IMO, you would have a more solid feed if you use a 5GHz analog or digital video transmitter/receiver (like the FPV RC plane and drone hobbyists use). You can then encode that on the ground at whatever bit rate and quality you desire. You won't suffer from packet loss or disconnects like you will with WiFi. Remember, the balloon receiver will be receiving a huge amount of WiFi traffic from that altitude and may have problems sustaining the link even with a directional antenna. There is no way to filter out unwanted WiFi or other traffic in the RF sense, only in the protocol, which will be swamped with unwanted garbage.