Thursday, July 30, 2015

Building a new antenna at N6QW ~ Part V

A Potential Major Setback ~ the Rohn H950 Mast

We all have heard the caution about the unbelievable low price and you get what you pay for and most likely the physical product does not satisfy the requirement. I believe I am at that point with the H950 mast. I was at first unsure and more confused about how to actually assemble the mast which I found later involves moving a section beyond a point on the next larger section and aligning a notch on the inner pipe with a hole drilled in the larger section.
 
Once aligned you simply place a cotter pin through the assembly so that the inner section notch rests on the cotter pin and then a clamp mechanism is aligned with another hole in the larger section to force against the inner section. The intent here is to have the cotter pin prevent the inner section from slipping down and the clamp's purpose is to make sturdy the two section. Nice theory.
 
I substituted 5/16 steel bolts with nuts for the cotter pin and assembled 4 out the five sections -- the 5th upper most section would be like trying to use wet spaghetti as a car jack tire iron. Just too flimsy! I am OK with just the 4 sections as I think the rotator plus the section of mast above the rotator could easily raise the height to 30 feet. I am OK with that.
 
I tried to "walk up" the just the four sections and immediately saw that there was a lot of play in each of the mechanically coupled sections. This does not bode well because even with adequate guying the rotational forces introduced into the mast could be significant. I really am beginning to question having 30 pounds of weight acting through a 30 foot long moment arm and the implications to the structural integrity especially about the clevis assembly base. Added concerns involve the thought of pushing up some 60 pounds of weight  through a mast assembly that at this juncture does not appear to be robust.
 
So we are left with several options after having discounted the push up approach.
 
  1. Option 1 is to mechanically assemble the four sections, install the beam,  rotator and finally all of the cables plus guy wires while the assembly is lying on the ground. By using a block and tackle mounted on the roof, the assembly could be pulled into place. That solves the push up concern but the flimsy bolted mechanical joints would still be an issue
  2. Option 2 would be like Option 1, but have all of the mechanical joints be welded. That said the flimsy mast itself could suffer a bend, kink or failure during the raising process. (Same applies with Option 1).
  3. Option 3 would be like the original  thought of a push up mast. My back yard has access that could be used by a bucket truck that could raise the mast section by section. This would add a lot of cost to the project. We would still have the flimsy mast issue
  4. Option 4 involves a different mast structure. SpiderBeam has an aluminum mast made for their hex beam and it was written up in QST. It cost 3X the Rohn H950 --guess there is a message there.
Needless to say I am pretty bummed out! Before purchasing the H950 I contacted the distributor about my plan and asked if there were any concerns  -- well you know distributors all they want to do is sell product. I was told my installation would be a fairly typical use of the mast. The ad says you should not use it for a beam --Dah! But the distributor said a 2 element beam would be OK. Like I said the distributors will do anything to sell product. Guess I need to find an alternate distributor.

All this leaves me wondering just what would you do with a Rohn H950? It would probably work OK if you were supporting a 6 Meter dipole made out of #30 wire.
 
Stay Tuned de N6QW