HF Theory

Text version: A radio wave's frequency and wavelength are locked together: wavelength in feet is roughly 984 divided by frequency in MHz. At 2 MHz one wavelength is about 492 feet; at 30 MHz it shrinks to about 33 feet. Using the calculator's WF-16 factor, the half-wave wire cut is about 234 feet at 2 MHz and 15.6 feet at 30 MHz. The Frequency slider updates the wavelength and WF-16 half-wave wire readouts so you can see this trade directly: higher frequency, shorter wavelength, shorter antenna.

Text version: No antenna radiates equally in every direction. A half-wave dipole sends most of its energy broadside (at right angles to the wire) and almost none off the wire ends — the top-down plot looks like a figure eight. A vertical whip radiates evenly in all compass directions but has a dead zone straight overhead. An NVIS dipole mounted low sends energy nearly straight up so it rains back down over the surrounding region. An inverted-V falls between the dipole and vertical: mostly broadside, but with the nulls partly filled in. Practical rule: point a dipole's broadside at the station you need, and never expect contacts off the ends of the wire.

Text version: HF signals travel far because the ionosphere refracts them back to Earth. Three things set where the signal lands. Launch angle: a shallow angle (near the horizon) returns far away, up to thousands of miles; a steep angle near 90 degrees comes back close to you (this is NVIS). Frequency: low frequencies below about 5 MHz are absorbed by the daytime D layer, mid frequencies refract from the E and F layers, and frequencies that are too high punch through the ionosphere into space and never return. Between your ground-wave range and where the sky wave first lands is the skip zone — a ring where nobody can hear you at all. Steepening the launch angle is the field fix for closing that gap.

Text version: SWR (standing wave ratio) measures how well the antenna matches the feedline. At 1:1 essentially all transmitter power reaches the antenna. At 2:1 about 11 percent reflects back; that is generally acceptable. At 3:1 about 25 percent reflects and the antenna needs attention. By 5:1 nearly half the power is being reflected, and modern radios will fold back power or risk damage. High SWR does not just waste power — the reflected energy makes extra passes through the coax, multiplying feedline loss. Trim or tune the antenna until SWR is below about 2:1 at your operating frequency.

Text version: The ionosphere is built by sunlight, so it changes completely between day and night. By day, four layers exist (D, E, F1, F2). The low D layer absorbs signals below roughly 10 MHz, so the higher bands (roughly 10–30 MHz) do the work. At night the D layer disappears and F1 and F2 merge into a single high F layer: the lower bands (roughly 2–10 MHz) come alive and travel very long distances, while the highest bands often go dead. Field rule of thumb: higher frequencies by day, lower frequencies by night.

Text version: Polarization is the orientation of a radio wave's electric field, set by the orientation of the antenna: a vertical antenna transmits a vertically polarized wave, a horizontal wire a horizontally polarized one. When both stations use the same polarization, the receiving antenna captures the full signal. Ideal antennas at a 90-degree cross-polarization have zero coupling; real installations commonly see 20 dB or more loss on line-of-sight paths, so a strong signal can drop to a whisper. This matters most for ground wave and VHF/UHF work; on long ionospheric paths the polarization gets scrambled in the ionosphere, so the mismatch penalty largely disappears. For nearby stations, match your polarization to theirs.