I know you don't want to bury cable but people have been using ethernet over coax with success... You could maybe get some direct-bury RG6 and a couple coax-to-cat6 ethernet adapters and try that.
You could just run a length of cat 5 cable if you are willing to do a cable. Use the telflon version of cat 5, as I think it holds up better to sunlight and being buried in the ground. In theory, you can have up to 100 meters of Cat 5 for a run. This would make a faster and more secure connection.