HF stands for high frequency and it’s typically achieved with a High Frequency Ham Radio Transceiver. Other common frequencies are usually considered to be line of sight, so they’ll work as far as your eye can see…once they hit the horizon and the earth begins to curve they’re done cause they keep going out, they don’t follow the curvature of the earth. With a lot of these frequencies people will use repeaters to increase that distance. Which is great, I use them too…but that is an outside sourc