Sunday, 18 October 2015

Delicate work on the mast

I figure one of the reasons I'm doing a hollow mast is to hide things in it. Specifically hide the sail track, halyards, and VHF radio antenna. I've been spending a little time building a prototype antenna, and working on the details of halyard management and sail track.

First the antenna. I based the design off this five element collinear, using 086 semi-rigid coax for the 1/2 wave sections and a 1/16" brass rod for the 1/4 wave section on top. By using 086, which is about 2.2mm diameter, I'm able to insert the whole antenna in a 6mm diameter polyethylene sleeve to seal it up nicely, and then put the whole shebang in the 1/4" slot that I've run up the antenna.

Here's detail of one of the joins between 1/2 wave sections, along with the polyethylene sleeve (actually air line):

I'm having all sorts of difficulty getting a match out of this arrangement. I was expecting to have to trim the radiators to account for the dielectric constant of the sleeve and wood, but it looks like the whole thing is just miles from 50 ohms. A network analyser plot is shown below:

Not so good. The marker is in the middle of the marine VHF band, at 158 MHz. The only promising resonance appears to be due to the feed line, at 90 odd MHz. I'm debating whether to design a stub match to brute-force match the antenna to 50 ohms, or else to abandon the idea of hiding an antenna in the mast and instead run RG-223 up the slot I've routed and bring it out the front of the mast above the jib halyard.

So while vaccilating I did some other work on the mast. First I cut some exit slots for the jib halyard and main halyard:

Next I cut a 12mm diameter dowel from Jarrah, and used it to plug the top and bottom of the sail track. By using Jarrah here, I get the best possible strength for screwing the halyard block and (possibly) the vang eye. I might yet run the vang off the tabernacle.

With the top and bottom of the sail track plugged, I then had to machine an escape for the slugs, just above where the boom will attach:

Finally I've started making reinforcing blocks for where the hardware attaches. The one shown is where the spreaders attach:

This one has both halyards run past it, so I've got a large ramped slot so the 8mm halyards can run past freely. The next one up will be where the shrouds and jib halyard attach, so it'll have to be made just so to guide the jib halyard to its sheave. Similarly the block for the top, which will (sort of) seal up the top of the mast, will have to include a path for the main halyard to exit via a sheave.

I'll also fill in the bottom of the mast, with Jarrah for strength, from the foot to about 200mm above the boom. I'll put a very gradual taper in this block to ensure I'm not creating a stress riser, plus a slot so that I can bring the antenna lead out the bottom, plus allow water to drain.


William Chitham said...
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William Chitham said...

Lovely stuff, glad to see the mast is even more elaborate than the bench.