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Wind Damage To The Biological Antenna Support Structures

Getting home from vacation a couple of weeks back, a huge branch was found in the rear area of the antenna farm.  One ginormous limb had fallen from one of the white oaks holding the north-south 40 meter dipole.  It was on the opposite side from that antenna, but must have crossed paths with the 80 meter folded dipole on the way down.  The bad news is that the 80m antenna was on the ground, as well as six or eight radials from the nearby inverted L for 160m.  The good news was that only the line holding the antenna itself had failed, and not the line over the branch, and the nearby 40m vee was not effected.

On most of these hard to hit branches, the approach that usually works best is to use two lines for the center load-bearing supports.  Once a pilot line is shot over a branch, a heavy line is run up and over to make a loop.  At the joint in the heavy loop line, a pulley is attached.  The actual antenna support line is then run through the “pulley”.  Haul the pulley up to the desired height, paying out antenna support line as it goes up. 

Note: The “pulley” is usually just a simple welded steel ring, attached to the heavy loop with a swivel.  Real pulleys often bind – never a problem with the simple steel rings, although the steel ring may cause the line to break more often.  The swivel is to offset some of the twist a line can take on when being hauled up 50 or 60 feet.

Using that method, the heavy line over the branch can usually be pulled up and left alone, reducing the amount of line “sawing” that is caused by raising and lowering a hard-to-tune antenna.  More importantly, the line over the branch breaks less frequently.  Easier to pull the ring down and send up another support line than to shoot a new pilot line.

So – One dead folded dipole.  Worse, its always been my favorite antenna for 80 meters.  It has a broad bandwidth, and seems to do a great job in the domestic contests.

Making lemonade out of lemons, it was a great time to do long delayed maintenance.  The line over the branch was at least three years old, maybe four.  Worse, its probably too lightweight for the spot – a large branch at about 70 feet.  It was a hard shot to hit, so the heaviest line available. about 10mm,  was pulled into place as a replacement line for the pulley support.  The antenna pull line was also replaced, using 6mm line.

With the new support lines ready, one other minor issue was repaired.  The antenna was built directly from the cookbook dimensions, from an article in the antenna book chapter on portable antennas.  (Reproduced with permission in the Cary Amateur Radio Club newsletter, the Feedline).  The tuning option used was the open stub of twin lead, since the twin lead was here back in 2002, but no suitable capacitors.  Since then the parts bin has been augmented with suitable panasonic capacitors.

The twin lead stub functioned properly, and was trimmed only slightly from the cookbook dimension of 37’4″[11.28m] .  The “third leg” was always a bit of an additional problem to deployment, as it needs to be stretched away from the feed line rather than coiled as might be possible with a coax stub.  Plus the extra weight.

The stub was removed, and the junction was repaired physically.  240pf of 3kv capacitors were pulled from the parts bin and used to replace the stub for impedance matching.  The cookbook called for 289pf, but I took the time to experiment.  The 240pf value was arbitrary, but it paid off.  the original antenna had a 2:1 SWR bandwidth from 3550 to about 3920, with the sweet spot of 1:1 at 3730. The modified version using 240pf for matching lowered the 1:1 point to 3675, and the 2:1 bandwidth extends from 3500 to about 3860.  The autotuner in the FT-920 has no problem providing a good match across the entire 80m/75m allocation.  With 100w, my 80m results have always been competitive.

When re-installed, it was also possible to get the apex a bit higher than before.  Nice.  Hopefully it will survive a few more seasons before needing additional work.

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