I am afraid that there is no single antenna that will work well across the entire frequency range covered by the RSPdx. That’s the main reason that it has multiple antenna ports. For operation from VLF to 300 MHz, I can strongly recommend the Bonito Megadipole MD300DX. It is NOT cheap, but it’s performance is excellent. For frequencies above 300 MHz, a Scanking Royal Discone 2000 is probably a reasonable choice if you want just one antenna to operate up to 2 GHz. However, please be aware that this will give you no gain and so the performance will be modest at best. If you want the best performance then I am afraid you will need dipoles cut to the specific band of interest.