Masthead Complete

The mast has a lot going on at the top and I didn't want to start peppering it with holes to mount the various bits of hardware that are going up there, so I got a plate lasercut out of 6mm Aluminium - now I have discovered laser cutting, I'm finding new uses for it everywhere I look.

With the plate safely on my workbench I could then trial fit, align (critical for the wind sensor), drill and tap plates for the three components I wanted to fit:

• NASA marine Supernova Combi
• VHF antenna with windex
• Wind sensor

I anodised the plate and mounted to the mast top with 2 x M6 machine screws, which meant I only needed to tap two holes in the mast and could place them where I wanted them. Much easier. The cables run up a PVC pipe installed in the mast and exit through a hole I drilled in the top - I thought about putting in cable exits or grommets but couldn't find any the right size, so I decided just to drill it out, round the edges with a dremel, wrap the cables in chafe protection (sprial cable wrap, left over from the office), and will putty up the rest of the hole with Sugru - this way I get a perfect fit and can just redo if I never need to change the cables. Not that water ingress reallty matters with a deck-stepped mast I suppose, but if I can keep it dry I may as well.

That's the plan anyway, as of today I haven't been able to run the cables a my cheap nylon electrican draw tape was overcome by friction about 11m into my 14m mast. Just ordered one made of sprung steel, fingers crossed it will do the job.

Antenna Cable

Most VHF antenna (including the one I bought) come with 20 odd meters of cable, usually RG58, which is a 50Ω cable 5mm in diameter. My mast is 13.6m, assume a bit below deck for routing to the VHF and we have 16m of cable. RG58 at 150MHz has a cable loss of about 20dB/100m, so 3.2dB loss for my setup.

Well, the ISAF Category 2 regluations require a VHF with "...a masthead antenna, and co-axial feeder cable with not more than 40% power loss" (3.29.1(b)ii).

40% power loss is 60% signal which is 10 * log(0.6) / log(10) = 2.218dB, so 3.2dB is too high. Fortunately there are other options - I bought a reel of H155PE from eBay, which apparently is the dogs bollocks of 5mm cables for this sort of frequency - the catch is it's 5.4mm and has different diameters for the central conductor and dielectric, so it doesn't fit my antenna plug. Grr. Added to my "sell on ebay when done" list.

A bit more hunting and I found LMR-195, which has the same dimensions as RG-58 and a 14.6dB loss at 150MHz over 100m, or 2.33dB over 16m. That's 1 - 10^{(-2.33 / 10)}, or 42% cable loss. That'll do, pig, that'll do.

Postscript 1 Sep 2016

My antenna is not performing as well as I hoped, and needs testing. Hopefully it's a fault I can remedy with the current setup. With hindsight, fannying around with the cable was the wrong approach - I should have bought an antenna that takes 7mm RG-8X, which has much lower loss. I will replace the antenna when next is practical, probably with a Metz Manta. Once again, a bit more research before buying would have saved me a lot of effort. After this week that should really go on my gravestone: "Here lies Mike. He would have done better with more research up front."

Postscript 25 Oct 2016

I isolated the fault to the antenna or the cable, so replaced both with the RG-8X and Metz Manta. Although with the boat out of the water and the mast currently down, I can't test the new antenna.