Barry you might find the following links of some value:
http://www.ggrweb.com/article/gulley.html
http://www.tuc.nrao.edu/~demerson/helixgain/helix.htm
There are many sites dealling with Helix antennas, you could snoop around for a while and pick up some good info.
These above are not designs for 21cm, but they will give you a rough idea.
Jerry raised the point that these antennas would need to be driven; at the smallish size/weight these helix antennas are, they could be mounted on the front of an OTA; driven, guided by the mount.
A receiver? well as John said they don't come cheap. Andrews have a scanner that will go that high for around $499. I think the pc radio mentioned elsewhere on ISS would give real value for money because you could graph the signal strength/bandwidth of your received signal whereas a scanner would maybe hiss and crackle, but not much more. Poor return on your investment.
cheers,
Doug