I figured the same thing but this antenna isn't wired to any + or - . There's what looks like a transistor on the board. From that a wire goes down to the antenna base on the phone, so long as that wire is touching the antenna base the LEDs will light up, if that wire is pulled away from the antenna base the LEDs turn off.
I'd even be interested in putting together my own sort of flashing keypad if I could, hard wired to any + or - source. Any info you could give me on either one would be great.