From their web site:

SPECS: Size: 1.6 x 1.1 x 0.35" (40 x 27 x 9mm)
Power Requirement: 4.8 - 6V
Current Drain: 80mA (at no signal)
Weight: 0.35oz (10g) with case
0.25oz (7g) without case

So, as most receivers, either 4.8 or 6 volt pack will work.

-tychoc