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, the quarter-wavelength Patch Antenna, which leads into the Planar Inverted-F Antenna (PIFA).

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 antenna 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 antenna 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 Antenna

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 gt;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

The Planar Inverted-F antenna (PIFA) is increasingly used in the mobile phone market. The antenna is resonant at a quarter-wavelength (thus reducing the required space needed on the phone), and also typically has good SAR properties. This antenna resembles an inverted F, which explains the PIFA name. The Planar Inverted-F Antenna is popular because it has a low profile and an omnidirectional pattern. The PIFA is shown from a side view in Figure 4.

Figure 4. The Planar Inverted-F Antenna (PIFA).

The PIFA is resonant at a quarter-wavelength due to the shorting pin at the end. Well see how the resonant length is defined exactly in a minute. The feed is placed between the open and shorted end, and the position controls the input impedance.

In PIFAs, the shorting pin can be a plate, as shown in Figure 5:

Figure 5. The Planar Inverted-F Antenna (PIFA), with a shorting Plane.

In Figure 5, we have a PIFA of length L1, of width L2. The shorting pin (or shorting post) is of width W, and begins at one edge of the PIFA as shown in Figure 5. The feed point is along the same edge as shown. The feed is a distance D from the shorting pin. The PIFA is at a height h from the ground plane. The PIFA sits on top of a dielectric with permittivity as with the patch antenna.

The impedance of the PIFA can be controlled via the distance of the feed to the short pin (D). The closer the feed is to the shorting pin, the impedance will decrease; the impedance can be increased by moving it farther from the short edge. The PIFA can have its impedance tuned with this parameter.

The resonant frequency of the PIFA depends on W. If W=L2, then the shorting pin runs the entire width of the patch. In this case, the PIFA is resonant (has maximum radiation efficiency) when:

Suppose that W=0, so that the short is just a pin (or assume W lt;lt; L2). Then the PIFA is resonant at:

Why does the resonant length of the PIFA depend on the shorting pin length W? Intuitively, think about how a quarter-wavelength patch antenna radiates. It needs a quarter-wavelength of space between the edge and the shorting area. If W=L2, then the distance from one edge to the short is simply L1, which gives us Equation [1].

What about when W=0? Since it is the fringing fields along the edge that give rise to radiation in microstrip antennas, we see that the length from the open-circuited radiating edge (the far edge in Figure 5) to the shorting pin is on average equal to L1 L2. You can convince yourself of this by measuring the distance from any point on the far edge of the PIFA to the shorting pin. The clockwise and counter-clockwise paths always add up to 2*(L1 L2), so on average, resonance will occur when the path length (L1 L2) for a single path is a quarter-waveleng

剩余内容已隐藏，支付完成后下载完整资料

PIFA天线

天线的设计者总是通过灵活的方式来改善天线的性能。在贴片天线的设计中，常会在不同的位置引入短路端（介于地和贴片之间）。以下两个四分之一波长贴片天线的例子可以说明短路端（使天线成为平面倒F天线）如何提升天线性能。

四分之一波长贴片天线

如图一所示，四分之一波长天线在远端短路了。

图1.终端有短路针的四分之一波长的贴片天线

由于贴片在末端短路，贴片天线末端的电流不再必须为0A。这使得这种天线实际上和半波贴片天线有这相同的电压和电流分布。但是，产生辐射的边缘场在远端被短路，因此只有在接近传输线的场可以产生辐射。最终，虽然天线增益下降，但贴片天线保持了和半波天线相同的性能，同时尺寸缩小了50%。

在馈电点短路的贴片天线

如图二所示，短路针也会用在贴片天线的馈电点处。

图2.馈电点处有短路针的半波长贴片天线

你也许会想，短路针会使能量传输不到天线上。但是，因为天线是高频器件（通常工作频率大于1GHz），短路针实际上相当于向天线引入一个并联电感。天线上的等效电路如图3所示。ZA为天线的电阻，短路针引入的等效电抗为jX。

图3.图2天线的等效电路

并联电抗提升了天线的共振频率，尤其是两个器件并联会导致它们的导纳增加。因此贴片天线的导纳会增加一个1/(jX)部分。用这种方式，谐振频率会发生改变。

除此之外，如果短路针上有少量电流流过，而不是完全接地，短路针会变成容性。这引入了另一种优化天线的设计参数。

平面倒f（PIFA）天线

平面倒F（PIFA）天线正越来越多的运用于手机市场。这种天线谐振于四分之一波长（从而减小了在手机上所需的空间），同时也具有较好的电磁波吸收比值。由于这种天线类似于倒下的F，所以称为倒F天线。由于平面倒F天线具有较低的外形和全方位模式，所以很受欢迎。倒F天线的侧视图如图四所示。

图4.平面倒F（PIFA）天线

由于在末端有短路针，PIFA天线谐振于四分之一波长。我们将会看到如何在一分钟呢确定谐振波长。馈电点置于开路和短路端之间，并且其位置决定了输入阻抗。

在PIFA天线中，短路针可以是一个平面，如图5所示

图5.带有短路面的平面倒F天线.

在图5中，贴片长L1,宽L2。短路针宽W，如图5所示在PIFA天线的边缘。馈电点如图所示在同一边，与短路针相距D。PIFA天线距离地面h。PIFA天线置于相对介电常数为的材料上。

PIFA天线的输入阻抗可以通过调节短路针和馈电点之间的距离D来控制。馈电点距离短路针越近，阻抗减小；馈电点与短路端越远，阻抗增大。可以调节PIFA天线这一参数来改变它的阻抗。

PIFA天线的谐振频率取决于W。如果W=L2，并且短路针有整个贴片的宽度，在这种情况下，PIFA谐振（有最大辐射频率）：

[式1]

假设W=0，即短路端只是一个针（或是Wlt;lt;L2）,这时，PIFA谐振于

：[式2]

为什么PIFA天线的谐振波长与短路针W有关呢？直观地说，这是由四分之一波长的贴片天线辐射方式决定的。它需要在边缘和短路端存在四分之一波长的空间。如果W=L2，那么从边缘到短路端的距离则是L1，即等式1所示。

那么当W=0时呢?因为是边缘的边缘场提高了微带天线的辐射，我们把开路端（图5中的远端）到短路端的长度平均看成L1 L2。你可以测量从PIFA远边的任意一点到短路针的距离。顺时针的路径加上逆时针的路径总是2*（L1 L2），所以平均下来，当路径长度L1 L2是四分之一波长时谐振。

总之，我们可以大致通过这些参数推算PIFA的谐振波长：

[式3]

更具体地说，假设L1=0.1米，L2=0.05米，W=0.02m，=4.那么谐振频率是多少？可以通过等式4计算出

[式4]

在等式4中，考虑到我们用了天线基础方程之一，相应的波长波速和频率如下

[式5]

PIFA天线的电容性负载

假设我们想要进一步减小PIFA天线的长度，我们可以做什么呢？通常是在PIFA天线中运用电容性负载。在这一技术中，我们向PIFA天线的馈电点和开路端之间增加电容，如图六所示

图6. PIFA天线的容性负载.

为什么这可以达到效果呢？如图6，在馈电点的右边，有一个接地的短路电路。只要考虑阻抗，有一小部分波长的短路电路可以被看做一个并联电感。相似地，图6中开路电路和馈电点左边的臂可以被看做是电容。从馈电点到短路针的距离或者开路端的距离分别决定了PIFA天线的电感和电容。在某种意义上，要想抵消电容和电感，这些长度需要知道。

因此，如果我们要缩小PIFA天线的尺寸，如图6，我们要减小馈电点左端的电容。为了补偿这点，我们增加了一个平行电容，然后从阻抗的角度，其他部分匹配，并且PIFA能够辐射。

这一技术可以成功奏效，但同时要小心：这一技术也减小了谐振频率，同时PIFA的带宽也减小了。不能只减小尺寸，用电容取代它而指望其他部分不变。总之，天线工程追求一个平衡，你不可能不牺牲一些参数而提高另一方面的性能。

现实中的PIFA天线

三星galaxy s是美国的一款工作于CDMA网络下的智能手机。这意味着它的工作频率会在850ＭＨｚ到１９００ＭＨＺ之间，需要一根收发天线和一根只接受天线。这款手机的ＦＣＣ图如下

图7.三星Galaxy S上的天线形式和位置.

如图７所示，这款手机有６根天线。蜂窝数据收发天线在底部的蓝色方框中，多用蜂窝数据天线在左上方区域。ＧＰＳ天线在顶部，ＷＩＦＩ天线在右下部分的绿色框中。这款手机还有ＷｉＭａｘ天线，一个作为收发天线，另一个作为多用天线。

这些天线都是ＰＩＦＡ天线。一个大的板子支撑电路板和触摸屏，同时也是所有天线的地。需要注意的是，即使ＦＣＣ把一些特别的区域标记为天线，但是是整块地面构成了天线。这意味着，如果你切去地板，那么手机将不能很好地在８５０ＭＨｚ时辐射。

剩余内容已隐藏，支付完成后下载完整资料

资料编号：[29082]，资料为PDF文档或Word文档，PDF文档可免费转换为Word