I am building an antenna that specifies the use of Dacron rope to help support the spreaders, but that is not readily available in Trinidad and Tobago.
What will be a good replacement with regards to UV resistance?
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While UV resistance is one quality, you should also be concerned with the other qualities such as yield strength, weight, moisture absorption, and stretch.
Nylon and polyester ropes can often be found in a UV resistant form. In general, ropes that are used for maritime purposes, other than pure hemp, have the qualities simular to those required for antenna applications.
I have always used 3/16" or 1/4" white nylon rope. It gets stiff after awhile, but doesn't lose its strength even after 5 to 10 years outdoors.
Stay away from yellow polypropylene rope. It is not UV resistant and starts deteriorating in just a few months.
At low frequencies, below 50 MHz or so, you can use steel wire with porcelain isolators every 0.35 wl or so (at the highest frequency of interest.) You could use many other kinds of isolators, cheap things you could make yourself. Steel is strong and stiff....