= Slotted-line waveguide measurement
MIT's documented <slotted-line waveguide measurement> shorts one end of a <rectangular electromagnetic waveguide> and excites it through a probe coupled by <capacitance>. A second such probe moves through a longitudinal slot, aligned with the wall <electric current> to reduce disturbance. Its output locates axial <standing wave> nulls. The book's standing-wave setup chooses a <frequency> above the first <waveguide cutoff frequency> and below higher-mode cutoffs. Lowering it below the first cutoff instead reveals spatial decay away from the source. The text supplies apparatus and an observable comparison, rather than a table of measured null positions.<Electromagnetic Fields and Energy by Hermann A. Haus and James R. Melcher/Rectangular Waveguide Modes>[Haus and Melcher, Electromagnetic Fields and Energy, §13.4, Example 13.4.1, Demonstration 13.4.1 and Fig. 13.4.5; chapter PDF printed pp. 31–33]{ref}
The inference can be checked without assuming that detector output is proportional to <electric field> amplitude. At fixed measured <frequency>, record several minima at coordinates $z_0,\ldots,z_N$; then estimate $\lambda_g=2(z_N-z_0)/N$ when the minima are consecutive. This uses positions alone. Compare the result with the <guided wavelength> prediction using independently measured $a$ and $f$. The $40\,\mathrm{mm}$ spacing above is a synthetic example, not a reading from the MIT apparatus.
Additional proposed controls are to keep probe insertion fixed, repeat the scan in both directions, recheck source <frequency>, and compare shallow probe settings for shifts in the minima. These test probe loading, mechanical backlash and drift. A deliberately reflecting termination supplies the <standing wave>; varying its position should translate the pattern without changing adjacent-null spacing in a uniform single-mode region. Source-adjacent <evanescent waves>, unintended reflections, additional propagating <normal modes>, detector noise and finite positional resolution can spoil that inference. A disappearing remote signal alone does not distinguish below-cutoff decay from loss or poor excitation, and a scan at one <frequency> does not measure a pulse's <group velocity>.
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