weather balloon link budget calculation

hi friends...........
how can we get the parameters of the antenn we design for link budget calculation.?i using a yagi antenna at ground station and dipole antenna at space station

You can handle the link budget like any RF or microwave link design. The advantage of your situation is that there is no ground along the path, so your link is a "free-space" one.

For a Yagi antenna you only need to input its gain, and for the dipole you can take its gain equal to one (0dB). Knowing the transmit power, propagation loss over a maximum distance, you will get the input S/N ratio at the ground receiver. Adjust it for your modulation type to get a good received signal or video.