DIY Communications Projects - The NVIS Antenna Prepared By: William Furniss - WF9B Contact Info: Cell: 1-(801) 787-1809

email: william.furniss@gmail.com

The Case for NVIS - Antenna Build Part 1

NVIS is an acronym that stands for “Near-Vertical Incidence Skywave.” The NVIS Antenna was used extensively by Armies and Navies (and Search and Rescue groups World-wide)since World War I as an effective means of communication over a relatively small local area using High-Frequency (HF) radio signals.

Why use HF? Why not use 2-Meters or even the 70-CM band? Unfortunately, Dual-Band Hand-Held Radios have a disadvantage in some situations. The problem is that in the 2-Meter band (144MHz to 148 MHZ, also known as VHF) and the 70-CM Band (420 MHz to 450 MHz, also known as UHF) happen to live in the part of the electromagnetic spectrum with wavelengths that are energetic enough to punch through the ionosphere. This means that unassisted UHF and VHF signals (unassisted meaning Simplex, or non-repeater assisted) are limited to line-of-sight communications - You generally aren’t able to “bounce” signals off the ionosphere and back to the earth in order to communicate with another radio operator over the horizon, or even just a couple of blocks away unless their antenna can “see” or “hear” yours. (There are circumstances where “skip” with UHF or VHF signals can be achieved, but not but that’s another discussion.)

Supplies Required for This Project:• About 150’ of Braided Antenna Wire

(thewireman.com #811)• 75’ - 3/16” UV Resistant Dacron Rope

• 4 ea. 48” Fiberglass Military Surplus “Camouflage Net Support Poles” (also known as “Mast Poles” on eBay

• 1 Camouflage Net Pole Spreader Bar• 1 Spreader Bar Nut Assembly

• 1 SO-239 “Chassis Connector”• 5 ea. Heavy-Duty Tent Stakes• 20’ RG-8X Low-Loss Coax Cable

• 2 ea. PL-259 Coax Connectors• 1 Ea. Female Barrel Connector for

PL-259 Connectors• Solder, Solder Flux, 100W Soldering

Iron or Butane Torch

(Please refer to the comprehensive list at the end of this document)

Completed NVIS Antenna Mast-Head(Sealed With Hot Glue)

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What this means is that UHF and VHF communications can sometimes be difficult beyond a relatively small local area.

Now, imagine a scenario where you find yourself attached someday soon to a small Search and Rescue group assigned to locate and rescue folks in a mountainous or canyon-like area. Communications could be a challenge without the right equipment.

What you need, is a method of communications that works from mountain tops to canyon bottoms without the need for a repeater.

Enter the NVIS Antenna System!

The NVIS (Near-Vertical Incidence Skywave) antenna is designed to transmit radio signals on the 40-Meter, and 80-Meter bands* (in the High-Frequency part of the electro-magnetic spectrum), and to send them almost vertically into the

atmosphere. Because radio waves in this part of the spectrum are reflected by the ionosphere, a signal transmitted by the NVIS antenna is reflected back to the ground in a cone-shaped pattern that’s anywhere from 600 to about 1000 miles across. Since signals are reflected back in a cone-shaped pattern, you can be in the bottom of a canyon and talk with someone on the other side of the mountain WITHOUT A REPEATER!

Now that you know a bit about why an NVIS antenna can be useful, let’s discuss how to build a strong, yet portable NVIS antenna yourself for less than $100.

HOW TO BUILD YOUR OWN NVIS ANTENNA - PART 1

The antenna design described on this and the following pages uses a few Military Surplus parts, some inexpensive brass hardware, antenna wire, and some UV Resistant Dacron Rope. FIGURE 1 above shows the two main parts of surplus equipment needed for part 1 of

this project - a camouflage net “Spreader Bar” and a “Spreader Bar Nut Assembly” that we’ll repurpose to make a strong, protected mast head for your very own NVIS antenna. Ready to get started?

STEP 1 - Remove the Spreader Bar Nut Flange:

The first step is to remove the flange on the inside diameter of the spreader-bar nut as shown in FIGURE 2 above. The reason this is necessary will be shown a bit later.

The simplest method to remove this cleanly is to simply grind it off with an angle grinder. I used one equipped with a layered sand paper head. Be careful to keep the grinder level as you trim - you want the ground end to be as flat and level as possible. You know you’re just about there when you start to see daylight at the inside edge where the flange meets the threads as shown in FIGURE 3 above. Work slowly.

HOW AN NVIS ANTENNA WORKS: The NVIS (or Near-Vertical Incidence Skywave) antenna works by transmitting a signal nearly straight-up where the signal is reflected back by the ionosphere. The “footprint” of the reflected signal is roughly a cone shape anywhere from 600 to 1000 miles in diameter. The principle of the near-vertical signal/near-vertical reflection is similar to the principle of the Mortar - a “high-angle of attack” allows the signal to be received just as easily in the bottom of a canyon as on the top of a mountain. The NVIS works best on 80-Meters, and 40-Meters, but experiments by Glen NA7M and others reveal that an NVIS antenna also resonates well on 2-Meters!

Spreader Bar and Nut Assembly STEP 1 - Remove Nut Flange

FIGURE 1 FIGURE 2 FIGURE 3

Remove Flange With a Grinder

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STEP 1 - Remove the Spreader Bar Nut Flange (Continued)

Once you’ve ground-down the flange, the head should look as shown in FIGURE 4 above.

STEP 2 - Cut-Off The Round Bar End

Using a hack saw or a cut-off wheel on the grinder, cut off the round end of the spreader bar. Leave about 1/4” to 1/2” of material beyond the edge of the round part so that you can trim it down later as shown in FIGURE 5 above.

Once you have finished the cut, trim the edge with the grinder to form a round shape as shown in FIGURE 6 above.

STEP 3 - Drill Center Mount Point for Your SO-239 Connector

There are a number of different suppliers of SO-239 connectors, none of whom seem to build to a single standard dimension. Rather than declare a specific hole size, you are encouraged to identify the specific size hole needed for your specific SO-239 connector. This can be accomplished using a set of calipers, or using the “close enough” approach of laying the bit along side the SO-239 connector to identify the correct size bit. Make sure the bit size is sufficient to allow for any “shoulder” or flange where the center meets the square base. You want the hole to be large enough so that the entire center fits into the hole easily so that the SO-239 connector’s square

mounting plate is flush when it is bolted into place later.

Once you’ve selected the proper bit size, drill a hole completely through the round base as show in FIGURE 9 above. (PLEASE NOTE: It is NOT critical to locate the exact center when you drill the hole for the SO-239 connector. “Close enough” to center will do.)

Test the fit of the SO-239 by inserting it into the hole you just drilled - make sure it’s a good fit and that the square base is flush with the plate when seated in place. Use a file or sand paper to enlarge the hole as necessary. It’s OK if the connector is slightly loose in the hole, as long as none of the 4 mounting holes on the connector’s square base are visible from the center-pin side.

Head With Flange Removed STEP 2 - Cut Off “Toe” of Bar Grind Off Extra Material

FIGURE 4 FIGURE 5 FIGURE 6

FIGURE 7 FIGURE 8 FIGURE 9

Measure Head of The SO-239 Connector You’ll Be Using

Choose The Proper Drill Bit (Yours May Be DIfferent)

STEP 3 - Drill Center Mount Point for Your SO-239 Connector

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Make sure to test the fit of your mounting bolts in each of the four mounting holes on the SO-239 connector before moving on to Step 4 as shown in FIGURE 10 above. If they won’t fit, use a drill bit to carefully enlarge the mounting holes to fit.

STEP 4 - Mount the SO-239 Connector on the Round Plate

With the SO-239 held in-place in the center mounting hole, drill holes for the four brass mounting bolts as shown in FIGURE 11 above. In this case, we used #8 1-1/2” brass bolts. The decision to use brass hardware is preferred specifically for to mount the SO-239 connector because of the metal’s improved conductivity.

(Stainless Steel hardware can be used as a substitute.)

Once you have mounted the four mounting bolts, you should add two nuts per bolt, secured with a dab of red thread locking compound. Tighten the bolts as securely as possible without damaging the bolt. You might also consider using serrated lock-washers instead of the second locking nut.

Once the bolts and nuts are securely tightened, grind off the extra length of two of the four SO-239 mounting bolts as shown in FIGURE 12 above. The fourth bolt will be used to mount a conducting wire later.

STEP 5 - Mount the Spreader Nut Assembly

With the SO-239 secured on the mounting plate, you should mount the Spreader Nut Assembly. This is an important step, so be sure to get it right. First, unscrew the Spreader Nut from the Spreader Nut Base. Turn the Spreader Nut around and reassemble as shown in FIGURE 13 above. Next, center the Spreader Nut Assembly over the mounted SO-239 connector as shown in FIGURE 14 above. It’s important that you center the Spreader Nut Assembly as carefully as possible so that you can avoid damaging the heads of the SO-239 mounting bolts. This is why we ground off the Spreader Nut Flange back in Step 1.

Mark the correct location with a pencil, and then unscrew the Spreader Nut from the assembly.

Check Fit of Mounting Bolts in SO-239 Mounting Holes

STEP 4 - Mount The SO-239 Connector on Round Plate

Tighten Mounting Bolts and Grind-Off 2 of the 4 Bolts

FIGURE 10 FIGURE 11 FIGURE 12

FIGURE 13 FIGURE 14 FIGURE 15

STEP 5 - Mount The Spreader Nut Assembly

Center Spreader Nut Assembly Over SO-239 Connector (IMPORTANT!)

Mark Location For Reference

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STEP 5 - Mount the Spreader Nut Assembly ! (Continued)

With the correct location marked from the previous step, you should drill the mounting holes for the Spreader Bar Nut. Rotate the drill holes 45 degrees relative to the placement of the four mounting bolts from the SO-239 that were secured earlier as shown in FIGURE 16 and FIGURE 17 above.

Offsetting the bolts will avoid problems with conductor wire placement later. This also distributes strain forces in the mounting plate, thereby reducing the chance of damage in a mast fall later.

With the Spreader Nut mounting holes in place, insert the brass mounting bolts and tighten following the same process described in the SO-239 mounting instructions above leave two of the four Spreader Nut mounting bolts long for use in mounting lead wires in a later step.

Next, mount four brass bolts at the edge of the mounting plate to serve as the wire attachment points for the antenna leads. These should be mounted in-line with the brass mounting bolts for the SO-239 connector. In this case, I decided to use to heavier-duty 1/4” brass bolts as the wire mounting points simply

because of the greater strength. With the 1/4” bolts as mounting points, the Spreader Nut mounting bolts were cut off as shown in FIGURE 18 above.

The next step is arguably the most critical, and the most difficult in the whole construction process.

STEP 6 - Prepare and Solder #6-32 x 1” Brass Bolt to Center Pin of SO-239 Connector (Critical Step)

In order to create a threaded base to which you can attach wire leads in a later step, you must prepare and solder a 1” long #6 x 32 brass bolt. The bolt

Drill 4 Mounting Holes for Spreader Bar Nut

Rotate Spreader Bar Nut Bolt Placement 45 Degrees

Relative to SO-239 Bolts

Add (4) 1/4” Brass Bolts Near Plate Edge As Attachment

Points for Wire Antenna

FIGURE 16 FIGURE 17 FIGURE 18

FIGURE 19 FIGURE 20 FIGURE 21

Test the fit often as you remove the brass material from the end of the bolt

STEP 6 - Prepare and Solder #6-32 x 1” Brass Bolt to Center Pin of

SO-239 Connector (Critical Step)

STEP 7 - Mount Conductor WIres, Final Trim and Seal

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STEP 6 - Prepare and Solder a #6-32 x 1” Brass Bolt to Center Pin of SO-239 Connector (Continued)

must first be prepared in order to fit into the “solder pocket” of the center pin of the SO-239 connector.

One simple method to do this is to place the bolt firmly in the jaws of a power drill, and hold the bolt against the flat edge of a metal file. You only need to prepare approximately 1/4” of the end of the bolt in order to fit the solder pocket. Test the fit often as you file away material. Stop once you have removed enough brass material to fit the pocket with just a bit of “wiggle room” for a soldered connection around the bolt.

(IMPORTANT) Apply a small amount of Rosin Soldering Flux or use Rosen-Core Solder in order to help achieve a strong soldered connection between the bolt and the center-pin of the SO-239 connector. When you’re ready to solder, You must work quickly and carefully in order to ensure both a strong connection, and to avoid damaging the SO-239 insulation material. The rosin flux will help to draw the liquid solder into the joint, and strengthen the connection. Test the connection between the bolt and the center post to see if you need to file away any excess material.

Using either a small butane torch, or a 60-Watt or higher soldering iron, (100 Watts + is preferred) CAREFULLY pre-heat the bolt end for a few seconds, and set the bolt into position. Hold the solder at the connection between the bolt and the center pin, and then apply

the tip of the soldering iron (or tip of the flame if you are using a butane torch) to the bolt on the side opposite where you have the solder held in place. This causes the heat conducted through the bolt to melt the solder, and not the heat source itself. In other words, using conducted heat helps to ensure the joint has reached the proper temperature to ensure a solid connection.

With the center pin and bolt soldered in place, you’re ready to finish the construction of the head assembly. To do this, simply measure the correct length for the wires from the center pin to the connector posts at the edge of the mounting plate, and from the long bolts you left in place from the SO-239 connector. Slip some short lengths of heat-shrink tubing over the wires before crimping and soldering ring terminals in place. Once the ring terminals are attached, use a small torch or a lighter to shrink the tubing in place. This serves to protect the ends of the wire, and it adds a measure of protection against sun and rain. You should have two connections running from the center pin to the edge connections separated by 90 degrees as shown in FIGURE 21 above. In this way, you will have one wire lead in each direction to connect to the antenna head - one connected to the center post, and one connected to the “shield post” or outside of the SO-239 connector via the long bolts you left on the SO-239. One set for the 80-Meter leads in one direction, and one set for the 40-Meter leads 180 degrees apart.

Completed NVIS Antenna Mounted on 4 - 48” Military Surplus Fiberglass Camouflage Netting Support Poles

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STEP 7 - Test and Seal: Once you have completed the steps above, you are prepared to test for electrical continuity and to seal the assembly.

Be sure to test the electrical continuity of the antenna head assembly by using a volt meter.

Turn the volt meter to “OHM X 1K” and touch the black and red meter leads together. You should see movement on the meter indicating 1 OHM. If so, your meter is working, and you can now use it to test the connections on your antenna head.

First, touch the heads of the plate-edge connection bolts for the two wires coming off the center pin of the SO-239 connector. You should immediately see the meter jump to “1.” Next, touch the meter leads to the connection bolts for the leads coming off the SO-239 mounting bolts. Again, you should see the meter jump to “1.”

Next, touch the bolt head for one of the leads from the center point of the SO-239 and one from the bolts you left long from the SO-239 connection. YOU SHOULD SEE NO REACTION ON THE METER AT ALL. In this case, “no news

is good news.” If you don’t see any reaction on the meter when you do this, it means your antenna head is electrically isolated between the center pin, and the “shield” of the SO-239 connector. This is exactly what you want. Congratulations! You’ve completed the hardest part!

If you see the meter jump when it is not supposed to (as described above) it means that you have an electrical “short” in the wiring. This is most commonly caused by a small piece of solder caught between the center pin and the edge of the SO-239 connector.

STEP 7 - TEST and SEAL(Test Meter by Setting to “OHM X 1K” and Touching Leads Together)

FIGURE 22 FIGURE 23 FIGURE 24

Touch Bolt Heads for Leads Coming Off of Center Pin of the SO-239 Connector - (Should Read “1” on Meter)

Touch Bolt Heads for Leads Coming Off of Mounting Bolts of the SO-239 Connector - (Should Read “1” on Meter)

Touch Bolt Heads From One Center Lead, and One Shield Lead - Meter Should Read “0”

FIGURE 25PLEASE NOTE: Normally, you would test for electrical connectivity BEFORE sealing the mast head, (which I did.)

Unfortunately, I didn’t think to take pictures of that process until after the mast head assembly was sealed with hot glue.

FIGURE 27

Seal Center Point with Clear Silicone or Hot Glue.

Indicates “1” on Meter Indicates “1” on Meter Indicates “1” on Meter

Indicates “0” on Meter