Monday, 29 February 2016

Wimo 7ele 2 metre beam repair and antenna matters


Some time ago during a storm my Wimo 7ele 2m beam shifted and had to be re-installed.

I mused at the time that about fact that it was mounted off centre might have been factor. I also mentioned before that it whistles in the wind.

Well during a short spell of still air I luffed the mast over and rebalanced it. Or, to put it another way, unbalanced it.
With the mast tilted over you can see the greenhouse. This close building means that there is only (roughly) 1 metre space on one side and a 6m beam will not fit sideways, nor any antenna with more than a 2m boom will fit lengthwise. Unless I move the mast (moving the greenhouse is not allowed).

That Wimo antenna has a fixing which allows it to be placed anywhere along the antenna except the middle. In the middle is a boom joiner, so the fixing does not fit there. I held the antenna along the boom until I found the weight balance point and supported it there. This is with the idea that (at rest at least) all the antennas should as far as possible be weight balanced around the mast, taking strain off the rotator. Great idea.

However, the weight balance is not the same as the wind balance. The elements are round section and the boom is square section, presenting different resistance to the wind. Net result, there seemed to be too much antenna beyond the mounting point. Or so it looked. And the boom is very light, meaning the antenna developed a bit of a "droop" towards the front.

Whether all that caused it to shift I cannot say. But shift it did. While working on it this time I took some photos of the resultant tear in the mast.
You can see the marks above the mount. I left it at the lower height ...

That is a strange mounting bracket, providing as it does for slight elevation of the boom. I like that as most meteor scatter operators do indeed put in a slight elevation, but then only one bolt fits on the top. Indeed they only provide two bolts. So it all seems a bit open to movement.

I moved the mounting point 400mm towards the front of the antenna. You can now see the boom joiner on the right, so I am near the middle of the boom. Once back up, there has been no whistling in the wind. It looks better balanced front/back, though of course with more elements towards the back it is clearly unbalanced for weight. But will it rip off again? I hope that it stays where I left it.

It does have a slight droop towards each end. Longer Wimos have double fixings and supports. Maybe this one needs those too. Aw no, if I have better supports maybe a longer antenna should go with them ...

I have been thinking again about the 1m limit to one side. This has resulted in my 6m band antenna having to go onto a different mast and therefore it is a very small and light antenna. I could do better with a 6m antenna on the main mast. I would move the Wimo to the secondary mast (it is small and light enough for that). But what 6m antenna is only 2 metres wide? A half wave for 6m is of course about 3m wide, so no conventional beam will fit.

Two solutions that I have thought about are quads and Moxons. I could build a quad (none of the commercial ones fit) but they are also tall and would be too close to the 4m antenna. A Moxon would just about fit (2.180m wide is just about possible, resting on the greenhouse glass!).

I am thinking about building a 4 element Moxon like the ones Vine used to sell. Not sure about that. They are a very unconventional antenna and I am not sure if the claimed gain figures can be realised in real life.

We shall see.

Antenna structures for us ordinary humans are all about what is practical and what we can afford plus what we can squeeze in. For the great DXers such issues are mere quibbles. I am determined, if I do build a Moxon, to spend nothing on materials and use the bits I have lying around. This is not just my miserly attitude to life; I do not want to spend any more money on this.

Do I really value 6m so poorly?



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