Quarterwavetransformaattori

Quarterwavetransformaattori, known in English as the quarter-wave transformer, is a short section of transmission line of electrical length one-quarter of a wavelength (λ/4) used to match impedances between a source and a load at a specific frequency. The design uses a line with characteristic impedance Z0 chosen to transform the load ZL into the source impedance ZS.

For a lossless line of length λ/4, the input impedance is Z_in = Z0^2 / Z_L, so to achieve Z_in = Z_S, choose Z0 = sqrt(Z_S Z_L). This yields an exact match at the design frequency. A numerical example: if Z_S = 50 Ω and Z_L = 100 Ω, Z0 ≈ 70.7 Ω, and a λ/4 line of that impedance will present 50 Ω to the source.

Limitations: The transformation is highly frequency dependent and most effective near the center frequency; small deviations in length or impedance can degrade matching. Real cables introduce losses and dispersion, reducing performance. For wider bandwidth, designers use multi-section quarter-wave transformers or other matching networks such as LC networks or baluns.

Implementation notes: Quarter-wave transformers can be realized with coaxial lines, microstrip, or stripline and are common in RF front-ends, antenna interfaces, receivers, and transmitters. They are typically used at a center frequency where the line length equals one quarter of the wavelength in the chosen medium.