The ornamental design for a signaltransmitter, as shown and described.
FIG. 1 is a front, right side, top perspective view of a signal transmitter showing my new design;
FIG. 2 is a rear, right side, bottom perspective view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a right side elevation view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken lines immediately adjacent to the claimed portions are boundaries, all other broken lines are environmental structures; all the broken lines form no part of the claimed design.