The radiation from the horn antenna is transmitted in the direction of the
glass tube filled with neon gas. It is reflected from a metal baffle at
the right and interferes with the incident radiation to produce standing
waves in the tube. At the maximum electric field regions the gas ionizes
and glows. The distance between glowing regions is half a wavelength.
Hence the distance delineated by the dividers is one wavelength, the
distance marked by two intervals between lighted neon.
Left click on photo to see it in its actual size.