If you do a search for "ARDF" you'll find a lot of information on hand-held DF receivers for 80m.
There are international competitions that involve running several miles through the forest
looking for up to 5 transmitters on both 80m and 2m. Receivers need to be compact and
easy to use.
Plans for a simple 80m DF receiver were published in QST for September 2005. (Ignore the ugly guy
on the cover who is modeling it.) It's direct conversion, and certainly not the best circuit,
but mostly adequate. Unfortunately the kits are sold out and I don't know if the board is still
available. I've built several versions using Ugly construction, though my recent versions
have been a superhet design that is, if anything, easier to build and works much better.
I've used such receivers several times for tracking down RFI. I've managed to tune them
up as high as 40m. Above that I built a simple 5 - 15 MHz loop that plugs into a receiver such
as the VR-500 and allows me to DF on other bands. Building a loop for separate receiver is
pretty simple. (And if you can hear the noise on the AM band, just use the build-in loopstick
antenna on a pocket portable receiver.)
When I was first asked for the loan of such a 80m DF receiver to track down RFI there were
only 3 that I knew of in North America - and I happened to have 2 of them at the time. Now
there are many more, over 100 at least. They are scattered around the country - I know there
are several in the Boston area. For other bands you'll have to modify them or build a custom
Meanwhile, this page has a list of resources for receivers and some links:http://www.ardf-r2.org/equipment/
There are many more circuits available - I have paper copies of some European articles, and
there are always newer designs appearing. I can help you track down an 80m receiver to
borrow, or come up with a design if you need it for other bands.