Tuesday, December 3, 2013

Mast Failure

About a week ago the Site B antenna mast suffered a major failure and came down. It did so without any drama, hardly making a sound and causing no damage to the house and grounds. Of course the multi-band inverted vee it supported is also down.

If you follow the link in the previous paragraph you will find a picture of the antenna mast soon after it was installed. It consists of ~6 meters of aluminum boom from a old Cushcraft 6 meters yagi and 2 sections of army surplus telescoping fibreglass mast below the boom and nested, with shims, into the Schedule 40 steel pipe that is bracketed to the house. The mast is guyed with ropes and a pulley up top is used to raise and lower the antenna.

Care to guess where the failure occurred? The adjacent picture is a big clue. I posed the two sections of mast side by side.

The exact point of failure was the lower-diameter end of the bottommost fibreglass mast section, the one that is mated to the steel pipe. As you can see the fibreglass broke at the collar that rested on the pipe rim. The moderate amount of on-the-ground testing of the fibreglass -- vertical and bending loads -- I performed did not identify this failure mode.

I have no one to blame but myself since I judged the material suitable for the application. This highlights a problem with surplus material: no spec sheet and no documented history of use. Either the mast was stressed beyond its unknown spec or it had been previously compromised. Unlike materials like steel it is difficult to determine which is the key factor by simply looking at it.

My original intention was to use metal but I had difficulty locating steel of aluminum tubing or pipe of the required strength and able to mate with both the bottom pipe (1.9" O.D., 1.61" I.D.) and aluminum boom (1.375" I.D., 1.5" O.D.). Since the fibreglass mast did the job with simple aluminum shims that is what I chose to do. I only expected it to be in use until the spring.

As I said from the start, the mast was deliberately built to be cheap, light and (supposedly) strong enough for the job at had. I wanted to cap my investment in this short-term experiment and avoid the risk of serious damage should it fail. By these criteria I succeeded, except for the longevity.

The combination of wind load and bending stressing are the likely culprits. I suspect, but cannot prove, that these loads caused gradual weakening of the fibreglass as it pressed against the steel rim of the pipe.

Unfortunately the snow we've had in the past week makes it unsafe to work on the roof. All I could do by ladder was to disassemble the mast and inspect the rest of it for damage. I believe I can get it ready for reinstallation with only a little work. A brief thaw will also be needed.

The biggest loss is the antenna. It's physically perfectly alright, just unusable for the present. This limits my operating due to the loss of 30 meters and inability to fill the side nodes of the the other dipole on the higher bands. At least it had the grace to fail after the CQ WW CW contest.

My options to getting the antenna back up include:
  • Installing the aluminum boom directly into the steel pipe. That will cost me 2.5 meters of height.
  • Find a length of steel pipe that is suitable for mating to both the aluminum boom and steel pipe. So far I haven't had any luck. I'll keep looking. Of course this is why I went with the fibreglass in the first place.
  • Move the antenna to the Site C tower. The maximum height I can manage there is 12 meters. The problems include interaction with the 40 meters delta loop and suitable tie points for the ends. The purpose is to mount the antenna at approximately right angles to the TH1vn dipole (up 11 meters), which severely limits my choices.
With the early arrival of cold and snow the fundamental limiting factor is the weather. Unless we get a thaw the antenna will stay on the ground. I am still on the air though with fewer bands and less azimuth coverage.

1 comment:

  1. The military fiberglass sections were designed for use as supports for camouflage netting, usually two or three sections are used with a fibreglass spreader over the top to hold the netting. These are not designed for supports for antenna masts. They are actually designed to break off in the case one has to move in a hurry. Depending on the urgency factor, one just starts the vehicle and drive off. In a none urgent situation, one carefully takes things down....

    Currently, Princess Auto carries both the fiberglass sections and the aluminium sections. The aluminium sections work very well for temporary and portable use. On eBay, you will find some aluminum base sections that can be used raising the mast sections, and also a tripod base unit. These still need to be guyed for use. There are complete military aluminum mast sections complete with base units, guy ropes, and spikes/hammer, etc.... from the military collector events in the USA.

    Thank you for sharing your experiences. It is appreciated.



All comments are moderated, and should appear within one day of submission.