For long distance propagation, the vertical radiation angle of the energy from the antenna should be:

For long-distance high-frequency (HF) communications, signals must reflect off the ionosphere and return to Earth as far away as possible. A low vertical radiation angle, typically less than 30 degrees, ensures the signal strikes the ionosphere at a shallow angle. This shallow geometry maximizes the distance the signal travels before returning to the ground. Conversely, steep angles (above 45 or 90 degrees) cause the signal to travel nearly straight up. While this is excellent for short-range Near-Vertical Incidence Skywave (NVIS) communications, it fails for long distances because the signal either penetrates the ionosphere or returns to Earth very close to the transmitter.