KC2G's MUF map (Maximum Usable Frequency).
KC2G's foF2 map (NVIS critical frequency).
N2LVI's Quick Guide to HF Propagation Using Solar Indices.
Propagation - eHam - propagation info and links.
Understanding Solar Indices - ARRL.
PSK Reporter - get an idea of what's going where by band and digital mode.
WSPR Rocks - alternate (better in my opinion) WSPR query / mapping site run by VK7JJ.
WSPR Live - do analysis on real time WSPR spot data.
Real Time Lightning and Thunderstorm Maps.
Ham Radio Science Citizen Investigation.
Amateur Radio Propagation - K9LA, a lot of good info on 160m, HF and VHF propagation.
Ionospheric Radio Propagation.
WWV (Colorado) and WWVH (Hawaii) transmit time signals on 2.5, 5, 10, 15, 20, and 25 MHz. The female voice time announcement is WWVH, the male voice is WWV. Sometimes you can hear both voices, one or the other depending on time of day and frequency.
Aircraft Scatter - https://w3sz.com/AircraftScatter.htm.
"Revolutionary" Cover Story in RadCom July 2022 - Article about a new solar cycle model.
NVIS (Near Vertical Incidence Skywave)
NVIS is useful at lower frequencies both during the daytime and at night for reliable local and regional communication:
- 0 to 400 miles (650 km) range.
- NVIS is most useful in areas where line of sight is ineffective due to terrain and/or distance.
- Radio waves travel near vertically towards the ionosphere where they are refracted back down with-in a circular region up to 400 miles from the transmitter.
- NVIS Frequencies are between 1.8 and 8 MHz, above 8 MHz probability decreases dropping to near zero at 30 MHz.
- Optimum frequencies tend to be higher towards the tropics and lower towards the arctic regions. They are also higher during high sunspot activity years.
- Usable frequencies change from day to night, typically 3 to 7 MHz during daylight and 1.8 to 4 MHz at night.
KC2G's foF2 map shows the critical frequency for NVIS communication. Below this frequency the F layer refracts signals directly back down in a 400 mile radius, above it the band "goes long" and local stations become weak while DX stations become stronger.
Horizontal antennas (dipoles, loops, doublets etc) between 1/10 and 1/4 wavelength high are optimal as the ground acts like a reflector producing high angle radiation, which declines slightly up to about 3/8 wavelengths then drops off. Lower heights will also work well but gain starts decrease due to ground losses. Dipole NVIS performance can be enhanced with the addition of a reflector below or a director element above for example.