Friday, June 3, 2011

Wire Antenna Tension Breaker

Wire Antenna's enemy #1 is the wind (Corrosion being #2, Lightning is a topic of entirely different discussion).
In high winds, tall trees sway a lot - the taller the tree is - the bigger is the amplitude. What makes the matter worse is the fact that different trees sway with different frequency and amplitude due to the specifics of each tree - height, canopy, etc. The wind could also blow from different directions for each tree if they are far apart.
Wire antennas are often stretched between tree-tops where the swing is in it's maximum.
In other words, during strong wind almost everything works against the antenna, putting it at a great mechanical stress.
One solution for long wire antennas is to let it sag - the sag could provide enough slack in high wind situation so the antenna is never tensioned to the maximum thus reducing the chance for break. Such approach works fine for long-wire, end-fed antennas.
When it comes to dipoles, one would want the antenna as high as possible. In addition, preserving the flat-top geometry of the antenna also helps the radiation pattern so people tend to tension them a lot.
In order to protect my G5RV from breaking due to tensile stress and to reduce the unnecessary sag in calm weather at the same time, I made a "tension breaker" (it is more of a "fuse" actually)
The idea is very simple - to create an artificial "weak point". If high wind occurs and the sway of the tree-tops puts the antenna under excessive stress, the "tension breaker" opens at a predetermined tension load, releasing more slack in the antenna rope and relieving the stress by letting the antenna to sag. When the weather calms down, the "breaker" could be easily "reset", stretching the antenna back to it's original state.

The "tension fuse" is located near one of the antenna rope's anchor points. The actual "fuse" is two lengths of "50 lb test" Spectra Line braided (yellow) filament between two Quick-Links (each rated for 220lb load). The filament should break at a load >100 lb (2 x 50lb). A test sample broke at ~120 lb. I am using AWG #12 wire for the antenna (tensile strength ~ 220 lb) and 3/16' double-braided polyester antenna rope with break strength of 770 lb). Even if I de-rate the breaking load for the whole antenna because of the antenna insulators, knots, soldering etc., the filament "fuse" is still going to be the weakest link. Once it breaks, it will release a slack of approx. 6 ft of antenna rope (bottom-left on the picture). To "reset" it, I have prepared a couple of extra "tensile fuses" which can be installed between the Quick-Links in 5 min.
The trick is to have such weak point to break only at a load dangerous for the antenna and withstand the load of moderate wind conditions while keeping the antenna tensioned for minimal sag.


Anonymous said...

... or you could use a weight at each end of the antenna, providing constant tension in the line independent of the anchor's position.

Simply use nylon rope fastened to the end of each element, passing through a pulley in a fixed position (can still use a rope to position pulley) and then a cannon-ball anchor on each tension line.

No "fuse" needed!

Andrey E. Stoev said...

Of course "there is more than one way to skin a cat". The weight aproach will work just fine (and it is a good solution) if there is a way to install a pulley at the top-most point (buildings, masts, towers or trees that can be climbed). Since my rope goes over a tall tree (100+ ft), directly over the canopy, excessive movement of the rope back and forth (counter-balanced by the weight), will cause a damage to the rope (and maybe the tree)due to the constant friction with the tree branches. I find "the fuse" to be a simple and inexpensive way.