Frequency and wavelength

Repetition in time and space

Frequency counts cycles per second and is measured in hertz. Wavelength is the distance between equal phase points of a travelling wave. In vacuum, the speed of light connects them: λ = c/f. A calculation using metres and seconds requires frequency in hertz.

1 MHz means one million hertz. With c = 299,792,458 m/s, 145 MHz gives a wavelength of about 2.068 m. Doubling frequency halves wavelength. A radio band’s name in metres is a convenient designation, not an exact match for every frequency within that band.

Half and quarter waves

Many antennas and matchingImpedance×Impedance is complex opposition to alternating current: it includes resistance and a frequency-dependent reactance. Antenna input impedance is therefore more than a DC resistance reading.Example & full explanation → lines use lengths near half or a quarter wavelength. These relationships are starting points for a design. A real wire has diameter, surroundings and sometimes insulation. Its physical resonant length therefore need not equal the ideal free-space fraction exactly.

The wavelength calculator accepts a factor for derived conductor lengths. With k = 0.95, for example, the ideal half wave is multiplied by 0.95. The displayed free-space wavelength itself remains unchanged. For coaxial cableCoaxial cable×Coaxial cable carries a signal using an inner conductor and a concentric outer conductor. The dielectric and construction determine characteristic impedance, losses and propagation speed.Example & full explanation → use its documented velocity factor instead; these factors have different physical origins.

Calculate, build, verify

Check units and frequency before cutting material. Allow a small tuning margin in an initial build and measure in the intended position. The calculator does not determine bandwidthBandwidth×Bandwidth is the width of a frequency range. A receiver filter limits the analysed range; the signal’s occupied bandwidth is a separate quantity.Example & full explanation →, gainGain×Gain describes the ratio of output to input level. In a receiver it can occur before or after digitisation and is often expressed in dB.Example & full explanation → or a guaranteed feed-point impedance. Those questions need further models or measurements of the actual installation.