It significantly speeds up all of your signaler's radio communications. If you have a signaler and plan any quick action support, buy an antenna first.
The antenna is rather fragile and needs to be treated with care e.g putting a sandbag in front of it. Keep it in the back of the field- it'll stay up longer and save you TP in the long run.
Antenna can also be found from Mobile HQ half track upgrade.
Where should you place it?Edit
It is best to keep this in the lower half of the field. Try and place it where no explosives or tank fire can reach it. Try placing it in a pillbox (it can still be blown up, though) with a signaler and an officer and watch the fun!