First off you may want to look at guying that tower a bit better.Fifty feet in a wind equals one heck of a lot of leverage and it can/will pull those little 2 ft. stakes right out of the ground unless they are cemented in.
If you want to run the coax underground I do suggest some sort of conduit. I have all my cables running thru 4 inch sewer pipe about 10 inches below the ground.The run is not that long, only about 25 feet, but it keeps things from getting hit by the mower and it also makes it a LOT easier to run new cables to the tower or pull old cables no longer needed back out as well as looks better.It also keeps moles etc from chewing the cables. Run a length of rope thru the pipe when you put it together that is twice as long as the pipe so you can use it to pul cables thru later as you add them. If you do run PVC pipe or whatever you should find the lowest point in the run and drill a hole 1/4 - 1/2 inch in size in the bottom and fill this area with some sand. This will allow the water to drain out. No matter how well you think you have sealed the ends water will form inside the pipe due to condensation. I noticed water dripping down the inside of the basement wall about a year or two after I buried my cables. The entire 25 ft run of 4 inch pipe was FILLED with water and was backing up into the house. The low point is at the tower end but I have the pipe coming up out of the ground about three feet or so and making a 180 degree turn to keep water out.The water was about two feet up inside the pipe. I had to dig down to the elbow where it turns up and drill a hole. I then dug a bit of soil out and filled the hole with sand before replacing the sod on top. I never had another problem with water in the conduit.
For grounding, there is not much that will protect you in the event of a direct strike. Things usually just get vaporised and that's the end of it.
The idea is to make it as hard as possible for lightning to enter the house. First you should install at least three ground rods,one for each tower leg and bond them to the legs with at least #6 wire.Bond all the rods together at the top as well. The rods should be copper and driven in as deep as you can get them. I use 1/2 or 3/4 inch copper water pipe for my rods. Flat copper strap is best to use as it has more surface area that a piece of wire as well as less inductance. Contrary to what a lot of people think braided copper strap is not the best idea as the braiding causes some inductance which opposes current flow from a lightning strike and we want the lightning to flow thru the connection to the ground rods.All connections should be clamped tightly and never use solder on them. In the event of a strike the solder will simply vapourise and cause the connection to blow open. You may use bronze or silfoss rods to weld the connections as it has a MUCH higher melting point but honestly a good clamped connection to clean metal is sufficient. I use either stainless hardware or the silfoss rods. Silfoss is a trade name for oxy-acetylene welding rods that contain a bronze alloy and silver.The coax cables shield should be connected to the tower at both the top and bottom. You can use hose clamps for this but be carefull to waterproof the cable after and do not crush the cable by over tightening the clamps.You can install a lightning suppressor at the base of the tower and connect the ground side of it to the ground rods.Running the cables underground offers a bit of decoupling from the energy from a strike.At the base of the tower make a one turn loop in all cables before running them to the house but do not let the cable touch itself anywhere along the loop. This creates a bit of inductance which lightning does not like and forces it to look for an easier path to ground which should be directly into the ground rods.At the house end of the run it is a good idea to install another lightning supressor or at least ground the shield of the coax cable at the point of entry with another ground rod. this ground rod should also be bonded to the main electrical service ground. Above all, disconnect all coax cables during a storm.
BTW, for 22 years I worked in commercial broadcasting and looked after a total of 13 towers in our system for both AM and FM. We employed the above protection and only twice do I remember being off the air due to lightning. One time I sat at the base of the mountain and watched the FM sight get hit five times
before going back home due to it being unsafe to enter the site.
When all was said an done the following morning the only damage done was to the FM transmitter,there was a lot of VHF and UHF stuff there as well but unharmed, and was confined to blowing out the three phase rectifiers in the power supply and shorting the final tube six ways from Sunday. Not bad for being hit God knows how many times that night.
