This blog is part of a series on the problems with our HF rotor, in 2020. In the previous two parts, we found out that our old rotor for the HF beam was broken, and replaced it with a spare we had. The result was a rotor that turned a little, but not all the way.
Our initial thought was of course that the mast was off-center. After the wind had calmed a few days, we tried to improve the centering, but never got it far enough so it would turn all the way. The best we could get was around a +/-90 degrees turn.
We also made another troubling observation. The innermost threads on some of the U-bolts holding the HF-yagi bracket in place, were quite worn. The bracket had slid down the mast-pipe, and was resting on the top block of the mast-pipe mount. What had kept it from rotating all the way, was the boltheads of the bolts securing the top-block.
At first, we thought the tightening had been difficult because of the hard wind when the mast was put back up, but we quickly realized the worn threading made it literally impossible to fasten the bracket tight enough to have it handle the weather. The outermost portion of the threads was still fine, so despite being a bit loose in the wind, we could be sure the Yagi wouldn’t fall off completely. As before, we ended up throwing a rope over the hind reflector and tying it securely, to prevent it from causing too much damage to the top-block of the mast.
It was clear now that major maintenance work was needed.
With winter 2020 came new corona-restrictions and a new lockdown. This was just as the vaccine was being carefully rolled out, but it would take months before students were eligible for the first dose. There was no way we would be able to do something all-hands with the rules in place, so we had to wait until restrictions relaxed closer to the summer. Therefore we spent the spring semester planning the and ordering new (properly dimensioned) U-bolts for the bracket.
We decided on the following game plan:
- Bring down the mast like before
- De-attach the antennas from the mast
- Remove the bracket from the Yagi
- Drill holes for the new U-bolts
- Do additional maintenance where seen fit
- Fix the centering of the mast pipe before adding any antennas back
- Verify that the mast pipe turned all the way
- Re-attach/mount the antennas
- Hoist the mast back up
It was hard to find the right time to do this: it had to be after the restrictions were lifted, such that we could gather a large enough group of people, but also at a time when we had enough people in town. This was a small window of time between the end of the exam season and the beginning of summer vacation.
A good number of ARK’ers showed up to help, and things went smoothly. The biggest challenge was the temporary storage of the Yagi, partly due to its size and the building’s ventilation system being in the way. After some antenna acrobatics, we were able to pull it up onto the upper roof for safekeeping while we did the rest of the work.
We also performed some other maintenance: for example, LB3SG did some great work installing a loose sliding disk on the mast top block. This will prevent the Yagi-bracket from getting caught on the bolt heads, should it sag again. The bracket was also sprayed with galvanic paint to prevent corrosion. Other things include re-doing water seals on coaxial connectors, inspecting various bolts, as well as making proper grooves in the mast pipe to stop the pipe from slipping in the rotor mount.
Despise the workload, we managed to get it all finished before the end of the night. The edge of one of the reflectors on the Yagi broke when putting it back, but this was quickly fixed. As an added bonus, after putting everything back, the beam would finally turn all around!
Later in the summer, we realized we weren’t completely done. Some of the bolts holding the support wires of the 40 m dipole on top had not been tightened, and during a storm, the dipole had turned 90 degrees. As the Student Society was getting a new roof, there was little we could do about this until later in the summer, when this was fixed by LB1HH and LB9WI.
The antenna was re-connected later in the year, but shortly thereafter there was another storm that shifted the Yagi some 10-20 degrees — we deemed that too little to fix urgently, since the 40m Yagi on top has a large enough lobe to compensate for this, and we’re really just using the Yagi for heading now. It has been working fine since, and we’re very happy with how it turned out.