Near Vertical Incidence Skywave (NVIS)
And The 40 meter Novice Sub-band.
Carl W. Davis W8WZ
You may have seen this article on eham.net.
Carl has kindly allowed us to repeat it here slightly edited to share with more Hams!
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. (See graphic in the Hamuniverse.com link above.)
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 ? 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 ?? 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.
THE ANTENNA SYSTEM COMES ALIVE!
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! I was very happy that I wouldn't need to do the antenna pruning/adjusting that I thought I would.
SOME GOOD POINTS ABOUT BCI AND QRN REJECTION USING NVIS ANTENNAS!
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.
SOME FINAL POINTS TO PONDER ABOUT NVIS ANTENNAS!
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.
73 Carl, W8WZ
Questions? Contact Carl at www.QRZ.COM for his email address.
Super NVIS project referenced in this article here