Most of what I have read on the subject suggests converting the solar radiation
to microwave by using photovoltaics and a microwave transmitter. Has there been
much discussion of converting the solar radiation to heat and then re-radiating
this in microwave ?

Below are two paper discussing highly directional radiation from a thermal
source.

http://www.opticsinfobase.org/abstract.cfm?URI=ol-30-19-2623
http://www.nature.com/nature/journal/v416/n6876/full/416061a.html

Here is an abstract:-

"We report the design of a tungsten thermal source with extraordinarily high
directivity in the near infrared, comparable to the directivity of a CO2 laser.
This high directivity is the signature of the long-range correlation of the
electromagnetic field in the source plane. This phenomenon is due to the
resonant thermal excitation of surface-plasmon polaritons."

I would have thought that a solar absorber and a heat pipe connected to a
microwave emitter would be simpler and lighter than going through an electrical
stage.

Pete.