This afternoon I went up the mast to replace my Raymarine radar scanner and I took the opportunity to take a few photos showing my radar mount base, and to capture a view into the conduit. The photos attached show some interesting things:

* I chose to mount my radar so that the beam was slightly above the spreaders. I did not want the metal spreaders to interfere with the beam - metal is very reflective to X-band radiation - so I made sure at the original installation back in 2002 to mount high enough so that the radar beam could pass undistorted above the spreaders. I also made sure that my radar reflector was NOT located at the height of the scanner. Note that for this upgrade to a new Raymarine HD radar I was able to re-use my original Raymarine scanner mount, even though the new radar scanner is about 2" larger outside diameter.

* You can note the location of my scanner mount base and the location of an approximately 2-inch dia hole cut into the mast and mast conduit. You can judge the location of this hole by noting the lower shroud tangs nearby.

* I took a couple shots into the conduit with my iPhone. It seems like the conduit is most likely ~ 2-inch PVC pipe. - AS I look more carefully at my photos I believe now that the conduit is aluminum, not PVC.

* After cutting it at the base of the mast, I pulled the old cable out through that hole at the spreaders, and as the old cable came out it brought one end of my new 50 ft plastic 'fish' that I got at Home Depot. I then used that fish to lead the new radar cable down the mast. The new cable has a connector on the bottom (non-scanner) end of slightly more than 1/2 inch diameter. All went well leading the new cable down inside the conduit until about 12 inches above the mast step where SOMETHING caused the connector/fish bundle (which I hadn't faired properly) to hang up. An application of s-i-g-n-i-f-i-c-a-n-t (read 'reluctantly high') pulling force finally forced the fish and connector through. I was pushing and manipulating the cable like crazy at the spreaders, while my buddy pulled strongly at the mast step. (The small 10-pin data connector may have been damaged by this reluctant force, but if so I will bring the individual wires to a terminal block near the mast step. I may do that anyway.)

* NOTE TO SELF: Take extra care to properly FAIR any small bundle going through the mast conduit.

* In one of the photos you see the new radar cable tie-wrapped to a shroud. After all that work I didn't want to lose my end of it while I installed the new scanner.

* Looking into the conduit you'll see a surprising amount of room. In my photos you'll see in addition to (1) my new radar cable, (2) a coax for the masthead VHF antenna, (3) a two conductor cable for the anchor light, and there is (4) a messenger line to the masthead that was left in place back in 2002, and which I will use in a few days to lead my new Raymarine wind sensor cable down from the masthead. I am confident that there is entirely adequate room remaining for the wind sensor cable, and possibly even for another small cable should anyone ever decide to, say, put a tri-light (or in another part of the world, a lightning dissipator?) up there. 

I hope this helps to visualize what's available to you folks inside your mast.