=  NOW 35 WPM  =  TEXT IS FROM JANUARY 2020  QST  PAGE 58 =

MICROWAVELENGTHS PAUL WADE, W1GHZ, W1GHZ ARRL. ORG  VISUALIZING MICROWAVES,
PART 2 WAVEGUIDE VISUALIZING MICROWAVES ON A COAXIAL LINE, AS SHOWN IN THE
OCTOBER 2019 MICROWAVELENGTHS COLUMN, CAN BE ILLUMINATING, BUT WAVEGUIDE IS
EVEN MORE INTERESTING.  WAVEGUIDE OPERATION IS FAR FROM INTUITIVE, BECAUSE
RF ENERGY TRAVELS THROUGH A HOLLOW METAL TUBE WITH NO CONDUCTORS, LEADING
SOME HAMS TO CONSIDER WAVEGUIDE MAGIC.  BECAUSE IT IS HARD TO SEE OR PROBE
THROUGH THE SOLID METAL WALLS OF A STANDARD WAVEGUIDE, I CONSTRUCTED A
WAVEGUIDE SHOWN IN FIGURE 1 USING COPPER MESH SHEET AMACOS WIREFORM METAL
MESH IN COPPER, OFTEN USED BY ARTISTS.  I LEFT A LENGTHWISE SLOT IN THE
CENTER OF THE BROAD WALL OF THE WAVEGUIDE, BECAUSE THE MAXIMUM ELECTRIC
FIELD, WHICH WE CAN DETECT AS VOLTAGE, IS IN THE CENTER SEE FIGURE 2.  IF
THE SLOT IS EXACTLY IN THE CENTER, THE VOLTAGE ON EACH SIDE OF THE SLOT IS
THE SAME AND THERE IS NO RADIATION FROM THE SLOT.  THE SLOT DOES NOT AFFECT
WAVEGUIDE OPERATION, AND WE CAN PROBE THROUGH THE SLOT WITH LITTLE
DISTURBANCE.  WE CAN SEE IN FIGURE 2 THAT THE ELECTRIC FIELD VOLTAGE AT THE
SIDES OF THE WAVEGUIDE IS ZERO, BECAUSE THE SIDE WALLS CONDUCT AND CREATE A
SHORT CIRCUIT.  THIS IS ONLY POSSIBLE IF THE WIDTH OF THE WAVEGUIDE IS MORE
THAN A HALFWAVELENGTH.  THE LOWEST FREQUENCY WHERE THIS OCCURS IS CALLED
THE CUTOFF FREQUENCY, BELOW WHICH NO ENERGY CAN TRAVEL THROUGH THE GUIDE.
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