I remember reading about this on the Pirate 4x4 forums a couple years back....full wavelength antennas don't do anything for you if the equipment isn't set up to take advantage of it. Most consumer-level electronics that use transmitters are set up to use quarter-wavelength antennas. The problem is that many of the antennas that come with the products are far too short to be optimal. There was a formula for calculating the exact length your antenna should be...one sec...
Found it:
Look up the FCC ID number to find out what frequency the opener uses, then divide 300 by the frequency (for instance 49MHz, you'd get 300 / 49 = 6.122 meters) for the wavelength. Then divide by 4 to find the quarter wavelength and make your antenna that long.
Hope that helps. I've had the same problem with my truck so I intend to try this over the weekend.