Near Vertical Incidence Skywave (NVIS)

and the 40 meter Novice Sub-band.

BY: Carl W. Davis W8WZ

Forty meters has always held a special attraction for me. The personality of 40 lends itself to casual rag chewing contacts across the Unites States and Canada. The novice sub-band was my first real introduction to HF since I got my ticket when ten meters was totally dead and there was not enough room in my parent's yard for an 80 meter dipole.

My first 40 meter antenna was a flat-top dipole center fed with 300 ohm television twin lead and up about 45 feet. I have been operating on 40 meters ever since.

Today, I am fortunate enough to live deep in Indiana farm country where I have freedom to experiment with antenna design to my heart's content. For 40 meters, I have an OFC Dipole very similar to the Fritzel FD4 in an “Inverted V” formation with the apex at 75 feet. I also have a gap titan DX vertical. And, until a short time ago, I had a half wave 40 meter dipole sloping south with the feed point about 40 feet above ground.

As I used those antennas, I realized that very rarely (if ever) did my sloping dipole ever outperform my Fritzel. So, I decided to take down the sloping dipole and use the wire and coax to make something new. But what???? I also had half a roll of 14 gage stranded insulated wire in the shed that could be used for this project.

My first thought was a phased array for 40. However, after looking into that, I realized that I would have to move some of my other antennas for that idea to work. Being lazy, and also not wanting to change the performance of my other antennas, I decided to do something less invasive. As I looked for ideas I came across a few web pages for NVIS.

Near Vertical Incidence Skywave involves placing a horizontally polarized antenna very close to the ground. The result is that your RF power is radiated largely straight up into the sky at a VERY HIGH angle of radiation. Also, there is limited ground wave propagation of your transmitted signal. Basically, you create a large “ball” of RF with the bulk of it going straight up into the sky. How in the world could that be useful to a radio amateur? I wondered.

The answer is that this type of antenna can fill a void that our more traditional antenna systems leave out. While NVIS systems will not work well for DX - the bubble of RF that they create results in a very strong RF signal to all stations within that bubble. The size of that RF ball or bubble is determined by the selected transmit frequency and to a lesser extent propagation conditions. 7 MC is the highest ham band that NVIS works well on. An NVIS system creates a strong uniform signal over a small geographic area.

In short: only close stations can hear you - but they will hear you well.

Since I had a very good DX 40 meter antenna, I decided to try out the NVIS concept.

The NVIS system I chose is very simple. It is described in an article by K5AXN which can be found on the internet at http://www.hamuniverse.com/supernvis.html. Basically, It consists of a 40 meter dipole hung in a flat-top configuration at an elevation of about seven feet. Underneath it is a system of 3 RF reflectors. The RF reflectors are 60 foot sections of #14 stranded insulated wire. One of these reflectors was laid on the ground directly under the dipole. The other two were laid on the ground parallel to the dipole. One was laid 6 feet South of the Dipole, the other was laid 6 feet North of it.

The antenna's need to be placed so low presented two problems for me. First, this design presented a safety concern. We all know that good amateur practice is to keep the antenna high enough so that it cannot cause injury to a passer by who may inadvertently touch it. Secondly, an antenna that low would be in the way when I did yard work such as mowing the lawn. Finally, how was I going to place the RF reflectors on the ground and still be able to mow the grass? I have never liked buried radials because they work themselves up over time and get tangled in lawn mower blades. So, I didn't even consider the idea of burying them. Instead I looked around my property for a place that this antenna system could work.

Directly South of my house is a small but thick woods. I decided to put my NVIS system in the woods. NVIS has been used by military radio operators for years. They have used this technique in dense forests, so I was confident that the idea would work. Although I did figure I would need to do much antenna trimming to compensate for its interaction with all the trees. This would be the safest location possible since no visitors to my house would accidentally go into the thick brush. Also, since I didn't need to mow the grass in the woods, the wire reflectors could lay flat on the earth, held in place with plastic tent stakes.

I tied the dipole to trees at 7 feet and supported the antenna at a point in the middle of each leg to prevent sag. To support them at those places, I cut one foot sections of ¼ inch PVC pipe. I drilled holes in each end. I threaded the antenna wire through the hole in one end of the PVC pipe. Then I hung the PVC pipe from tree branches by putting nylon twine through the end of the PVC pipe opposite the antenna, and tied it to a tree branch. I supported the center of the dipole by hanging it from a tree branch in similar fashion. I had purchased some ½” PVC pipe to use as a center support mast but found that I did not need it.

To my surprise, I found the antenna to be resonant on the same frequencies as it was when hanging as a sloping dipole in free space. In this case that was the lower end of 40 with the lowest SWR between 7.000-7.040 MC.

I made some contacts with the antenna this way. I found that it was less prone to BCI and atmospheric noise than my other antennas. It was very quiet. Amateur signals were also quieter than on other antennas. When I found that I had a much lower RST from other stations, I figured that the antenna was quiet because it was nothing but an air-cooled dummy load. At this point I was disappointed with the antenna, and figured that its ability to limit the BCI and QRN was the same as my attenuator's ability to do the same - but at least the attenuator didn't effect my transmit signal. But, I knew I was not finished with the project. I still needed to put down the RF reflectors. I just wanted to see how much difference they made, so I made some contacts without them.

Once I added the RF reflectors - the antenna system came alive! As anticipated they did have an effect on the resonant frequency of the dipole. They moved the resonant frequency right into the Novice Sub-band. Exactly where the BCI is worst and where I really wanted to use this antenna at! I was very happy that I wouldn't need to do the antenna pruning/adjusting I had thought I would.

Here is what I found using this antenna in the Novice Sub-band. First - it is a quiet antenna. Of all my antennas, it rejects the most BCI and QRN. The Broadcast signals are usually 10 - 15 S-units lower on the NVIS system. The Amateur signals are generally about 1-3 S-units lower on the NVIS system. Sometimes, the amateur signals have the same signal strength. Very rarely, are they louder. However, even though the amateur signals are usually lower in strength, they are easier to copy because the BCI has been drastically reduced. A typical contact goes like this: On normal antenna: Amateur Signal is S-7 and noise is S-6. That means that the signal I want to copy is only one S-unit above the noise. On the NVIS system the amateur signal is only an S5. However, the noise is only an S1 or S2. Thus, the signal I want to copy is now 3 to 4 S-units louder than the noise.

I have routinely gotten very good signal reports from the stations I work. The best DX I got with this antenna system so far was a station in Georgia, USA. He gave me 599 and I sent him 599. It is definitely not a DX antenna. However, it does allow me to have many loud contacts with stations within about a 500 mile radius of me. By rejecting the BCI so well, it also allows me to use frequencies that I would have otherwise just tuned past due to the QRM.

Do I suggest NVIS for others? If you already have a “normal” 40 meter antenna and want to expand - this is a great way to do it. If you are looking for an easy antenna project that doesn't involve a lot of tree climbing or tower raising - this is a great one.

If you want an antenna system you can place in the woods and free up your open space for other antennas, this is one of the few antennas that will do that well.

If you are a new ham limited to the Novice Sub-bands and just want a simple, inexpensive antenna system that will reject as much BCI as possible and let you practice your CW skills by making loud and clear contacts with hams within 500 miles of you - this is a good antenna for you.

Also, if you need a 40 meter antenna to check into local or state wide nets the NVIS system would be good for you too. You wouldn't be causing much QRM to stations outside of your net's service area. And, your signal would be uniformly loud to all of the stations within your net's service area. For that sort of application - this is a good antenna as well.

Disclaimer: This article isn't designed to be a technical discussion of NVIS. It is intended only to be a description of a practical application of NVIS on the 40 meter Novice Sub-band. For a better description of how NVIS works the author suggests that readers do their own technical research and personally experiment with NVIS systems.