I think you might need to ground the transmitter. To test this, wrap a piece of wire around the collar of the BNC connector of the antenna (the center pin is connected to the antenna, the shield/collar should be connected to ground inside the transmitter) so that it makes good electrical contact and attach a small metal stake (you can use a large screwdriver for testing) to the other end of the wire and stick the stake in the ground - yup vertical in the dirt. Then fire everything up and see if it works.
If that fixes it, it would be better to have a bigger stake - like 12 to 18 inches. For just Christmas, I think the industry standard 4 foot copper grounding rod would be a little excessive...