The design and implementation of microwave equipment is considerably more difficult than at 900 MHz and therefore generally is manufactured only by truly capable firms. Note that with the exception of Microtek's consumer line, these firms sell their products to law enforcement only, so don't waste their time if you are not government. One of the requirements to meet FCC criteria is the product cannot interfere with other users of the radio spectrum. If the signal is weak enough the chances of interference are very low. FCC specifies a maximum field strength permitted for a transmitter.
Field strength is created by a combination of transmitter power output and antenna efficiency.
You cannot increase transmitter power output to increase range and still be FCC legal. Likewise, you cannot improve the transmit antenna without also increasing the field strength beyond legal limits. Type accepted transmitters are not allowed to have removable antennas to prevent people from replacing the antenna with one having more gain or efficiency. The bottom line of all this is a 900 MHz type accepted device, regardless of the manufacturer, is limited to a relatively pipsqueak signal. The typical transmit range of a legal video transmitter on 900 MHz is around 150 feet in open air.