I'm new to the forum, but I figure this is a better introduction than anything else. I just finished building and testing a tower trailer for ham (amateur radio) stuff shown here:
A friend of mine received a visit from his neighbor who noted the radio gear on his roof. She asked him if he'd like to have a ~50' telescoping tower, all he had to do was come and get it. He called me in and that's what we did. It wasn't much of a bargain as it took several weekends over the course of a few months to get the thing off of her house and out of her backyard which was entirely occupied by a swimming pool and a gazebo.
My friend decided that he wanted to put the tower on a trailer for portable operation and ease of setup, so I made a few sketches and this is what we ended up with. It was welded up entirely with a Millermatic 135 using .30 GMAW and .35 FCAW for the thicker stuff. I used 2x6 .120 wall for stiffness and the ability to get good penetration with such a small welder. I also used some 2x2 .160 wall, and 2.5x3.5 .250 wall for the tongue. There's also some 1/4" and 5/16" plate in there. The total final weight is about 1500Lbs with the tower on a 3500Lb axle. The jacks, winches, axle, and fenders are standard commercial trailer stuff.
The most complicated bit was the hinge that the tower rides on. I was able to find a 1.250" rod at the scrap yard and a piece of 2" DOM that it fit almost perfectly into. The machinist knocked about .010" off the rod to smooth it out and I hit the inside of the pipe with a hone and then added a grease fitting. Unfortunately, I don't have any really good photos of the hinge. The fit was so tight, that there was no discernible axial deflection in the unguyed tower while it was fully extended and being buffeted by 30+ mph gusts of wind.
Obviously, it's not finished as is the case with most projects. There's still a few embarrassments that I'd like to grind on a bit. It'll also need to be prepped and painted. It has been tested (ARRL field day) and it works well. With jacks on four corners, it was pretty straightforward to get assembled, balance, and mount the axle after the fact. I think this contributed a great deal to the towing characteristics of the trailer. It tows like a dream, tracks straighter than my truck will go, and handles corners like a sports car. I was worried about ground clearance and considered adding casters on the back, but after towing it for a little over 700 miles I haven't bottomed out yet.
The MM135 was able to handle the .120 wall pretty well, but it really starts to suck wind on anything thicker. Good prep was critical and I was running it as hot as it would get most of the time. I also used the $50 HF chop saw that some folks have complained about. It's really not as bad as people have have made it out to be. It doesn't have much power, certainly not the 2HP that it claims, but I was able to cut most things accurately and in a reasonable amount of time. I just OA-cut the plate, which was a pleasure compared to the chop saw.