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W2IK's BASIC NVIS ANTENNA KIT
designed by Bob Hejl - W2IK all rights reserved
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NOTE: A re-designed W2IK-NVIS antenna system which was beta-tested by three EmComm groups (with radically new and simplier construction details) can be found at: W2IK's Emergency NVIS Antenna
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The "W2IK NVIS Antenna Kit" is shown in the picture above. NVIS is one key to sucessful HF emergency communications when you need to make reliable, short-range contacts. This type of antenna, if designed and deployed properly, will not create "skip zones". This makes it ideal for 40 or 80 meter emergency work. The RF pattern will resemble a half grapefruit with reliable signal coverage 400 or so miles in every direction.
Unlike my first kit design, I replaced the hammer-in support rope stakes with a nylon auger variety available at military surplus stores. At first I thought that they wouldn't be able to withstand the torque or pullout stress. A few tests showed me I was wrong. They are easy to use and easier to remove from either soft or hard soil while still supplying enough strength so that they won't pull out like a conventional stake might.
The picture also shows the kit without the coax cable and ground reflector system that were a part of my other pictures. I did this to give you a better view of the antenna, itself. The ground reflector system is very important in creating the RF waveform needed for NVIS signals. I have designed a reflector system which is easy to deploy yet suits the ground conductivity of our area. It took many field strength experiments to come up with the ideal reflector design.
There are three antenna supports, all of which are a little over 5 feet tall when assembled. This height, after field testing, I feel is not only RF adequate to create the "half grapefruit" lobe needed for communications within a 400 mile circular range but it also makes the antenna easier to handle and construct. With this low height above ground, this also means that it reduces the reception of both static crashes AND European broadcasters on 40 meters making it very valuable during emergency communications in south Texas.
If every emergency communications operation in south Texas, that used HF, were wise enough to also use the same type of antenna system it would make a very reliable communications web between all counties. I have also designed a NVIS antenna system which will create the same radiation pattern when it is installed on the roof of a building, such as at an emergency operations center. The reflector system is completely different to compensate for "pooling" and nearby metal structures.
I have updated this NVIS construction webpage with pictures and construction details . This antenna concept is very important during emergency communications on a local area level. Every EOC and HF field op who are serious in their work needs an NVIS antenna, such as this system, to maintain reliable HF communications during an emergency.
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Pictured Below:
Complete antenna system with 100 feet of RG8X coax.
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PARTS LIST:
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3 - 5 foot sections of 3/4" CPVC-4120
CPVC 100psi pipe (cut each 5 foot
section into 4 equal lengths)
9 - 3/4" CPVC couplers for above pipe
3 - 3/4"x12" thin wall copper tubing
(make sure the CPVC can slide in)
67 feet. Stranded, insulated wire 18ga.
1 - 9" section of 1/2 inch CPVC
Lowes p/n 50025 100psi
(this should slide into the Sch 40
PVC as listed above)
1 - Mating 1/2" CPVC "T" connector for above
4 - Auger-type nylon ground stakes
2 - 20 ft lengths of light duty nylon cord.
1 - SO239 "Barrel" connector
73 feet "zip cord" any gauge
6 - 5" pieces of 1/2" PVC tubing (any type)
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ANTENNA SET AT A LOWER LEVEL FOR DECREASED RANGE
(SEE TEXT AT END OF PAGE)
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HOW TO CONSTRUCT AND DEPLOY THIS ANTENNA:
(pictured referred to in construction text)
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FIGURE 1 - a few of the masting sections
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FIGURE 2 - One end of antenna wire and top mast section
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FIGURE 5 - masting section and copper ground sleeve mount
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FIGURE 3
Center Of Antenna
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FIGURE 4 - Reflector end
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Driven Element And Support System Construction
Take the 3 five foot sections of 3/4 inch CPVC pipe and cut each section into 4 equal lengths. This will now give you 12 sections that are all equal in length. Using CPVC glue, attach to NINE of these lengths ONE CPVC coupler to ONE end. This will give you NINE sections of pipe, all equal in length, with couplers on one end like the three shown in FIGURE 1. This also leaves you with THREE sections without any couplers.
All of these components represent the three antenna wire supports.
Take the 1/2" CPVC "T" connector and drill a hole on each side of the "T" as to accommodate the 67 feet of 18 gauge, stranded, insulated wire and pass one end of the wire through one drilled hole on the "T" and the other end of the wire through the other drilled hole so a 6 inch length of both ends are snaked through ONE of the pipe openings that run along the top of the "T". Remove 1/4" of the insulation from each wire end and solder ONE end to the center of the SO-239 and the other end to the threaded shell after filing away the nickel coating along one lip edge so the solder will flow. Make sure that the wire ends do not touch each other. The wires must be soldered so the barrel connector can be pressed into the "T". Refer to FIGURE 3 for a better detail. Draw back the excess wire out of the drilled holes of the "T'". Cement the short piece of 1/2" CPVC to the vertical portion of the "T". Fill in the void on the horizontal unused hole of the "T" with expoy so it encapsulates the connections in the "T". Then run out the entire wire and cut it in half so there will be 33.5 feet of wire coming out of each side of the "T" for each leg of your antenna. At each other end of the wire, attach one piece of the 3/4" piece of CPVC pipe (the ones WITHOUT the couplers) by drilling a hole at one end of each pipe and slipping the wire through it and then tying a knot as shown in FIGURE 2. This completes the driven element construction of this antenna.
Assembly of the Reflector System and It's Proper Placement
Take the 6 pieces of 5 inch long 1/2" PVC tubing and drill a hole completely through each side of each piece at a point 1/4" from the ends. Make the drilled holes parallel to each other. Then take the 73 feet of "zip cord" and seperate the wires so you are left with two 73 feet sections of insulated wire. Slip the one wire through one hole on each of the pvc pieces, attaching the wire to the end pieces but allowing the other pieces to slide. Do the same with the other length of wire on the other side of the tubing. Refer to FIGURE 4 for a picture of one of the end sections. What you have created is a 73 foot pair of wires with spacers that keep the wires 4 1/2 inches apart. This is the reflector system (which is, of course, longer than the driven element you've made)
It is meant to be laid on the ground directly below the antenna with the spacers keeping the wires apart from each other. This reflector system is "passive" and therefore not wired to any part of the active antenna wiring. Also, do NOT connect the parallel wires to each other. You can slide the 4 inner spacers along the parallel wire length to keep them apart. The three antenna support masts, when assembled and antenna wire in place at the top of each support, will fit within the parallel wire spacing which remains on the ground. Make sure that the entire reflector system (the parallel wire) is aligned so that it over shoots each end of the antenna supports by the same length on both sides of the driven element. Do NOT assume that the earth below the antenna will make an adequate reflector system. Tests both by myself and others have proven that an actual wire reflector system creates a much more effective radiation pattern with more RF energy "mushrooming" up and out in the proper direction. It is also important that the reflector system be made from INSULATED wire and not bare wire.
How to deploy this system
Find an open area where you can deploy this antenna. You'll need a total length of about 80 feet. Make sure it is no where near where anyone can come in contact with it or serious injury might result. We assume no responsibility for any injury due to antenna deployment. Keep it away from power lines or any place where people might wander in and touch it, disturb it or walk into it. Remember, this antenna is basically a 40 meter dipole antenna placed at a height of about 5 feet with a reflector "element" laying below it.
Find the exact spot where you wish the center support (the "T" assembly will be here) and carefully using your foot, gently push one of the 3/4" x 12 inch copper tubing sections in the ground until it is embedded in the soil with 4 inches exposed above the soil. Into this tubing insert one section of the 3/4" CPVC assemblies with the coupler facing up. To the end of that coupler add another coupler section, then one more until you have built a mast of 3 sections. In the top coupler of this mast, insert the last piece of CPVC that DOES NOT have a coupler. In to the top of this, slip in the "T" section you have made (it's FIGURE 3). This point is now the center of the antenna.
Now uncoil the antenna wire away from this "T" with each wire going in opposing directions, the same way a normal dipole antenna would. When you reach the end, give the antenna wire a SLIGHT pull so it ever so slightly bends the center support in that direction. Where this happens, insert another piece of copper tubing in the ground the same way as was done to the center. DO NOT assemble the support at this time. Do the same with the other end of the antenna. Now install the ground reflector assembly by taking the center "T" off for a moment, slipping the parallel wires on each side and replace the "T" back in the support. Run the reflector towards each end and make sure each end of it goes over each of the copper tubing ends. NOW, assemble the end support masts, one at a time and insert the CPVC wired end of the antenna into each end. This will create a very droopy, wobbling dipole with the center at 5 feet, the ends 5 feet and a reflector under it.
Now take one 20 foot piece of nylon cord and tie a small loop at it's center. Slip that loop over the end of one of the end support masts. The attached antenna wire will prevent it from running down the CPVC. Gently pull back and at the same time seperate the two lines you've created and using two auger stakes, secure the ropes to them the way you would anchor a tent.
This will pull the support back a bit. Now do the same with the other side. In pulling this support you will also remove the droop in the antenna. The CPVC tubing can take the flex, so don't worry. You may have to do some adjusting but with a little practice you'll soon be putting the antenna up faster and better each time.
From the center connector, make a choke by tightly winding about 8 turns of RG8X into a coil at one of it's ends and securing the coil together with tape or wire ties. Screw in the coax, with the coiled coax end into the antenna, and allow the cable to dress STRAIGHT DOWN along the center mast before you dress it away from the antenna. This will put less side bending stress on the center mast.
Before you go inside to operate, dress the reflector assembly so it is straight under the antenna and each end is equally beyond the end antenna supports. Use the sliding spacers as needed to keep the wires parallel.
It's now time to hook up the coax indoors. Remember... since this antenna is so close to the ground, each time you set it up the actual resonance will be different so you need to use a TUNER at all times. You will probably be using this antenna on 40 meters most of the time, hence the close-to-40 meter dipole measurements, but you will also find it very adequate for emergency use on 80 meters. Again, you'll need a tuner.
There are different designs of NVIS antennas, however I find that this design is one of the best and easiest to use during emergency set up and communications. It can easily be broken down and packed in a briefcase.
And now another unique detail of this design
If, due to closer static crashes OR too intense reception of unwanted international broadcasts, you can LOWER this antenna by removing one section, each, from the three supports! This also starts to limit the range of it's use, however it might be needed under such conditions. You can even remove TWO sections from each of the three support masts to make it even lower. Just be careful as it might become a trip hazard. In any event, and at any height deployment, you might wish to tie bright caution tape on the dipole wire so it can be easily seen.
So, here you have the basic design of the W2IK NVIS Emergency Antenna system. Build one, play with it, modify it to fit your needs. Do it now... before an emergency has you saying "I wished I had built an NVIS antenna."
To be updated as needed.
design by Bob Hejl W2IK
all rights reserved
feel free to add this as a link to any
of your emergency group's web listings as long as
design credit remains intact
CHECK OUT THESE TWO
OTHER ANTENNA WEBPAGES:
and
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