The design comprises the features of shape and configuration of the radio transmitter unit as shown in the accompanying drawings.Drawings and photographs of the design are included wherein:Figure 1 is a front view of the Radio Transmitter Unit embodying the design;Figure 2 is a rear view thereof;Figure 3 is a right-side view thereof;Figure 4 is a left-side view thereof;Figure 5 is a top view thereof;Figure 6 is a bottom view thereof; andFigure 7 is a front and left-side perspective view thereof.