One of the most efficient antennas to listen to low frequency signals (<500kHz) is the loop antenna, which has two main nulls, allowing for high directionality. The antenna itself is comprised of lamp wire loops of wire tuned with a variable capacitor I picked up from a ham swapfest. I designed the antenna to be resonant at ~300-400kHz to listen to weak NDB beacons used for navigation. 

Many of these beacons are located in remote areas of Russia and Alaska and operate on very low power (<500W). As these signals are pretty weak, I picked up some second-hand parts from more ham swap meets and built a preamp which runs off a 9v battery. When testing this at first inside my apartment, I couldn't hear anything. I soon discovered that QRM from my apartment complex's shared ground propagated through my power supply and into my radio, drowning out these weak signals. As a result, I have to operate mobile, far from the noise of Boulder, running my receiver off a LiFePo4 battery. 

I've heard many different beacons, and at the lower end of my receiver's range (100kHz), I've heard strong broadband emissions from equatorial lightning. This entire project cost less than $100, and was a complete success! I had a blast and learned a ton.