The discone has no gain. It is designed to be a very wide band antenna,
not a gain antenna.

Whatever antenna you put up, if you are mainly interested in 800 meg
reception put the BEST coax you can. Stay away from the thin RG-58 like
the plage. As mentioned, there is a loss in the coax. However, there is
considerable variety in the amount of loss with differnt coax. AND the
higher the freq (800 meg espically) the higher the loss using a given
coax. (coax A may be fine for VHF, but terrible for 800 meg, while coax B
may be excellent for VHF, and OK for 800 meg) Also, if you can arrange it
try to make your coax run as short as physically possible to minimise
loss.

Basically, you get what you pay for when it comes to coax.

Richard in Boston, MA, USA