{"id":1613,"date":"2010-07-11T17:03:05","date_gmt":"2010-07-11T21:03:05","guid":{"rendered":"http:\/\/w4kaz.com\/qth\/?p=1613"},"modified":"2010-07-11T17:03:05","modified_gmt":"2010-07-11T21:03:05","slug":"wind-damage-to-the-biological-antenna-support-structures","status":"publish","type":"post","link":"http:\/\/w4kaz.com\/qth\/?p=1613","title":{"rendered":"Wind Damage To The Biological Antenna Support Structures"},"content":{"rendered":"

Getting home from vacation a couple of weeks back, a huge branch was found in the rear area of the antenna farm.\u00c2\u00a0 One ginormous limb had fallen from one of the white oaks holding the north-south 40 meter dipole.\u00c2\u00a0 It was on the opposite side from that antenna, but must have crossed paths with the 80 meter folded dipole<\/a> on the way down.\u00c2\u00a0 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.\u00c2\u00a0 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.<\/p>\n

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.\u00c2\u00a0 Once a pilot line is shot over a branch, a heavy line is run up and over to make a loop.\u00c2\u00a0 At the joint in the heavy loop line, a pulley is attached.\u00c2\u00a0 The actual antenna support line is then run through the “pulley”.\u00c2\u00a0 Haul the pulley up to the desired height, paying out antenna support line as it goes up.\u00c2\u00a0 <\/strong><\/em><\/p>\n

Note:<\/strong> The “pulley” is usually just a simple welded steel ring, attached to the heavy loop with a swivel.\u00c2\u00a0 Real pulleys often bind – never a problem with the simple steel rings, although the steel ring may cause the line to break more often.\u00c2\u00a0 The swivel is to offset some of the twist a line can take on when being hauled up 50 or 60 feet.<\/em><\/p>\n

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.\u00c2\u00a0 More importantly, the line over the branch breaks less frequently.\u00c2\u00a0 Easier to pull the ring down and send up another support line than to shoot a new pilot line.<\/p>\n

So – One dead folded dipole.\u00c2\u00a0 Worse, its always been my favorite antenna for 80 meters.\u00c2\u00a0 It has a broad bandwidth, and seems to do a great job in the domestic contests.<\/p>\n

Making lemonade out of lemons, it was a great time to do long delayed maintenance.\u00c2\u00a0 The line over the branch was at least three years old, maybe four.\u00c2\u00a0 Worse, its probably too lightweight for the spot – a large branch at about 70 feet.\u00c2\u00a0 It was a hard shot to hit, so the heaviest line available. about 10mm,\u00c2\u00a0 was pulled into place as a replacement line for the pulley support.\u00c2\u00a0 The antenna pull line was also replaced, using 6mm line.<\/p>\n

With the new support lines ready, one other minor issue was repaired.\u00c2\u00a0 The antenna was built directly from the cookbook dimensions, from an article in the antenna book chapter on portable antennas.\u00c2\u00a0 (Reproduced with permission in the Cary Amateur Radio Club newsletter, the Feedline)<\/a>.\u00c2\u00a0 The tuning option used was the open stub of twin lead, since the twin lead was here back in 2002, but no suitable capacitors.\u00c2\u00a0 Since then the parts bin has been augmented with suitable panasonic capacitors.<\/p>\n

The twin lead stub functioned properly, and was trimmed only slightly from the cookbook dimension of 37’4″[11.28m] .\u00c2\u00a0 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.\u00c2\u00a0 Plus the extra weight.<\/p>\n

The stub was removed, and the junction was repaired physically.\u00c2\u00a0 240pf of 3kv capacitors were pulled from the parts bin and used to replace the stub for impedance matching.\u00c2\u00a0 The cookbook called for 289pf, but I took the time to experiment.\u00c2\u00a0 The 240pf value was arbitrary, but it paid off.\u00c2\u00a0 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.\u00c2\u00a0 The autotuner in the FT-920 has no problem providing a good match across the entire 80m\/75m allocation.\u00c2\u00a0 With 100w, my 80m results have always been competitive.<\/p>\n

When re-installed, it was also possible to get the apex a bit higher than before.\u00c2\u00a0 Nice.\u00c2\u00a0 Hopefully it will survive a few more seasons before needing additional work.<\/p>\n","protected":false},"excerpt":{"rendered":"

Getting home from vacation a couple of weeks back, a huge branch was found in the rear area of the antenna farm.\u00c2 One ginormous limb had fallen from one of the white oaks holding the north-south 40 meter dipole.\u00c2 It was on the opposite side from that antenna, but must have crossed paths with the 80 meter folded dipole on the way down.\u00c2 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.\u00c2 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.<\/p>\n

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.\u00c2 Once a pilot line is shot over a branch, a heavy line is run up and over to make a loop.\u00c2 At the joint in the heavy loop line, a pulley is attached.\u00c2 The actual antenna support line is then run through the “pulley”.\u00c2 Haul the pulley up to the desired height, paying out antenna support line as it goes up.\u00c2 <\/p>\n

Note: The “pulley” is usually just a simple welded steel ring, attached to the heavy loop with a swivel.\u00c2 Real pulleys often bind – never a problem with the simple steel rings, although the steel ring may cause the line to break more often.\u00c2 The swivel […]<\/p>\n

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