[Oremem/Idamem] Fw: NVIS Techniques

[Virginia Franklin]

Hello Bob,
----- Original Message ----- 
From: Barry Landson 
To: 'Virginia Franklin' 
Sent: Friday, September 10, 2004 8:52 PM
Subject: NVIS Techniques


Hello Bob,

This is the e-mail on NVIS for retransmission to Oregon MARS.

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            NVIS Summary

                        Near Vertical Incident Shy wave Communication (NVIS) was apparently developed by the German Armed Forces and used frequently in World War II. It was picked up (or separately developed) by the Soviet Union forces and used extensively at least after World War Two. United States involved seemed to date to the Vietnam War although I am sure it was used earlier, but I haven't found a reference for Korea.



            The purpose of the technique is to use HF for close in communications out to about 400-500 miles although some references report longer distances out to 800 miles. The physical principle involved is to transmit RF energy nearly vertical and reflect that energy off the appropriate ionosphere layers for retransmission directly back to earth. The result is an omni directional circle of communications with a radius of 400-500 miles.



            The result of this capability is continuous coverage of a large area independent of terrain. In that sense, it is superior to VHF and above which is terrain sensitive. On the other hand, one has to contend with the normal problems of noise, etc. in HF communications. The technique can be achieved with very simple antennas including mobile antennas and, in some cases; merely laying an antenna on the ground can be quite effective. This is the reason this technique has been so widely adopted for military tactical operations.



            The technique can also be very effective in State MARS operations and is used in Northern California MARS from whence I came.



            The best reference that I have found dealing with this subject is listed below. Reading the book was quite informative, however the bottom line is the best simple antenna is a dipole cut for frequency mounted .1 to .25 wavelengths above RF ground. RF ground can be the earth's surface in the case of saltwater, but is typically below the surface for other types of material. The variability of the earth's composition can be reduced by placing a single counterpoise below the dipole antenna. This is another wire about 5-10% longer than the dipole placed underneath.



            In my case I use a B&W folded dipole which, depending on length, can span the HF spectrum from 1.6-30 MHz. Therefore it is very broad banded but also must less efficient than a single dipole cut to frequency. In fact the B&W is about half as efficient as the dipole. Hence I use an amplifier.



            As a concluding note, advanced digital techniques can largely overcome many problems with HF use. Hence NVIS techniques combined with advanced digital techniques can usually assure high quality communications in a relatively large area. It will be interesting to see how digital voice comes along in this regard.





            The reference information is as follows.



            Fiedler, David M. LTC (NJ ARNG) Ret.

            Farmer, Edward J. P.E. (CA SMR)



            Near Vertical Incidence Sky wave Communication

            (Theory, Techniques and Validation)



            World Radio Books

            P.O. Box 189490

            Sacramento, California 95818



            The price is $14 plus shipping. Please note that that there is a web site at www.wr6wr.com for ordering. Click on products and then books.





            That is all for now.



            Barry

            AA7ZC/AARØZC