1. -- Not really. If by antenna you mean the thing that sticks out of a handheld/backpack radio. Well, a radio tower is technically a huge radio antenna.
--- You can broadcast with both, but the tower will be able to recieve/send more signals and over greater distance than the portable one.
2. -- Depends on which army. Usually there's the kind that gets mounted in vehicles, one that is carried as a backpack, and some times they might use personal radios for SWAT type situations. There is also a large hand-held device used in some places, but I don't know if that is in use anymore.
-- See above
-- For the personal ones, not very far, a couple of hundred meters at the most. The backpack ones can probably send/recieve over several kilometers if the circumstances allow for it. The vehicle mounted ones can most definately send/recieve over several kilometers.
-- Not sure here, I'd say a couple of hours for the personals, several hours for the backpack ones (24h, 48h, I'm not sure) and indefinite for the vehicle mounted ones as they recharge from the vehicle's battery, and that is recharged when the vehicle starts it's engine.
3. -- Yes, if he is talking on the right frequency/the chopper is listening on the right frequency.
-- A standard radio frequency that both of them are tuned into.
4. -- There are certain radio frequencies that are reserved. Some for police use, some for paramedics, some for firemen, some for coastguard etc., and there's a general emergency frequency. And then each army has a set of frequencies that they have reserved, no one is allowed to broadcast on them except army people. I have no idea which frequencies though, so don't ask
-- Yes, if they tune into the right frequency.