This paper studies a technique to save power consumption required for repeated transmission of advertising-based data packets used for a narrowband wireless communications system on the sub-GHz bands such as low power wide area (LPWA) systems under slow fading channels. The time interval and the frequency channel used for each repeated packet are considered as the key parameters that will affect the probability of packet success transmission, eventually the power consumption of devices. The correlation models of fading channels depending on the speed of a mobile node and the multipath delay profile are developed based on the fundamental communications theory, and they are utilized for designing the key parameters of repeated transmission. A burst transmission scheme, to repeat the identical data packet with a combination of a short time interval and a long time interval, is developed to save the power consumption by decreasing the total number of transmitted packets compared to a regular-interval transmission scheme. Simulation results show that the time interval dominates the performance of the probability of packet success transmission under time-varying channels with the mobility at a walking speed. It is also shown that a careful allocation of frequency channel used for the repeated packet can reduce the number of repeated packets to achieve a specific probability of packet success transmission under a slower fading channel, i.e., a quasi-static channel, which leads to the saving of the battery power consumption.