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Antenna designers are always looking for creative ways to improve performance. One method used in
patch antenna design is to introduce shorting pins (from the patch to the ground plane) at various
locations. To illustrate how this may help, two instances will be illustrated.
Quarter-Wavelength Patch
A quarter-wavelength patch shorted at the far end is shown Figure 1.
Figure 1. Quarter-wavelength patch with shorting pin at end.
Because the patch is shorted at the end, the current at the end of the patch is no longer forced to be
zero. As a result, this antenna actually has the same current-voltage distribution as a
half-wave patch antenna. However,
the fringing fields which are responsible for radiation are shorted on the far end, so only the
fields nearest the transmission line radiate. Consequently, the gain is reduced, but the patch
antenna maintains the same basic properties as a half-wavelength patch, but is reduced in size 50%.
Shorting Pin At the Feed to a Patch
A shorting pin can also be used at the feed to a patch antenna, as shown in Figure 2.
Figure 2. Half-wavelength patch with shorting pin at the feed.
You may be tempted to think that the shorting pin would zero out any power delivered to
the antenna.
However, because patches are high frequency devices (typically used at >1 GHz), the shorting pin actually
introduces a parallel inductance to the antenna impedance. The equivalent circuit of the above
antenna is shown in Figure 3. The antenna impedance is given by ZA, and the shorting
pin introduces a reactance equal to jX.
Figure 3. Equivalent Circuit of antenna in Figure 2.
The affect of the parallel inductance shifts the resonant frequency of the antenna. In particular, the
two components in parallel would result in their admittances (Y=1/Z) adding. Hence, the admittance
of the patch has a 1/(jX) added to it. In this manner, the resonant frequency can be altered.
In addition, the shorting pin can become capacitive if instead of extending all the way to the ground
plane, it is left floating a small amount above. This introduces another design parameter to
optimize performance.
Planar Inverted F-Antenna (PIFA)
The PIFA antenna is increasingly used in the mobile phone market. This antenna resembles
an inverted F, which explains the name. It is popular because it has a low profile and
an omnidirectional pattern. The antenna is shown in Figure 4.
Figure 4. PIFA Antenna.
The PIFA is resonant at a quarter-wavelength, due to the shorting pin at the end. The feed
is placed between the open and shorted end, and the position controls the input impedance.
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