Direct RF undersampling is an effective technique for miniaturization of HTS on-board unit. It can realize a reception bandwidth up to half of the sampling frequency fs/2 theoretically. However, if it expands reception bandwidth until fs/2, roll-off part of an anti-aliasing filter effects seriously on folding noise. In this paper, we propose a new folding noise model for broadband considering a roll-off part of an anti-aliasing filter. Based on the proposed model, 28 GHz band direct RF undersampling broadband receiver is designed and evaluated. As a result, the reception bandwidth is achieved 94% of fs/2.