AN2731 Freescale Semiconductor / Motorola, AN2731 Datasheet - Page 7

• Manufacturer Part Number: AN2731
• Description: Compact, Integrated Antennas: Designs and Applications for the MC13191 and MC13192
• Manufacturer: Freescale Semiconductor / Motorola

Impedance Matching
Figure 5. The Folded Dipole
All the different dipole types, loading techniques, and feeding networks total up to an enormous amount
of possible combinations, each with its own advantages and disadvantages. Selection of the correct design
for your application is best found using case-by-case assessment.
4.1
Monopole Antennas
If one part of a dipole antenna is removed and replaced by an infinite ground plane, the remaining half of
the dipole “mirrors” itself in the ground plane, much in the same way that one sees their own reflection in
water.
For all practical purposes, the monopole behaves as a “half” dipole. That is, it has the same doughnut
shaped radiation pattern, the radiation resistance is half that of the dipole (37 Ohm), it can be bend and be
folded like the dipole, and the same loading and feeding techniques can be applied.
However, one very important difference remains in that the antenna feed point is not balanced, but single
ended. Because of this and because most RF circuits are of the unbalanced type, this antenna type has been
immensely popular and a lot of variations of the monopole theme exist, most designed to match 50 Ohms.
Figure 6. Monopole Above a Ground Plane, Showing the “Mirror” Antenna
It is important to note that the “whip” is only half the antenna and that the remainder is made up of the
ground plane, or counter weight, as it is sometimes called. In a practical application, the ground plane is
often made up of the remainder of the PCB (ground and supply planes, traces, and components).
Compact Integrated Antennas, Rev. 1.2
Freescale Semiconductor
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