| re .1:
Your numbers are a bit off -- according to my Marine Exchange catalog
1/4" Sta-Lok insulators are $125 each, 9/32" are $160 each. Prices are
similar at Coast Navigation.
re .0:
Well, the usual insulator carries a load of a few hundreds pounds at
most. A 9/32" rigging insulator must carry a load of 10300 pounds before
failing and must provide insulation in a salty environment. And not many
of them are made and sold. The price doesn't seem all that unreasonable,
especially given the cost of a mast falling down.
As to your other questions, any good marine electronics dealer can give
you the answers.
Remember: If it's marine, it ain't cheap!
|
| I just installed two insulators on Halcyon for the Bermuda Race next week.
I don't remember the brand, but his backstay is 3/8" diameter, and they
cost almost $300 for the pair.
I've done the job several times and it's not difficult. The hardest part is
making sure you are measuring absolutely correctly! We double and triple
checked this.
I designed it so that the distance from the upper insulator to the ground
counterpoise in the stern of the boat, above which is an ICOM AH-2 antenna
tuner, would NOT be a multiple of a half-wave length at frequencies I wanted
to use for transmitting (receiving is not critical).
Here are some of my design calculations:
there are no marine frequencies (for transmitting) between 8.288 MHz and
12.33 MHz. If you make it so that the half-wave length is the geometric
mean between those two you should be safe.
I added a couple of other constraints: I wanted the next harmonic (i.e., a
full wave length) to be also clear since it would be harder to match an
antenna tuner there as well (but not as hard as for a single half-wave
resonance.) The Marine frequencies there are 22.0 MHz and higher.
I also wanted to avoid the 15 mtr ham band at 21 MHz.
All in all, I ended up picking a length of 51.4 feet as being equally clear
from critical frequencies.
I used the antenna formula for half wave: 492/f = half wave in free space,
in feet. With capacitive effects from the end, the dipole formula
including end effects comes to 468/f in feet. (f in Mhz). I picked the
worst of the two formulas for calculating the locus of the lengths that
have equal margins to the 'forbidden' frequency ranges.
The result is that it tunes on all frequencies that I've tried from 1.8 MHz
through 28.5MHz (ham bands) and a sampling of the maritime frequencies
around 8.2 and 12.33. Tuning is fast without having to do repeats.
The bottom insulator was chosen to be about 6' above the deck, and an
elaborate harness of ground counterpoise straps that bonded a 4x6 ft sheet
of aluminum grille, the stern pulpit, and forward to the gas tank and
engine, and the two toe rails that go up the side of the boat. The center
of mass of the counterpoise was about 2' below the deck, which is where
we measured the effective half wave length from.
The ground system seems quite good, as there is only one interference effect
that shows up: that is in the autohelm. I haven't been able to get rid
of it yet with bypass capacitors and ferrite torroids. We'll need to
bring a 100 MHz + scope aboard to get any further, and won't be able to
do it before the Bermuda race next week.
BTW, I'm also going on Halcyon that Rich Buchanan referred to in a previous
note.
A note of caution on applying the insulators:
1. if it costs only $35, something must be wrong. I'd beware.
2. it helps to see how it's done once and to have some mechanical aptitude
as well, so that you can do a correct job. There are ways to do an
incorrect job. The usual advice given in literature is that you let
a rigger do it so that he can be responsible. We did it ourselves
on Mistral (used in the Bermuda 1-2 race last year), and now. On
Mistral's there was some confusion between metric and 9/32" size and
we had to do some machining of the part to make it fit right. But in
the end, the fit was beautiful. THe one we did for Halcyon took a
couple of hours and went without a hitch.
3. On sawing the stay, put tape (electrical is fine) on each side of
place you want to cut before doing the cut. That way you won't have
the stay becoming unravelled prior to the cut. File burrs off the cut
end so that the strands will move and bend properly into the fitting
as it is tightened.
BTW, the April or May Ham Radio magazine has an article on antenna
lengths and ground systems as well.
The first year we used a ssb on Mistral, we put up an antenna from the
head of the mast, down to the stern quarter separate from the backstay.
It was about 4' from the backstay at the bottom, and maybe about 1'
from it at the upper insulator. That was cheap, used nylon line and
light insulators to support the wire, and worked well. It's only
problem would be the extra windage from the separate line. It was a
bit in-elegant.
Don
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