The comprehensive performance of cognitive broadcasting networks employing Fountain codes (FC) and maximal ratio transmission (MRT) is investigated in the present paper. More precisely, the secondary transmitter (ST) employs Fountain code to effectively broadcast a common message such as a safety warning, security news, etc., to all secondary receivers (SRs) via underlay protocol of cognitive radio networks (CRNs). Different from works in the literature that are interested in studying the outage probability (OP), and the ergodic capacity of the CRNs. The present paper, on the other hand, focuses on the characteristics of the number of needed time slots to successfully deliver such a message. Particularly, we derive in closed-form expressions the cumulative distribution function (CDF), the probability mass function (PMF), and the average number of the required time slot to broadcast the message to all SRs. Additionally, we also provide the throughput of secondary networks (SNs). We point out the impact of some key parameters, i.e., the number of SRs and the number of transmit antennae at the secondary transmitter, on the performance of these considered metrics. Numerical results via the Monte-Carlo method are given to verify the accuracy of the derived framework as well as to highlight the influences of some essential parameters. Furthermore, we also compare the performance of the proposed networks with state-of-the-art and simulation results unveiling that the considered system consistently outperforms works in the literature.