Howdy Ian and Malcolm. I've been experimenting with radio over the last few months. As always, the type of rig is pretty dependent on what you want to look at. The targets I considered myself were the Sun, Jupiter and other deeper sky targets like galactic core etc. Cutting a long story short I went with solar observation, but the basics can be adapted to other sources as well.
I am scanning the RF spectrum from 25-88MHz looking for artifacts from solar events. So the rig is essentially a "
Solar Radio Spectroscope". It consists of a wideband discone antenna, antenna amp (can use a satellite finder which has an amp built in and just has a signal strength meter on the front). Then USB TV dongle and software (aka Software Defined Radio - SDR). There's more info in the link above, which is on my blog. I've also added another post with more technical specs
here.
The whole lot cost me less than $200 and is giving me plenty of data to play with, even with a very quiet sun. Happy to elaborate if you want to know more...
Tony