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Near-vertical Incidence Skywave Antenna

DIY version of the military AS-2259 NVIS antenna for regional comms on HF.

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Near-vertical incidence skywave (NVIS) is a high-frequency (HF) radio propagation mode whereby signals are beamed nearly straight up to bounce off the ionosphere and cover an approximately 300-mile radius. The current project describes a DIY version of the US military AS-2259 NVIS antenna, built with common home center parts. It is easily erected by one person and can be quickly deployed and dismantled for field operations.

With amateur radio, reaching out to people on the other side of the planet is no big deal. With the right antenna and favorable propagation conditions, hams can chat over thousands of miles using less power than a 40-watt light bulb. This relies on using the ionosphere to skip signals over the horizon using traditional antennas with a low takeoff angle. This results in a dead zone out to around 300 miles or so, as the signal completely misses antennas in this range.

To get around this limitation and use the high frequency (HF) bands regionally, near-vertical incidence skywave (NVIS) propagation can be used. This mode requires an antenna with a nearly vertical takeoff angle which bounces the signal off the ionosphere almost directly back down to earth. This covers a radius of about 200-300 miles around the antenna, allowing regional communications on the HF bands.

There are several designs for NVIS antennas, most of which are simple dipoles mounted within a fraction of a wavelength of the ground, sometimes using simple long wire reflectors to shape the beam. These are easy to construct but take a lot of space, and are not particularly portable.

The US military developed a man-portable NVIS antenna, the AS-2259, to allow NVIS-mode HF comms in the field. Once abundant in the surplus market, these antennas are hard to find now, so this project aims to build a similar antenna using common DIY parts.

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  • A good mast design, but...

    Dan Maloney01/04/2016 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 Maloney01/02/2016 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 Maloney01/02/2016 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.

View all 3 project logs

  • 1
    Step 1

    Loading coil construction:

    The loading coils are air-core coils of 14-gauge THHN stranded wire. PVC pipe is used for forms. The 40 meter coils are made with 14 turns on a 3/4" PVC form, and the 80 meter coils are 19.5 turns on a 1-1/2" PVC form.

    The cores are each about 6" long - not a critical measurement. Drill a hole big enough for the wire about an inch from the end of the pipe, and another hole an inch from that. Feed your coil wire through the second hole, and crimp or solder on a spade terminal. Put a brass screw and washer through the other hole from the inside and capture the spade terminal under the head.

    Wind the coils as described, drill a hole at the end of the coil for the wire tail, and pass the wire back into the core. Leave a 7" pigtail. Drill holes in the center of the PVC caps for 3/16" eyebolts and screw them down. In one cap drill another hole for the pigtail. Thread the pigtail out the hole, glue the caps to the core, and terminated the pigtail with a spade terminal.

  • 2
    Step 2

    Dipole elements:

    Cut two 38' lengths and two 25' lengths of antenna wire. Leave a little extra for tuning and terminating.

    Since the antenna wires are also going to be the guy wires, I wanted to make sure the electrical connections were not going to be relied upon to take the guying forces - that's a little much to ask of an 8-32 screw. So I used crimp ferrules for 1/8" wire rope to form a loop in the antenna wire that passes through the eyebolt of each coil. I left a pigtail to connect the radiator to the coil.

  • 3
    Step 3

    Mast construction:

    Start with two 7.5' lengths of 1-1/14" EMT conduit. Drill four holes 90° apart about an inch top from the end of one tube. Thread the eyebolts into the holes and snug them up. Attach the threaded connector to the end of that pipe - this is going to be the top of the mast.

    Drill holes in one of the PVC caps for the brass screws. Make your connections to screws with the coax, feed the coax down the pipe, and thread the cap on the top of the mast.

    Install the coupler on the end of the other pipe and feed the coax down it. On the other end, install the other threaded connector to the pipe and attach it to the conduit body. Add some strain relief for the coax inside the junction box.

    Thread the pipe nipple into the other end of the conduit body, pass the coax out, and mount the SO-239 connector to the other PVC cap. Solder the coax to the terminals.

    For the swivel, wrap the split-ring hanger around the nipple and tighten the screws somewhat - it still needs to turn, preferably in the direction that will tighten the nipple in the conduit body. Add a threaded rod to use for a stake, and you're done.

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