Many preppers communicating within the same state will need to rely on what is known as NVIS or Near Vertical Incidence Skywave. A basic premise of radio wave propagation in the High Frequency (HF) spectrum is that radio waves must bounce off the earth’s atmosphere and return down to earth a great distance away. When radio waves hit the atmosphere at acute angles (<90 degrees), they reflect back down nearer the transmitter. When radio waves hit the atmosphere at oblique (>90 degrees), they reflect back down farther from the transmitter. However, the more acute the angle, the more likely the atmosphere is to just let them pass out into space without reflecting them. Whether or not the atmosphere reflects radio waves hitting it at acute angles depends on the frequency, with lower frequencies being more likely to be reflected and higher frequencies being more likely to pass through into space. The maximum frequency that would bounce back at a nearly vertical incidence (very acute reflection for statewide communications) is known as the NVIS Maximum Usable Frequency (MUF). Below is a map showing the current MUF for a given point on earth. This changes from day to day, hour to hour, so monitor this map so you begin to understand what frequencies you should be using a various times of day.
The following images show which frequencies would be best to use to communicate via radio from a given location by using the science described above.
Communications from San Francisco, CA
From Boulder, CO
From Minneapolis, MN
From New Orleans, LA
From Boston, MA
The following are wider coverage maps that show the best frequencies to use to communicate from a given location to oversees locations:
From Seattle, WA
From Washington, DC
