Tuesday, July 6, 2010


Although a little known fact, experiments in the microwave spectrum date to the very early days of radio pioneering. A number of Heinrich Hertz's early experiments with "Hertzian waves" during the late 1800s were at wavelengths which translate to frequencies of between 400 and 800 MHz. Guglielmo Marconi's early European experiments in radio utilized simple spark-gap equipment with small coils; the accompanying receiver also used basic "hooks" of wire. Translating the physical dimensions of this primitive gear to its corresponding wavelength and frequency yields an rf spectrum of approximately 1.5 to 3.0 GHz. Microwave communications have, indeed, been with us since the early days of radio activity.

Continuing toward our present period of time, we find a somewhat crude version of the magnetron tube developed during the mid-1920s. This unique tube used a strong magnetic field, created by large magnets surrounding the device, to deflect electrons from their natural path and thus establish oscillation in the microwave range. Because specific technology wasn't yet available for putting the device to use, however, the magnetron laid (basically) dormant for several additional years.

The European continent was also reflecting significant pioneering efforts in the microwave spectrum. A 2-GHzlink was operated across the English Channel during the mid 1930s. During the 1940s, the cavity magnetron was devised and placed into use with the first RADAR (RAdio Detecting And Ranging) systems.

Ensuing evolutions during subsequent decades produced the klystron, reflex klystron, the traveling-wave tube, the Gunn diode, and the recent GaAsFET transistors. The difficulties in developing these microwave devices revolve primarily around electron transit time for each cycle of wave propagation. Stated in the simplest of terms, electrons leaving the cathode of a tube (and traveling toward that tube's plate), must transit a path shorter than one-half wavelength. This situation is not of consequence in low-frequency devices; however, an alteration of design is required for microwave operations. The lighthouse tube (by General Electric) , and acorn tubes were introduced to fulfillthis need. By directing electron flow in more direct patterns while reducing stray and interelectrode capacitance, these devices allowedmicrowave operations at frequency ranges that were previously not feasible. As knowledge expanded , higher and higher frequencies became practical. The restrictions of stray capacitances and transit times were overcome, and "tuned circuits, " such as they are for these extremely-high frequencies, were incorporated directly into the new devices.