actually you can do quite a lot with simple HF kit
and a directional antenna is not really required but simple dipoles will suffice to start
with an HF receiver you can also add a preamp for LF if the receiver goes low enough too or add a simple VLF converter
space radio on shortwave can be easy to start
you can also build an interferometer using a satellite TV LNB and dish too
ill see if i can find some links and post some info too
rgds
Simon ZL4PLM