NVIS BANDS

The ONLY amateur bands that work for LOCAL NVIS communications are 160m, 80m, 60m and 40m. Signals of all other bands (higher) will not refract back to earth within a radius close enough to the transmitting station to be considered “local” when their angle of incidence is near vertical. These signals will instead pass through the ionosphere and off into space.

For successful nvis operation a general rule of thumb is that 40m is used during the day and 80m is used at night. This is due to the critical frequency being higher during the day and lower at night. This rule is a good starting point but there are many factors and exceptions that must be understood to have a higher rate of success in using NVIS communication throughout the different seasons of the year (i.e. spring, summer, fall, winter), and during the changing ionospheric propagation levels throughout the 11-year sun cycle.

Out of all the bands 80m is by far the most important. It is not only used as THE nighttime band throughout most of the seasons and 11-year sun cycle but is also sometimes the ONLY option for daytime communications when the critical frequency is below 7mhz and therefore does not allow for 40m NVIS propagation.

Unlike the physical impossibly of “curing” a low critical frequency with more power, D-layer absorption can be overcome by increasing the power output of the transceiver. It is therefore possible to use 80m during the day for NVIS comms even when a strong D-layer is present.

In theory 60m would be a great option for daytime nvis comms when the critical frequency is too low for 40m but in practice the strict operational limits on 60m keeps it from being used as freely and robustly as 40m or 80m. However, it may be desirable in some situations (as long as power limits, yield to primary users are adhered to) as new regulation changes have created a new 15 kHz wide, non-channelized segment (5351.5–5366.5 kHz) with a 9.15-watt ERP limit.

As of the writing of this blog (February 2026) we are barreling towards the lower part of the sun-cycle and the critical frequency is becoming too low to use 80m for nighttime nvis comms after about 10pm and until about 6am. The only option for this time-frame (10pm-6am, winter) would be 160m. As soon as I build a 160m antenna and experiment with other stations I will post my findings.

In summary, to have the highest probability of successful nvis LOCAL day/night seasonal communication throughout the 11-year sun cycle, an antenna that can operate on 160m/80m/60m/40m is essential.