A 3-Band Dipole for nvis

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Before I go into the details of this antenna, I want to clear the air with this caveat; there is no such thing as an "NVIS antenna". NVIS, or Near Vertical  Incidence Skywave, is a propagation path. An antenna suited for NVIS is based on HOW it is configured. To use the NVIS path, we must configure the antenna to put our RF up into the atmosphere anywhere from 70° to 90° straight up. Vertically oriented antennas do not work for NVIS as there will be a null in that straight up path. In a nut shell, a horizontal antenna (dipole or end fed) positioned around 1/10th of a wavelength off the ground is the better suited position. This way, the ground reacts to send our RF up, and not out at a ~20° to 30° angle that we typically want for long haul or DX contacts. We need the RF to be returned to the Earth inside of what would be the 'skip zone', or that dead spot between the antenna and the first bounce back to Earth.

My antenna of choice to use in the field for NVIS communications is my version of the US Military's popular AS-2259 sloping dipole. The military version of this antenna is a crossed dipole setup; that is, the two dipoles are separated 90° apart, and all four legs are also the means by which the antenna is supported. The legs of the dipoles are 25' and 38' long, and a tuner is placed at the base of the supporting mast, which on the military antennas is also the feedline. This makes the antenna resonant in the ~9 MHz and ~6 MHz range, respectively. That's fine for the military, but for us hams needing the most efficient antenna we can get on 80/60/40 meters, those or not very good lengths. Yes, you can use a tuner and "get a match" and make it work, but it's not nearly as efficient as a resonant dipole.

Enter the BNC-2259. Yeah, it's corny, but just play along. :) My antenna consists of two dipoles; one center loaded for 80m and tuned to a frequency of 3.810, and another that is linked for 40m and 60m. If I need to use the digital portion of the 80m band to send and receive Winlink emails, I can clip on short wires to the end of each leg that physically lengthens the dipole so that it resonates in the digital/CW part of the band. On the other dipole, when linked it is usable on all five 60m channels. When unlinked, the 40m dipole is tuned to 7.225 MHz, and I can use the digital/CW part of the band without changing anything or needing a tuner.

To make this an "NVIS antenna", it is set up with the feed point 15' off the ground. I use a 19' SOTABeams Tactical Mini mast to set this one up. The mast is supported by a Home Depot/Husky brand work light stand. This mast slides over the upright tripod pole very nicely. The two dipoles are separated from one another by 90°. Paracord is attached to the ends of the wires, and become the self-supporting guy lines for the antenna. I can have this antenna set up in about 8 minutes. If the wind is blowing, I will attach some weight to the tripod to keep it planted to the ground until I can stretch the wires out and attach then to the ground stakes. 

Don't laugh too hard, but this part is made from a plastic closet rod socket I found at Ace Hardware. (Keep in mind.... as built, this antenna is for temporary, portable use. It was never meant to be permanently installed. If you do plan to permanently install it, use a suitable material here.) I drilled 4 evenly spaced holes just large enough for the dipole wires to fit through. For all my portable antennas, I only use this small lightweight wire from SOTABeams. I also drilled a hole to feed a 15' long piece of RG8X coax through. The coax is terminated with a PL-259 connector, and I keep a barrel connector attached so I can hook up the coax that runs to the radio. The hole in the center of the hub is perfectly sized so it stops at the 15' point on the mast. Then as with any dipole, one leg of each dipole is soldered to the braid of the coax, and the other legs are soldered to the center conductor of the coax. I use zip ties to help reinforce the point where the two wires attach to the coax and takes the strain off the solder point. Some with the coax. I then coated everything liberally with liquid tape. 

Loading coils were added to shorten the 80m legs to about 45' long each. An 80m dipole in an NVIS configuration would normally be around 115' to 120' long. My goal with this antenna was to keep the footprint to less than 100' diameter. I used this online calculator to determine what specific microhenry value would be needed for the coils based on coil location, wire size, former diameter, and the center frequency. The calculator also gave me different options for location of the coils, and different former sizes I decided to place the coils in the middle of the legs. I also used a coil inductance calculator downloadable from the site to determine how many turns of wire on the former was needed to reach the required uH value. With that info in hand, and knowing the uH value I needed, I attached the 80m wire (yellow wire) at the hub, marked the wire 22.5; from the hub, drilled a small hole in one end of the former, and began winding the coils. I only needed to adjust the number of turns by about 1/2 to reach the desired uH value, as determined by my LC meter. I then drilled another hole in the former and pulled the rest of the wire through.

The linked dipole is the easy part! You will need to attach two wire for the 40m dipole that are about 32' long. But first, you will need to remember that with the 60m legs linked, the overall length of this dipole will be about around 42' long each. And after adding on 5' to 7' of paracord, you will be approaching about 50' total length. So I added long enough cord to each leg so that while tuning the 40m legs, the overall dipole was guyed at around the same point on the ground that it would be guyed at if it were complete and linked for the longer 60m antenna. I tuned the 40m legs first for the center frequency I wanted, 7.225 MHz. Then I attached these SOTABeams linked dipole insulators. I love using these pieces as they extremely lightweight and very inexpensive. Then attach the remaining wire for 60m that will get you an overall length of about 43' for each leg. I use inexpensive automotive bullet connectors for all my antenna links. Attach, then connect the links, then tune the antenna for 60m channel 3. This tunes the antenna for the center of the band.

The antenna is very lightweight, and easily fits inside a 1 gallon Ziplock freezer bag, including four tent stakes. This would be a great antenna to keep in an ARES go-kit, which is exactly where this one lives. Other than it's large footprint of just under 100' in diameter, I have yet to find a downside to this antenna. Because it is self-guying, it is easy to set up. I takes right at 8 minutes from grabbing everything and walking out to the location I will set it up at, to being ready to plug the coax into the radio and be on the air. It requires no tuner, which means no additional equipment or extra weight. And it is a full size antenna, so you are going to get the most performance possible out of it.