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• A good mast design, but...

  Dan Maloney • 01/04/2016 at 11:55 • 0 comments

  I decided to fly my multiband inverted-C dipole from the mast, to get it out of the heavy foliage it's currently in. I lowered the mast - a task that required a helper once I untied the first guyline - and attached the dipole. Getting wires and guylines sorted out wasn't trivial, but I was able to walk the mast back up and secure it, again with help.

  The one caveat here is the coupler between the two mast sections. It loosened up somewhat while I was working on the mast on the ground, and I failed to tighten the nuts before I put the mast back up. So there's a little kink in the mast now. I tried tightening the nuts from the ground, but they're just high enough that I can get much leverage. Don't feel like lowering the mast again, so I'll have to get a ladder or bucket to stand on for a boost.

  Still, it's pretty handy to have a 15' mast that I can raise and lower easily.

• Design basics

  Dan Maloney • 01/02/2016 at 17:36 • 0 comments

  The AS-2259 military antenna this design based on is pretty cool. The mast of the antenna is about 15' tall and breaks down into sections that can be easily bundled up in a bag. The mast is not just for support - it's built with coaxial conductors inside the mast that connect together when the sections are assembled. And the four elements are not just electrical parts of the antenna - they're also guy wires for the mast. So everything in the antenna has both and electrical and structural function.

  My version of the antenna is based on an antenna designed by Mike Melland (W9WIS) some years ago. It's basically two sloped dipoles, one for 80 meters and one for 40 meters. The feed points are on the top of the mast, and the two dipoles are oriented perpendicular to each other. Each element of the dipole is terminated with an insulator, and antenna rope is used to bring the total length of each leg to 45'. Each leg is anchored to the ground with a stake, and the antenna feedline goes up through the mast, which just rests on the ground.

  There are a couple of things I don't like about this design. As noted by N3AE and N3IDX, the dipoles are resonant well outside the 40 and 80 meter bands, meaning that an antenna tuner is absolutely necessary. I decided to incorporate their design for loading coils to increases the electrical length of the dipoles and make them resonant at 3 and 7 MHz. I didn't care for the structural design of their coils, though, so I changed them a bit.

  I also don't like using plastic pipe for antenna masts, having learned my lesson with the ABS pipe fiasco. So I decided to make this antenna less portable and more "luggable", and went with a mast based on steel EMT conduit. The pipes are nicely rigid, still pretty lightweight, and the array of connectors and fittings makes for a big steel Tinker Toy kit. Add to that the fact that a lot of PVC plumbing fittings are compatible with the threads, and you've got a nice system for quick and easy structures.

  One final design goal was to make sure the antenna could be erected quickly by one person. It's always nice to have help, but you can't always rustle someone up when you need them, so the ability to fly solo is key.

• Lesson learned: ABS is for LEGOs and 3D printing, NOT for antenna masts

  Dan Maloney • 01/02/2016 at 17:17 • 0 comments

  Version 1.0 of this was going to be a super-portable, lightweight design using three 5' sections of plastic pipe. I chose 1-1/2" ABS pipe because it was locally available, I could get a cap fitting with a flat top, and the matte black finish had a tacti-cool vibe. I won't bother going over the construction details for the mast because it was an utter failure. To say it was limp as wet spaghetti is an insult to pasta products the world over.

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