Ferrite rod antenna improvements
They had used an auxiliary coil to provide a positive feedback. This means that due to ageing and drifting, the receiver had become a transmitter. The first use was introduced by Nikola Tesla with his patent. 

It's a kind of weird functionality, like sending the received signal back into a coil antenna to get more received signal. I guess, more or less they had be projected with experimentation. The US submarines still use this antenna for VLF, the modern devices replaced this technology with hetherodyning to prevent transmission in case of malfunction.