I have been having issues with RFI across the HF spectrum at my QTH, and having killed the house power at the main breaker, the RFI was still present, so I decided to build a direction-finding (DF) antenna to attempt to locate the source.
A quick web search yielded a link to the ARRL, with design notes to build such an antenna.

http://www.arrl.org/files/file/Antenna%20Book%20Supplemental%20Files/24th%20Edition/Handheld%20DF%20Antenna%20-%20NA6O.pdf

I used what parts I found in the junkbox, and it took 2-3 hours to build. The tuning is quite sharp, so I may add a slow motion drive to make tuning easier.

The next step will be to wander the neighbourhood, waving the DF loop around, and likely get some odd looks from the neighbours.

DF loop antenna

Tuning unit with variable and fixed capacitors