I wondered about this as well. All of the information I can find, including DPD website, indicates that it is receive only. This is not to say that you could not transmit through it if it had a reasonably low SWR at the desired frequency. However, I doubt that it is broadband compared to a discone. It is supposed to be a half-wave antenna and on frequencies where it is resonant it might have some gain over a discone.
You can find designs for "X" antennas in amateur radio publications.