Have a question, as the coax cable will run under ground, how deep would the trench need for the pipe to lay in ?.....
Deep enough that it won't get hit by something. Code doesn't really have anything on low voltage communications cable, at least that I recall.
I'd say go down as far as you safely can/easily can and call it good.
I have one I ran down the side of my house into the back yard. I think I went down 18" or so. It's not in a location that it'll get hit by anything. 18" was as far as I could easily get without breaking my back. I did it all with pick and shovel, so I'm happy with it.
And on the antenna side, should I take this coax all the way up to the antenna, or put a break in the coax where it comes out of the pipe, and up to the antenna ?..........always concerned about the weight of the coax pulling down on everything...
If its a direct bury rated cable, it'll have a gel filling. Running that up vertical really far can result in the gel slowly oozing down the cable and pushing it out the lower end (probably in your shack).
I'd terminate the cable at the base of your tower/mast, a few feet up out of the ground. Put it in a PVC terminal box. Transition to a regular coax. That also gives you an excellent point for putting in a grounding point on the coax. You ideally want the coax shield grounded at the base of the mast/tower, and that'll be an easy place to do it.
Support the cable well along its vertical path. At the antenna, make a loop so the connector isn't under any stress.
I'd like to think this coax would never need to be replaced, .......
Done right, it shouldn't be an issue. I've got some runs of 1 5/8" Heliax that have been up since 1994. They still sweep just fine and show no signs of deterioration. Most important part is to make sure you properly weather seal all the outdoor connections, and you should be golden.
Industry standard weatherproofing tutorial:
