Short note: Probe coupling against loop coupling

Energy is fed into or taken out of a waveguide or cavity resonator with probe (electric) coupling or loop (magnetic) coupling. A coaxial line is usually used to launch the wanted mode.

Probe coupling

• The inner conductor of the coaxial line is extended into the guide as a short antenna (probe) of about λ/4 length. It goes through the broad wall.
• The probe is placed where the E-field is maximum, parallel to the E-lines. For TE₁₀ this is the centre of the broad wall, about λg/4 from the shorted end.
• It works like a monopole antenna. Coupling is varied by changing the probe depth or position.

Loop coupling

• The inner conductor forms a small loop whose end is joined to the guide wall.
• The loop is placed where the H-field is maximum (for example near the shorted end wall), with the plane of the loop perpendicular to the H-lines so that the most magnetic flux links it.
• Coupling is varied by rotating the loop: rotating it by 90° reduces coupling to almost zero.

 Probe (E-field)        Loop (H-field)
   coax                    coax
    ||                      ||
 ===||=========          ===||====|
    |  <- probe             (_)   | <- short
 E-lines ||||           loop near short wall

Both can also excite higher modes when placed at those modes' field maxima. A third method is aperture (slot) coupling through a hole in the common wall.