Are special antennas required for 802.11n connection?
No.

Antennas do have a bandwidth; for example, an FM Radio antenna will only efficiently cover around 80 – 110 MHz. If you wanted to listen to a signal that is broader than those 30 MHz, you'd need a different antenna.

802.11n does have 40 MHz channels instead of only the 20 MHz of a/g, but devices for WiFi don't just cover a single 20 MHz channel – they need to work on all WiFi channels, and thus, they are far, far broader from the start.