The lowest frequency that a standard RTL-SDR dongle can receive is about 24 MHz. However, by applying a hardware hack called the direct sampling mod, it is possible to use the RTL-SDR to listen to the HF frequencies.
Usually the direct sampling mod requires that you add a separate antenna port to the dongle, but Martin G8JNJ decided to take another route and instead use a diplexer to be able to use the same antenna port for both HF and VHF/UHF. A diplexer allows both HF and VHF/UHF signals to coexist on the same input port without causing interference to one another.
Along with the diplexer Martin added an impedance transformer, added additional coupling capacitors to the power rails and removed the IR LED components to make space for the transformer. Martin writes that the final modded RTL-SDR allows for tuning between 15 kHz to 1.8 GHz.