Reading the responses, some valid, some not*, I guess the best
thing to try is:

o illuminate a surplus TVRO 12 ft dish with the magnetron
- collect and focus side lobes
- 36+ dbi gain
o use a regulated power supply the decrease the bandwidth
o use some form of bandpass filtering such as cavity resonators
o point the thing at the moon and listen for echos
o experiment using it as a ground mapping radar

I vaguely recall seeing something in a late 80's (?) magazine, may
be 73 or CQ VHF, that built a digital data link from a pair of 2 mbit
PC network cards and a magnetron.

* FYI:
o GSM cell phone bands = 850/1900 MHz for the Americas,
900/1850 MHz outside the Americas, not 2.4 GHz.
o microwave ovens with the door closed already interfere with
most 2.4 GHz ISM band FCC Part 15 devices in close proximity