Re: turning on cisco aironet bridge without any antenna
According to Cisco documentation, you should not power up an access point without it being connected to an antenna, so as to not burn the antenna leads inside the AP. Other documentation suggests that if you are going to power an AP without an antenna being connected, the AP should be broadcasting its signal at no more than 5 mW. This is supposedly not enough power to burn the leads.
It will not hurt the AP to be powered up for the configuration and testing phase. I would not allow it to run for more than one hour without antenna's connected though. I have configured over 800 AP's in the last year and 95% were not connected to antenna's during configuration.
Also, you may want to consult Cisco's site for the wattage setting with using the 21 dBi dish. I think the max is 20 mW when using the dish, based on the 1W max specified by the FCC.