I have acquired one of those broad banded software defined radios, an Icom PCR-1000 to be precise, and all is well. I am enjoying listening to various MF, HF and VHF radio stations. However, there is a slight problem. Very slight, almost too small to even mention, more of an inconvenience than a problem. Still, if I am being inconvenienced, than others are too. This issue is with the antennas. My K9AY antenna works wonderfully from 500 KHz to 25 MHz or so. My discone antenna works wonderfully from about 30 MHz all the way up to about 1 GHz. In order to enjoy the full range of the receiver, I need to switch antennas. I have a small switch on my desktop, but it seems inconvenient to reach over and switch it when going from the AM band to the FM band or something similar. Therefore, I have decided that I need an HF/VHF receiver diplexer. One would think that such hardware is ready made for such instances. However, nothing I could find commercially would do the trick.
Thus, since I could not buy one, I decided to build one to add to my collection of receiver doo-dads and nick knacks. The design is relatively easy, a back to back low pass/high pass filer system with a 50 ohm impedance throughout. Something with a sharp cut off around 30 MHz or so: