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Saturday, October 12, 2024

Just About Done with Milton Repairs

As I mentioned in my first update Thursday afternoon, we had very little damage from Milton.  Power was out very little, if at all, and there were only a couple of things that needed repair. The big one seemed to be that an element broke off of my HF Log Periodic antenna (a Tennadyne T6) on one side of the boom, very much like what happened back during Ian two years ago.  The second was that our east side fence gate broke, throwing the metal piece that latches the gate closed about six to eight feet from the fence. 

The antenna fix ended up being different from the last one and while I thought I could just go find a piece of the 1/2" aluminum bar that I used to fix it last time, the unnoticed curve ball was that the tubing for this element is larger diameter than the one that snapped two years ago.  That one had an Inside Diameter of 1/2" while this one was 5/8".  I went looking for a piece of 3/4 or 1" bar I could reduce in diameter until it fit in the antenna tube, but didn't have one.  I absolutely didn't want to leave this one on the ground and order a piece of bar stock - that would take days to a week. That's when I found a piece I had originally bought to fix the antenna after Ian, a piece of metric-sized aluminum tube that was a bit oversized in Outside Diameter: 17mm OD, or about 0.670".  

My lathe has been obsoleted by Little Machine Shop, but it's SIEG SC4 type, 8-1/2 by 20", they sold as their model 3540. Yes, SIEG is the Chinese machine producer that makes the vast majority of the smaller home/"hobby" lathes in the 7x10, 8x12 and similar small sizes. This one is capable of doing automated cuts by using some controls on the carriage originally intended for threading, so to reduce this bar from that 0.670" OD down to 0.620 (or so) is a matter of setting up for repetitive cuts, put the cutter in position, throw a lever and just keep an eye on things. Pretty much. At the end of the cut, flip the lever, return the carriage to the starting point, move the cutting bit to the next cut's depth and repeat.

I turned it down to the point where the ends measured close to 0.625 and then pulled the tube out to check in the antenna element. It's not uncommon for a part to not come out to the same diameter over its entire length (at least not on my lathe ... or not on these "hobbyist" lathes), and while the ends fit well, I took off another .003" and it made a nice easy fit.  As with the last time I did this, I used a gun cleaning wire brush in my battery powered drill to scrub the insides of both parts of the element.  As with the last time, it was full of dirt that I believe was stuck in the tubes by mud dauber wasps.

Here's what it looked like put back together.  Before cranking the tower back up, I did a NanoVNA sweep of the antenna and it looked fine. Then I swept my other antenna on the tower, the VHF beam, and it looked unaffected, too.

The aluminum tube has a blue line drawn on it to mark the halfway point and you can see that in the gap where the two pieces meet. I smeared the (right) half of the tube with nail polish to glue the tube in place, then drilled a clearance hole for the 6-32 hardware: a screw, flat washer on the far side, lock washer on this side. Yes, that's a brass nut on a stainless screw. 

The other issue, the fence gate had much less progress. It looks to me like that gate needs to be rebuilt or replaced.  We have a "poly-something-or-other" fence and the gate latch was ripped apart.  I forgot to grab a photo of it, but it's this basic idea, except the highlighted part looks to be about twice the size of the highlighted one in this picture and holds twice as many screws on the side of the right angle closest to the camera. That's the part that ripped off the gate post and got thrown.

Where the screw holes are in this pic, ours has slots, 1/4" wide and 1/2" long.  Every screw was ripped out of the plastic post, ripping the screw holes in the door (4x4) wider.  I found the ripped off piece about 8 or 10 feet away from the gate.  The screws are self-drilling steel screws that are just under 1/4" major diameter (across the screw threads).  While a "standard fix" might be to go to larger diameter screws, the metal won't handle that and the slots would have to be widened, perhaps going to 5/16 or 3/8".  Not an outrageous job for the CNC mill, but not a one hour job either.  The issue is how much damage that the post this mounts to sustained.

The antenna is fixed and operating; the gate not so much. Our neighbors on two sides, backdoor and next-door, have trees in corners of their lots that overgrow our lot all the time. Where the three of our lots touch, I'm constantly pruning their trees and repairing next-door neighbor's fence. Milty brought more attention to that because excessive wind from the wrong direction could have put backdoor neighbor's tree through the roof of my shop. It turns out the tree came down in their yard. This time. Turns out I had trimmed those back between Helene and Milton.



4 comments:

  1. McMaster is your friend:
    https://www.mcmaster.com/products/metals/aluminum~/polished-multipurpose-6061-aluminum-rods/
    and can generally easily get it to you next business day.

    ReplyDelete
  2. How much difference is there in NanoVna results between ground level and elevated?

    ReplyDelete
    Replies
    1. That's hard to answer. If you overlay the plots, every single point is different, and the amount of difference varies with frequency. There's a similarity there, but it's hard to precisely describe. The easy part, though, is that when you're looking for a broken element it's very obvious.

      While it's from a different one port network analyzer, I showed before and after plots of the broken element in the post two years ago. The before plot shows a massive increase in VSWR when the element was broken.

      It's at the second link in the first paragraph, about " back during Ian two years ago."

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