Detection of the redshifted 21cm signal is a key to unlock the mystery of early cosmic evolution. The global 21cm signal from the cosmic dawn era can be measured with a single antenna. This can be achieved with a satellite on the lunar orbit, where the absence of ionosphere and radio frequency interference (RFI) enables high precision measurement. We describe here the design of a wide-band, frequency-independent disk-cone antenna for such a satellite, which is to be part of the lunar orbit array Hongmeng project. In this paper we will focus on science requirement, the design and fabricating method, simulation and measurement results of the disk-cone antenna. Maximum direction of the disk-cone antenna placed on the ground is between 60°-70° at 50 to 120MHz.