Yagi-Uda antenna
* The most common type of TV antenna
found on rooftops.
The rod lengths in a Yagi-Uda are
about a half wavelength each, and
the spacings of the elements are
about 1/3 of a wavelength.
The three types of element are
the driving elements,
the reflectors
and the directors
Only the driving element is
connected directly to the feeder;
the other elements couple to the transmitter power through the local electromagnetic
fields, which induce currents. The driving element is often a folded dipole,
which by itself would have a driving point impedance of about 300 ohms to the feeder;
this is reduced by the shunting effect of the other elements, so a typical Yagi-Uda
has driving point impedance in the range 20-90 ohms.
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