Below is a article I wrote for
American Partisan titled
Home Brew - NVIS Antenna. If you want to see the pictures and have access to the links click on the article otherwise read a-way below.
This is a down and dirty, under 1,500-word DIY article on how to build a NVIS (Near Vertical Incident Skywave) Antenna. There are several dozen articles on doing this available via the ineterwiz however hopefully this write-up simplifies the build for you. Might as well learn from my mistakes.
If you do not know what a NVIS antenna is and why it is a great addition to your antenna farm, go and read these four articles. The first being from NC Scout titled, Near Vertical Incidence Skywave – Simplified, then NVIS I, NVIS II, and NVIS III authored by Key Pounder and published on NC Scout’s site, Brushbeater.
At a 100,000-foot level though, a NVIS antenna is a dipole antenna that has a steeper transmit angle than a traditional horizontal flat-top dipole antenna and is located closer to the ground at between 8 to 15-feet Vs. a traditional 1/4λ height; let’s say 30 to 60-feet over tera firma. This allows you to transmit to other stations closer to your AO that you may skip over using a flat-top horizontal dipole antenna. Think of the skip bounced off the ‘F’ layer of the ionosphere being 25 to 50-miles Vs. 200 to 300-miles. The German Army developed this procedure during WW II to communicate with troops via the airwaves closer to the transmitting station.
Traditional dipole antenna radiation Vs. NVIS radiation {Image}
I started out building my NVIS antenna using the directions posted on the DX Engineering site as my blue-prints for the project. Although you can buy a kit from DX Engineering, I had plenty of hardware around the redoubt so I decided to save a few bucks. Below is my Mise en place for this build. Hams are the true scroungers of this earth.
PVC Schedule 40 pipe. Three sizes 3″, 2 1/2″, & 1 1/2″ On hand
Green 14-gauge, stranded, insulated wire From Home Depot
100-feet of Green Para cord From Tractor Supply
4-dog bones (Insulators) Ham Radio Outlet
Can of Camo Green Paint Local Hardware store
Misc. nuts, bolts, self-taping screws, PL-259 connectors, wire ring connectors, Self-amalgamating tape, etc. Local Hardware store
PVC Schedule 40 reducing couplers, 3″ to 2 1/2″ and 2 1/2″ to 1 1 1/2″ Local Hardware store
Here are the steps of my build:
1) Cut the three pieces of Schedule 40 pipe to 5-feet and joined them together using PVC reducing couplers. In the end the pole was just over 16-feet long.
2) Once this was accomplished, I pre-drilled the upper end (smaller neck) on the two reducing couplers then inserted three, #8 x 1” self-tapping stainless steel screws into each collar to secure the section of pipe into the reducer.
3) Once assembled and fastened I stood the 16-foot pole up. The rigidity of the pole was less than satisfactory so I took it down to stiffen. To stiffen up the poll I inserted 1” x 3-foot section of PVC Pipe into the 1 1/2—2” section/joint and secured the stiffening pipe with one 3” x 5/16 stainless steel bolt. Then took a 1 1/2” x 3’ section of the pipe and inserted it into the 2” – 1 1/2” joint. Secured with one 4” x 5/16 stainless steel bolt.
4) Happy with the stiffness of the pole now I wiped down the pole with isopropyl alcohol. Once that was accomplished, I took some 150-grit sandpaper and ruffed up the pole. After another wipe down with isopropyl alcohol I hit the pole with camo spray paint. Let it dry for two hours then hit it again with another coat of the paint.
5) With this all accomplished I cut 8-inches off the top of the antenna. At the 15-foot mark I drilled a 5/16” hole through the top of the mast for the bolts that will be used for the antenna wire connections.
6) I measured out two lengths of insulated stranded 14—gauge wire in 65-foot lengths (63-feet plus 2-feet). Once laid out I then measure 26-feet from one end and marked with tape. Then from the other end I measured out 39-feet. In theory you should end up close (within an inch) of where you marked the wire earlier. At that point take off 3-inches of the insulated cover. Once done duplicate another length of wire for the other side of the dipole antenna.
7) Inserted a 1 1/4” x 5/16” stainless steel machine screws from the inside out of the holes you drilled at the 15-foot mark. Then wrap the bare part of the aforementioned wire around one of the bolts once, soldered the wrapped wire around the bolt for security. Once soldered I secured the wire between two washers and a wing nut.
By now you should have one wire attached to one bolt with one end 26-feet long and the other end 39-feet long.
Similarly I attached to the other bolt the other length of wire. Now I had the 40-meter and 80-meter wire attached to
each bolt.
9) With that done, I measured from the bitter ends of the wires 1-foot and attached the dog bones by wrapping on its self (Not soldered, for future length adjustment if needed) to all four bitter ends. Then attached 25-feet of para cord to each 40-meter wire (25-foot section of wire) dog-bone and 9-feet of para cord to the 80-meter (38-foot sections of wire) dog-bones. After the para cord was attached to the dog-bones on the 40-meter lengths I measured back from the dog-bone 20-feet and marked the length. Similarly, on the 80-meter lengths I measured seven feet back on the para cord and made a mark. The reason for this is you want the marks on the para cord to be at the stakes you insert into the ground, so the wires are at the proper angle which in part makes it a NVIS antenna. These angles will give the antenna the optimal angle for transmitting.
10) To attach the coax to the antenna I made a ‘Y’ pigtail with a 12-inch piece of coax. One end of the coax I separated the inner core from the outer woven shield about 3-inches long. Tinned the core and then the shield. Then affixed one, 10/12-gauge 5/16th-inch ring wire connectors to the tinned core and the other ring wire connector to the woven shield. Often, I see folks just clamp down on a ring wire connector with pliers however I used solder for double security as this is a weak point of the antenna. The other end of the coax ‘Y’ I fastened a PL-259 connector. Once the pigtail was done, I fastened one of the ring wire connectors to one of the bolts that supported one side of the antenna wire and then the other one to the other bolt. You can see this in the next picture at the top of the frame.
11) To the PL-259 side of the pigtail I screwed in a SO-239 female to female barrel connector for a PL-259 male plug. Then attached 75-feet of coax cable to the pigtail. Last, I wrapped this three-joint connection with self-adhering amalgamated tape to try and keep moisture out.
With the help of my neighbor (A 14-year old General licensed ham operator) we Iwo Jima’d the antenna up, set the para cord guide lines with tent stakes. Once the antenna shaft was supported, we pulled out the antenna wires and set the stakes using the previously marked para cord. Fed the coax into my shack and then to my Kenwood, TS-590S transceiver. Fired her up, tuned up the antenna using the Kenwood’s internal auto-tuner and…Well… It would not tune. DAMN!
At this point the sun was setting so I called it a night. Headed to my computer and started searching for an answer.
After several days of scratching my dome, I ran across an article on the ineterwiz from Shawn T. Donley N3AE and Steven O. Urquiza N3IDX, who had similar poor results when they built their NVIS antenna. It came down to the fact, that they, as I, were using a longer coax cable than what the military typically uses by 50’ or so. The article went on to state that if you installed coils about 7-inches along each wire from the top of the pole it would tune up. In the article there were formulas for wrapping the wire around specific sized PVC pipe which is located on page 3 of the article.
I followed the instructions and wrapped my coils. Added an additional three or so feet to each length as the wound coils shortened each leg-wire of the antenna (with coil in place 40-meter length still needs to be 25-feet and 80-meter with coil in place still needs to be 38-feet in length). Put the antenna back together, hooked up the coax, and my neighbor and I raised the antenna once again. Re-staked out the guide lines and antenna wires. Ran inside to the shack, fired up the power supply, then the radio. Hit the auto-tune button on the TS-590S… VIOLA, the antenna tuned up on 40 & 75/80-meters. Literally, several minutes later I made my first QSO with a gentleman in Cuba. An unusual long distance for a NVIS antenna but I will take it.
Since that antenna build, I have used the antenna at this past year’s (2018) June Field Day weekend and countless other events. I really experienced the NVIS antenna’s value in this past Octobers (2018) Pennsylvania QSO contest. While it was easy to hit the far-flung counties of the state, with a traditional dipole antenna and its height, I couldn’t hit the more local counties without the NVIS antenna. When I switched to the NVIS antenna and called CQ, I was inundated by QSO requests. HAVE FUN!
Freedom Through Self–Reliance©
References:
Near Vertical Incidence Skywave- Simplified
NVIS Explained, I
NVIS Explained, II
NVIS Explained, III
DX Engineering
Shawn T. Donley N3AE and Steven O. Urquiza N3IDX
