Hi Jerry...
Well, you could of course buy the extra long cable that Davis makes--I don't recall exactly, well, I'll just look it up right now... it's 100'.
I have thought about splicing my own extension as well...
Provantage has the 100' extension for less than $36 (I didn't get a discount price quote), and it includes a waterproof coupler...
You won't notice that extra amount for the purchase you're looking at! I can help you spend your money!!
Unless you've already got the equipment to make a connection for 4 conductor cable, I'd apply that expense to buying the pre-made extension...
I wouldn't think having the cable underground is any worse than out in the open being exposed to all weather... but..... I might consider using conduit anyway...
Alright! You really are on the path!
Marc
P.S. Now that I'm thinking about it, you could probably find various lengths of 4-conductor plus waterproof connectors at Lowes or Home Depot--I'll have to check it out this weekend myself...