What’s Up With NVIS

My first experience with Near Vertical Incidence Skywave (NVIS) propagation was during Hurricane Frances, in September 2004.  We were bringing two tractor-trailers full of food and supplies to an area on the east coast of Florida.  I was the EmComm Specialist, and my function was to ensure that adequate communications were available. I was new to this, and this was my second deployment.  I recently purchased an Icom AH-4 to go with my IC-706 MKIIG.  I was looking for something that could quickly be deployed and recovered while providing a reliable linkage.  I also had some Hamsticks with me.  The antenna was about 30’ of 12ga THHN wire and the counterpoise was about the same length of bare flat-braided wire. Before we reached our destination, we stopped in Jacksonville, FL to allow the hurricane to pass before we proceeded south.  That evening, I deployed the antenna to test it. This was a hurried callout and I had little warning.

That evening, I called to check into our 75-meter EmComm net. I was amazed that I sent and received a 59-signal report from an antenna that was only about 6 feet off the ground.  The distance between the two stations was about 300 miles! Back then, I didn’t know about NVIS, but I proved the concept. It works and it works well.

Today, there is a renewed interest in NVIS with Georgia ARES and Georgia AuxComm. I wanted to present a Cliff Notes version of NVIS to help others get a leg up on this form of propagation. Throughout this series, my two main references will be Near Vertical Incidence Skywave Communication, Theory, Techniques and Validation by LTC David Fieldler and Maj Edward Farmer, and FM-24-18 Tactical Single-channel Radio Communications Techniques. When I use a reference, I will abbreviate the (NVIS) or (FM24-18). Italicized text is a direct quote. What is NVIS? NVIS is skywave propagation where radio energy is radiated at or near vertical at a low enough frequency to be reflected by the ionosphere back toward the earth.  This causes an omnidirectional pattern like a fireman’s fog nozzle pointed up (FM24-18). This pattern eliminates the skip zone allowing communications within a couple of hundred miles and because the path is near vertical terrain such as mountains can be overcome.

What is needed?  NVIS needs frequencies low enough to prevent them from penetrating the ionosphere (F1/F2).  This usually means frequencies in the 2 to 4 MHZ region at night and 4 to 8 MHz during the day.  This would include the 160-, 80-, 60-, and 40-meter bands (NVIS).  Note: It is important that tactical and emergency communicators develop strategies for implementing medium frequency (160-meter) NVIS paths (NVIS). There will be more about this in the discussion about propagation. Also needed are antennas that radiate at 75 to 80 degrees or greater from horizontal and that suppress groundwave radiation.  NVIS depends on skywave radiation.  Because of the time differences between skywave and groundwave radiation reaching the receiving station, there will be some cancelation in the form of fading (QSB) which may be enough to reduce effective communication. I will have more information about antennas in an upcoming discussion. NVIS isn’t mystical, magic, or arcane – well maybe a little arcane until now.  What successful NVIS operations need is a little effort from the participants.  There must be some skill with equipment, along with some knowledge about theory such as antenna design and propagation.  What it really needs is careful planning and an adequate communications plan along with practice. I have a couple of installments planned that should give the average ham enough specialized knowledge to be able to conduct successful NVIS operations. My one caveat for this section is that there will be times when NVIS will not work.  Your communication plan should accommodate that.