No antenna needed (counterproductive.)
Check render distance (must be 10 chunks or more.)
Where are you waiting? More than 24 distance from all spawning surfaces and less than 128?
If you can, go into spectator mode and determine that your flushing is operating properly and not stuck somehow.
If on server:
Your overworld spawns are divided between all players in overworld.
- This can't be helped except by building your mob farm not in the overworld, or having all players in the mob farm waiting spot, or having nobody else on.
The server render distance must be 10 chunks or more (player render distance doesn't matter)
- Ask an admin if you're not admin.
The 10 chunks thing sounds weird but is a long standing bug - shame, Mojang!