1) an artificial antenna (one which does not radiate radio energy, therefore does not communicate)
2) something (or someone) who absorbs energy but gives nothing in return
3) an male ejaculate that contains no sperm (that is, he has had a vasectomy)
1) he put a dummy load on the transmitter to test it (without it radiating and possibly interfering with other communications).
2) Nigel's a dummy load.
3) He delivered a dummy load into the box.