On Thursday, September 29th, in the immediate aftermath of Ian, I posted about the damage done to my HF antenna and the Aluminum tower they’re mounted to. This is a 20 foot tall Aluminum tower from Aluma, the big name in that market, mounted as a tilt-over tower to allow me to remove the antennas before a big storm. One of the 6 dipoles in the Log Periodic Dipole Array (LPDA but usually just called a Log Periodic or LP) snapped off close to the boom tubes they were mounted to, rendering the antenna severely handicapped. Instead of being usable on all amateur bands from 20 to 10m, (20, 17, 15, 12, 10), it potentially matched only on 20, 12 and 10.
The issue, then was to repair the snapped off elements. I talked about this more in my second post about things after the storm; a post that generated lots of conversations.
As I say in that post, I had started out in the direction of putting a splicing tube over the two snapped-off ends on each side and fastening them through the splicing tube with self-tapping sheet metal screws. Until a comment by Malatrope suggested a solid aluminum bar inside the two ends. Since the Inside Diameter of the tubes was ½” and I had two pieces of ½” bar stock long enough to make the needed bars, and no real machining would be involved, I opted to go that way.
I cut two pieces 6” long and after fussing with my selection of fasteners on
hand, joined each end into the element with a 1” long 6-32 screw with a flat
washer under the screw head and a lock washer under the nut. Before doing that I spent a long time scrubbing both the boom end and the broken off end of the elements. I used shotgun cleaning brushes, both a wire brush and a cotton mop brush, until no more dust blew out of the tubes.
The fix looked like this. I find it amusing that I can look at this
photo and know it’s the right side of the dipole, which is the east side when
the antenna is pointed north.
To demonstrate the fix, I have before and after antenna analyzer plots. These plots are from my older antenna analyzer, the AIM4170, (see Ham Radio Series post 4 for more on antenna analyzers). The reason I used this one and not my newer NanoVNA is that I have historical plots I can compare directly to with the AIM.
The curves to mentally highlight are the red ones, the Voltage Standing Wave Ratio – the VSWR or simply SWR that tell you if your radio or tuner will match to the antenna. For my purposes, anything above 3:1 is too high. First the before plot:
and the after plot
While the after plot has note at the bottom saying this was taken with the tower cranked over and the T6 pointed at the ground, they both were taken under those conditions. Note the big hump in the before plot went as high as an 8:1 SWR. The worst in the after plot is still over 3:1 in the 10m band. Since the antenna was pointed into the ground, with the shortest element (probably most effected on 10m) mere inches above ground for both plots, this is only going to approximate the SWR with the tower vertical and the antenna interacting far less with ground. I feel comfortable saying the repair is valid.
As I remarked several times in the comments to the second post about this set of problems, I see the antenna and tower issues as a short term and a long term problem. Repairing the antenna and the house bracket attachment are short term problems while replacing the house bracket attachment with something better is a long term problem. I have a tendency to start all thoughts about antenna projects as something to do “when it cools off” and I’d be happy if the house bracket fix lasted until January or February. For that fix, I’ll be replacing the 3/8 by 2” long lag bolts with ½ by 3-1/2” long versions. I don’t really know they will work but I also don’t really know they won’t work well enough to last until I come up with a better long-term fix.
The house bracket has been modified and another coat of paint applied (because
reasons). I intend to do that short term fix tomorrow.
Crossing my fingers you don't get another Big Blow before doing the Permanent Fix stuff...
ReplyDeleteThe chances of a big blow get a little lower every day, but the answer there is that if I know another one is coming, the tower gets cranked over again. The wildcard would be an unusually strong cold front that has strong winds they don't predict. Our tornado season is in mid-February through March, and I want to be done by then.
DeleteSeriously consider, if you can, some sort of backing plate and actual through bolts in the rafter. Lag bolts are not great for that type of stress, pulling and wracking stresses. Great for pushing stress. So... If you can... Not fender washers, an actual plate with holes drilled in it. That will spread all the stresses, pulling, pushing, sideways, wracking, fracking, over a larger area.
ReplyDeleteThe majority of towers I've seen have always recommended or required a house bracket. In my case, the tower is on the east side of the house, and both our prevailing winds and the first winds we've gotten out of any hurricane have come out of the quadrant between NE and SE. NE winds drive the tower directly into compression of the bracket; SE are more shearing.
DeleteThe winds that broke the house bracket free were from the west side.
The tower manufacturer sells a mounting pole not very different from the one I use to crank the tower over that has a support bracket at the 8' level. (Engineering drawing here if you don't mind a pdf and are curious) They emphasize no concrete required, mine is anchored in a three feet deep, two feet on a side poured concrete block. At this point, I'm really leaning toward this solution.
Someone mentioned anchoring the house bracket into the concrete wall. I'm concerned about that because it's concrete block, and I don't have any feel for how strong a cinder block is since the walls are about an inch thick. If it was a poured, solid concrete beam, that's different, but I'm not sure if I have that.
Looks good! Glad it worked. I like the spar support mast idea. It will flex a little bit, whereas the house does not move and thus increases the force both on the house and the mast at the point of connection. It is always better to let a structure move a little.
DeleteOne question though: when I lived in Florida, my sand was dry enough that digging a hole like that would have been impossible without using a coffer dam. By the time you dump a shovelful, another has slid into the hole until all you have is a big conical pit, and you are working right under your house's foundation. Do you have this problem? 73" is very deep...
The soil here isn't as bad as other places I've lived in Florida. We're on the sand ridge that the FEC railway was built on, specifically because it's the highest land around, but it's been holding oak, palms and other scrub growth long enough that the soil is grayish from organic matter and holds together better than some others.
DeleteAluma's marketing says that pole is mounted by digging that hole and filling it with something like "locally available dirt and gravel. It uses water to compact the soil and stabilize the system."
More to the point for me is that my pole is 10' tall, 4" schedule 40 aluminum and their pole is 4" schedule 40 galvanized pipe. On the other hand, it's rated for much higher loads than my tower, including a 75' tall "Extra Heavy Duty" tower. That's a 3 section crank-up tower; my 20' tower would be the smallest, uppermost section of that 75' tower.
Their pole is about 14' overall. Mine is 10'. But mine is in about 14 cubic feet of concrete with a rebar box, so a bit over 3 feet is in the concrete and the other ~ 7' is above grade. Their pipe is 6' in the ground and 7-1/2 feet above.
The more I look, the more it seems like I could use my aluminum pipe like their MP2.
With the transmission traveling the surface, outside of a lack of electrical bond the modification/repairs would not create enough difference to be detected.
ReplyDelete