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This will take up volume in space. Your first choice should be a parabolic reflector type that is tens of wavelengths in diameter. The reflector is wideband and only the feed restricts the frequency range.
Other types are narrow band and expensive for the gain you need.
Chapter "Reflector Antennas" in "Antenna Engineering handbook" by Jasik & Johnson
Chapter "Reflector Antennas" in "Antenna handbook" by Lo & Lee
your solution also may be Array Antenna such as patch Array, Horn Array,... or Reflectarray Antenna
not enough info ..... it would really help us to know for what freq you need it for.
for a MICROWAVE BAND (above 1000MHz) making antennas highly directive is easy
for freqs below 1000MHz it becomes much more difficult