The idea of a quickly erected field antenna support is appealing. I wanted something that didn't require guy wires or stakes, so that led me to various tripod kinds of schemes, but I hadn't come up with any good ideas for what the structural members should be. I had bought a screwdriver antenna, and I figured you could mount it on such a tripod for a nice field antenna.
While wandering through the aisles of the local home improvement/hardware store I noticed collapsible painter's poles. These are available in fiberglass and aluminum in a variety of lengths from 12 to 20 feet (extended length). They're telescoping 2 section poles and lock by means of either a clamp at the join, or an eccentric cam on the end of the inside member. The top has a plastic Acme thread about an inch in diameter with a fairly coarse pitch (2 threads/inch?). I bought 3 of the 12 foot units.
I figured you could take three of these poles and make some sort of tripod. The problem was finding something suitable for a socket on the top. One could just use a tapered sleeve (like on a window cleaning squeegee) and just jam the pole into it, but I wanted something a bit more secure. However, they don't sell appropriately threaded female fittings; at least, not cheaply. And I wasn't up to machining a tapered inside thread with my trusty Dremel and Xacto knife. I solved this problem by buying the cheapest paint roller handle I could find ($1.29). These things all have the requisite female thread. You can saw the handle below the metal part, leaving just the plastic handle, ready to use. Now I had my three sockets.
My first idea was to hook three sockets with hinges to some sort of flat triangular plate, so you could fold the tripod up like a photographer's tripod. I then moved to an idea where I drilled a hole through each handle and used a U bolt as a pivot, figuring that this would provide the ultimate flexibility in placing the legs. A bit of testing showed that this flexibility was a disaster. The big problem was too many degrees of freedom. The antenna support had nothing to hold it level. Looking at my photo tripod showed that they use a center tube and 3 struts to the legs to perform this function. Tripods also have fairly good hinges with only one degree of freedom. The other problem was that the whole assembly was quite bulky, even when folded up, and a pain to store and move around.
Next, I figured I would make a rigid thing with the sockets arranged at 30 degrees from vertical and nicely spaced at 120 degrees. Rather than folding the tripod, I would just unscrew the legs. Storage was now an issue of 3 long poles and one small tripod head. Here's a picture of the stuff lying on the lawn (click for a bigger picture) along with a screwdriver antenna.
The construction of the tripod head was fairly simple. The support is made of 3 4" corner braces (angle brackets) with one of the holes drilled out for a 3/8-24 (fine thread) bolt. The brackets were bent to be 120 degrees instead of 90 with a few minutes hammering on the vise. The paint roller handles (already conveniently drilled from the previous experiment) bolted to the other arm of the bracket with standard 1/4-20 hardware and a hose clamp provides the restraint for pivoting around the bolt. Here's a photo of the completed tripod head (click for larger). The wire is connected to a 3/8" ring lug to provide a convenient way to connect to the head. Note that there's no insulation between antenna and the tripod head, but the sockets for the poles are plastic, and an insulator.
And, a few photos of the assembled setup. The left photo, below, is a view of the tripod head mounted on the poles, but with the poles collapsed (6 foot poles). At this height, you can still reach up to put the antenna on the mount. You can then extend the poles in increments, moving the bases out. The poles and the couplings are rigid, but still flex, so you can sort of walk it up until the poles are fully extended, as shown in the right hand photo. (large view of head) (large view of whole tripod with antenna)
The telescoping tube aren't necessarily connected to each other, and they certainly aren't connected to ground (a plastic end cap on the rear end). I'm not sure about their RF properties. For testing, I made a parallel ground plane of wires that you could string along the poles. Electrical testing is still going on, since the feed line I had for the screwdriver antenna was only 15-20 ft long, so you had to basically sit next to one of the poles.
It's not the most rigid structure in the world, so that might need some work. It holds the antenna just fine, and stands up in a 10 mi/hr wind without being staked down. Part of it is the flexing of the metal brackets and the plastic couplings, part is just that the painter's poles aren't so rigid. A few internal braces (say some struts between the legs making a triangle) might make it a lot more rigid, but that would start to increase the complexity.
radio/antenna/tripod.htm - 14 Jan 2003 - Jim
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