissions, especially those
interested in finding noise sources and the like, but I´m just interested
in spotting and plotting those planes up there! =)

Any thoughts and suggestions appreciated, Ken