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When the plates of a neutralizing capacitor or ry similar tuning device approach each other, danger of a voltage breakdown increases. The introduction of a strip of dielectric material between the fixed plates increases the 'capacitance without necessity for moving the plates closer together.
For example, two 1 inch square plates ¼ inch apart have a capacity in air of about 0.9 pF Introducing a polystyrene strip in the gap changes the capacitance to almost 2.5 pF. A piece of plate glass shoots the capacitance up to almost 6.3 pF The dielectric may be made adjustable or may be fixed permanently once the desired capacitance is reached. Remember, however, to choose a material that has low loss at the operating frequency.
Frank Bronin, W4WOB.