Hi, I'm relatively new to A/V and microwave, but I guess there are two options:
1. Build an external RF amplifier for the wavelength/frequency of your system and use a standard antenna (like the one you're already using. This is a hassle, requires power, and is probably illegal whereevr you are, since A/V senders are usually sold with the highest output power they are allowed (10mw).
2. Buy/Build a 'bigger' antenna. Since most antennas used on wirelss A/V systems have a maximum gain of about 5dbi, you could build a directional antenna (like those pringle can ones on the net used for 802.11x) these can have gains up to 17dbi, and they're directional. So you can get your range without breaking the law.