Quadrifilar Helix Antenna Tuning Using Pin Diodes.
DEPARTMENT OF THE NAVY WASHINGTON DC
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An antenna is tuned in separate discrete frequency bands by changing the electrical length of the antenna. PIN diodes are placed at predetermined locations on the antenna coaxial cable radiating elements. When it is desired to shorten the antenna for a higher frequency band use, the diodes are biased short circuiting segments of the antenna. When the lower frequency band use is desire d, diodes are unbiased so that the diodes act like a very small capacitance shunted by a large resistance which is essentially an open circuit permitting the entire length of the antenna to operate.
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