balun divide

I want to disign broadband balun divide.the frequency up to 980MHz ,
power up to 50W.Could you give me some advise ?

you can use coax and balun core to get a broadband balun

use a trifilar toroidal transformer,the band can be very wide

I think you can try a planar balun, i did it and it was a good choice. you can try reading the motorola application note 1034 to the right choice of the topology and the article "Vertical balun and wilkinson divider" by Darvish ezzedine you can find on the net if you want to do it in a planar way. The advice is to built it in a vertical way to reduce the parasitic.
I hope this is useful for you

what is the bandwidth ?

I know that the balun always is narrow banded.