radar install on mast

[george123]

I will be installing the 18 " HD radar on the mast of a 34' sailboat. The height of the radar to the deck is 20' 6". The mast is deck stepped. I would like to make the connection at the deck. I will be using an extension radar cable. It is my understanding the extensions only come in 2m (6' 6"), 5m (16' 4"), and 10m (32' 9") lengths.
Is there 7.5 m cable available?
If this is not available can I connect a 2m cable to a 5 m cable that I would than pull up the mast? When I connect the two cables together should I be concerned about making the connection water tight? I would plan on connecting the 5m to the radar and connect the 2 m to the 5m. Because of the weight of the cable hanging in the mast should I tape the to together at the connect so that the tape will take the strain of the cable weight?
Once I have the mast wired I plan on installing the 10m cable that comes with the radar from the navigation station down below through the boat and up through the deck. When the mast is reinstalled I would make the connection to the extension cable and lower the mast onto the step plate on the deck. At the nav station down below I have a raynet cable with a joiner short cable that I would connect the radar cable to and provide power to the radar cable. The raynet cable goes to a e95 at the wheel.

One last question The boat had an old 18 radar scanner that connect to a Pathfinder RL70. Will the new 18HD radar mount in the same [censored] pattern?

Thanks
George