Near Vertical Incidence Skywave
A Book Review
By Richard Morrow, K5CNF
Most of the time, we want our signals to really reach out and head for the other side of the earth to contact a DX station, but there are occasions when we need communications on a more local level that cannot be served by the VHF repeaters. Now there are those who are thinking when could that ever be? There are some types of natural disasters are capable of taking out the repeater sites, such as hurricanes and really bad earthquakes. When this happens, the only thing left to use is the HF SSB communications that can do the job needed, which is to be able to bring in the required aid from surrounding areas.
Many times, particularly in the case of hurricanes, the emergency aid from Red Cross, Salvation Army, utility companies will be located away from the main path of a storm. This may be several hundred miles from the impacted areas. So the communications requirements are altered somewhat in this case. For those who have been fortunate never have been in a hurricane, a really big one like Hurricane Andrew will destroy anything in its path and flatten everything, towers and all. What is needed is a communications system that will reach out to the areas within a 400-mile radius to alert the waiting emergency units in the outlying areas. This is where the Near Vertical Incidence Antenna systems come in to play.
For those who are not familiar with the NVIS antennas, these were called cloud burners or cloud warmers, as the signal goes straight up and comes straight back down. This made for a tremendous signal close in to the transmitter site and out to about 300-400 milesjust what is needed for disaster communications when nothing is left standing after a really severe storm.
What equipment is needed to make up an NVIS system? Well that is what is this book is all about. The book is authored by LTC. David M. Fiedler (NJ ARNG) (Ret) U.S. Army (Ret) and Maj. Edward J. Farmer, P. E. (CA SMR), and is divided into three parts, with part one devoted to an explanation of how and why the NVIS works. There are many diagrams, charts and graphs of the different modes of propagation and how the NVIS mode differs from other modes. The principal difference is that the NVIS has no skip zone, which is what makes it so useable for close-in communications. There are many informative articles included in this book, many of them from articles written by the authors and many others that have been published in other publications.
The second part of this book is the how-to-do-it part and is another very interesting section. There are discussions on how the German and Soviet armies used this method of communications during World War II. There is a picture taken in the 1980s of a Russian BMP-60 with an NVIS antenna installed. The Russian NVIS usage is discussed in some detail and their reasons for why they use this method of communication is explained well.
Technical information on how to optimize the system for low power transmitters, 20 watts or less, is explained. There is more antenna design and information on how to design and optimize an antenna. Also there are several pictures of a mobile installation on a van that was used for testing a mobile NVIS antenna. The results of this installation are discussed in this section. Interestingly, the mobile radio used is a Kenwood 430S modified for military use and the matching Kenwood AT-230 tuner is used to tune the antenna. The frequency used is 4.520 MHz. The results were presented for analysis in an easy to understand format.
Many antenna are illustrated and their operation is described very well as is their construction. Dimensions can be figured out for the ham bands as these antennas are all within the construction capabilities of the average ham. By using the basic antenna length formulas, these antennas can be easily duplicated.
In the third section of the book, there are discussions on how the NVIS communications worked on D-Day and how it came to be used. It is a most interesting facet of the communications systems that were used that day. It is interesting to note that Dr. H.H. Beverage of Beverage antenna fame was the source of the solution for the communications problems that they were having at that time and as he recommended the use of horizontal dipole antennas close to the ground as a solution to the problem. The problem being that the vertical antennas being used were not working well as they had very little ground wave coverage and it appears that the skip zone was causing many problems with communications among the military command posts. The use of low dipole antennas solved that problem.
There is also a detailed report by the U.S. Marine Corps on testing that was done in the area of Camp Lejeune and outwards to 200 miles radius from there and this is covered in detail. There are several other tests that were run by other organizations and these are also covered in detail with charts and graphs. There are illustrations presented of the antennas that were used at the fixed locations as well as the ones used on the mobile units.
NOW I UNDERSTAND!
After I finished this most interesting book, I realized that in years past I had used many different NVIS antennas and not known it. Since I had done most of my operation on 75 and 40 meters, nearly all of my dipoles were not more than 15 feet high. So I got good reports during the day on 40 meters when I was a novice and the same on 75 meters. That antenna was a trap dipole that had the center at 10 feet above the ground and the ends were at a height of about 14 feet. It had a pair of W2AU traps and was made of lamp cord fed with RG-8 coax. I used a Heathkit AT-1 transmitter that ran about 25 watts and I was fairly pleased with things as they were. Keep in mind that this was in 1955. This antenna was also the one I used with a Central Electronics 10B SSB exciter that had about 3 watts out on 75 meters. I got decent reports all over Texas, Oklahoma, Arkansas and Louisiana. But that was as far as it went. No contacts were made any farther out, and now I know why. As a point of historical interest, I still have the W2AU traps and I think they are still useable.
Another interesting antenna situation took place several years ago when I had installed an Atlas SSB rig in my car. After I finished the installation, I went for a drive to see how it worked. I was getting 20 to 40 over 9 reports from stations in Dallas, Houston and San Antonio. Then someone pulled up beside me and pointed to the rear of the car. When I stopped I found out that the ball mount had slipped and the antenna was at a 45-degree angle to the horizon. After I got the antenna back to the vertical and tightened up, my signal had fallen off to S9 in the same places. A Lesson learned, but not realized until later after reading this book about the use of tilted whips and the effect on the signal.
IDEAL FOR SOME
I realize there are many who are interested in only local contacts out to no more than 200-400 miles for regional nets or just ragchew with the local cronies. For these purposes the NVIS is the ideal antenna system.
WHERE TO GET IT
The book may be ordered online from: http://www.wr6wr.com/products/book_booklist.html
Or via mail at : Worldradio Books. P.O. Box 1894900, Sacremento, CA 95818
antenneX ~ January 2002 Online Issue #57 ~
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