AAR: NVIS 9/11/26 – 9/13/26 Class Review, by Pater

Earlier this September I was lucky enough to attend Historian’s class on NVIS HF Radio. I had been wanting to take this class for a while, particularly because this skill set seemed to be well suited to setting up a regional comms network without relying on repeaters or other remote infrastructure. Our local CERT was currently using GMRS and VHF band repeaters as our primary, alternate, and contingent means of regional radio communication, and it seemed that an NVIS station would make a good emergency option if there was ever a situation where the repeaters were no longer operational.

Near Vertical Incident Skywave, or NVIS, is a method of transmitting in HF which can be used to create regional comms in an area without having to rely on any additional infrastructure beyond your radio and the antenna you’re transmitting with. The typical configuration of a High Frequency radio station is geared towards transmitting over long distances and making contacts with far off stations. This can be advantageous for communications across a continent but can also prove unreliable for regional comms due to the dead zone that is created between the end of the groundwave transmission and the start of the skywave transmissions returning to earth, an area typically occurring in a donut shaped ring between 30 to 60 miles out from the transmitting station. An HF NVIS station is intentionally configured to transmit shorter distances, reliably propagating for a few hundred miles around the station without a skip zone. This is achieved by increasing the angle at which the RF signals impact the Ionosphere and bounces back to earth. Due to the high angle, the RF signal degrades more quickly as it passes through the signal attenuating lower layers of the ionosphere at a higher rate than a lower angle transmission would over the same distance. The higher angle transmission also ensures that the signal returns to earth much closer to the station, which allows for reliable propagation of the signal throughout the area surrounding the station.

Historian started class Friday morning with a review of the three types of wave propagation, Skywave, Groundwave and line of sight, and how they related to NVIS transmissions. The goal was for the majority of our transmission to be propagated through skywave while keeping our ground wave and line of sight signal to a minimum. This was achieved by using horizontally polarized antenna’s and low-power transmissions. Transmitting frequencies used were the upper MF band and lower HF Band, depending on the time of day and the solar cycles.

Historian then went over the layers of the Ionosphere, D, E, F1 and F2 and how we could use them to our advantage to either increase our transmissions reach or attenuate our signal and keep our transmissions within a more localized area. The major key to achieving control of our signal was through understanding how the D layer functioned in relationship to our RF signal. When ionized, the D Layer acts like a cushion or filter, lowering the noise floor by absorbing random RF static, but also attenuating our signal each time it passes through the D-Layer, causing the signal to fade more rapidly. The D layer only becomes ionized when illuminated by the sun, hence why time of day is so important when making a radio transmission. If an operator wanted to minimize the distance their transmission traveled, there would be advantages to transmitting during the day when the D Layer would absorb their signal quicker.

That evening, Historian provided us with a demonstration of this effect, tuning in more distant stations as the sun set, the D layer de-ionized and the band went long. Early the next morning, just before sunrise, we observed the exact opposite phenomenon as more distant stations to our east faded out and the band went short.

Class continued with a brief discourse on WWII radio techniques and capabilities, particularly how both the Axis and Allied forces would DF radio transmitters to find enemy ships and spies. Using these hard learned lessons as a jumping off point, Historian then went over various ways of mitigating the risk of being DF’d while transmitting. Some of these techniques were particular to NVIS and MF/HF, such as transmitting while the D-layer was ionized or orienting your antenna to use terrain features to minimize your groundwave. Other techniques were more universal, varying frequencies and comm window times, transmitting and receiving on different bands, using burst transmissions to minimize time on air, setting different comms windows for the initial transmission and the acknowledgement.

On Saturday we spent most of the day building and testing our own multi-band antennas. The antenna was an inverted V linked dipole with a choke balun at the center and multiple different cable lengths that could be connected or disconnected with gator clips at the end of each element. By connecting or disconnecting elements the antenna could be tuned to transmit on the 80, 60, or 40 meter band. Finer tuning could be done by clipping shorter lengths of wire to the end of a particular element to improve its resonance at a given frequency. It rained intermittently throughout the day, which presented some challenges, but by that evening the antennas were complete.

On Sunday Historian had the class break into two teams for the practical application segment of the class. One team was to remain onsite while the other headed about 45 miles southeast and set up a second station. Before departing, we worked out an SOI to include the time of our comm window, a PACE plan for the frequencies we would be transmitting on, and the length of time we would stay on a particular power and frequency while trying to make contact before switching. Our plan was to make a voice transmission on the 80, 60, and 40 meter bands, starting at 10 watts and then stepping down to 5 and then 1 watt of power on each band. After making contact and stepping down power, we would set the antenna at half-mast and attempt to make contact at 10 watts down to 1 watt as we had done before. Our goal in this was to see how small our RF footprint could be while still making a successful transmission.

We were able to make connections at 80 meters with our antenna at full mast. 60 meters was closed when we attempted it, but switching to 40 meters we had better luck, and our signal actually increased in clarity as we proceeded through the exercise at reduced power. The issues we had at 60 meter seemed to have been related to solar activity that was interfering with our transmission. It seems the interference was still present when we started on the 40 Meter band but was beginning to subside, which would explain why our signal seemed to gain clarity even as we reduced our transmitting power. Once we were on the air, the prac-app went smoothly and we succeeded in our goal of making contact while transmitting with only 1 watt of power. When the remote team returned we concluded the class with an AAR of the exercise, discussing what went well and where there were challenges.

All in all, the class was excellent. The synopsis above is just a brief overview of the material we covered, the class itself was much more in depth. Historian is a top-notch instructor, an absolute wealth of knowledge, and a man not to be trifled with. He did an incredible job covering a large amount of material in a very condensed time frame, while keeping the class on track and moving forward. When students required further clarification on points made in class, he was more than willing to take the time after class to answer their questions. I was very impressed with his dedication to passing on all the knowledge he could to those who were willing to humbly take the time to listen and learn.

-Pater