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Improving Radio Performance in Wireless Applications Using Slot Antennas

Fri, 06/26/2009 - 6:54am
The slot antenna's main advantages are its design simplicity, robustness, and convenient adaptation to mass production using printed circuit boards.
By Gerardo Gonzalez, Pinyon Technologies, Inc.

Improving Radio Performance in Wireless Applications Using Slot Antennas
Figure 1a. and 1b. Examples of Pinyon's patented slot antennas.
The explosion of wireless technologies has spurred the improvement of most of the analog radio components from baluns to power amplifiers. Antennas are also part of the improvement of the radio portion of any system. The antennas chosen for data wireless standards such as WiFi, UWB, Bluetooth, Zigbee and other WLAN and WPAN technologies are variations of dipoles and F or inverted F antennas whether on ceramic or other materials. However, one antenna that is seldom used in the WLAN applications, but offers some compelling advantages is the slot antenna.

Companies such as Pinyon Technologies offer a patented slot antenna that overcomes most of the limitations of the old slot antenna designs. The military has used slot antenna for years. The slot antenna's main advantages are its design simplicity, robustness, and convenient adaptation to mass production using printed circuit boards. The size of the military slot antenna has always been a challenge for system designers. They can be large, but the control of the radiation pattern and high gain make them ideal for military applications.
Slot Antenna Applications
Using this type of slot antenna and integrating it onto the system board is an option that results in taking control of the systems performance. The company's proprietary, patented high gain antenna has a manageable small size for integration onto the system board. One version of the antenna is built on plain FR4 material on a two layer PCB. It is easily integrated on any systems board starting at two layers and going up from there. As an example, Pinyon's 2.4 GHz antenna measures 29 x 47 mm yet has a gain of 3.2 dBi. No special tolerances are required to manufacture it. The simplicity of the layout and the robust design make this antenna suited for integration on the system board.
Elements to Consider
When choosing an antenna, it is important to select for the following properties:

• Maximize gain
• Maximize polarization diversity
• Minimize PCB parasitic signals
• Manufacturability.
Antenna Technologies Differ
One difference in Pinyon's slot technology is that it is current driven versus the more common voltage driven antennas. The antennas good efficiency accounts for its high gain despite its implementation on FR4, a notoriously high loss material. This antenna has a double spherical radiation pattern with excellent directivity, giving the antenna a much stronger signal in the horizontal polarization as well the more common vertical polarization. Dipoles are incapable of detecting any horizontally polarized signals. Rejecting any horizontally polarized multipath signals puts the systems at a significant disadvantage.
Slot Antennas Offer Some Unique Advantages
The advent of 802.11n was specially targeted to detect multipath signals with spatial diversity. By using the Pinyon antenna, the system now also has polarization diversity. One more beneficial property of this type of antenna is how it can reject noise outside the specified operating bandwidth. It functions as a band pass filter.

When implementing the Pinyon antenna for example, some design considerations are prudent to maximize gain by taking care of the layout and making sure enclosures don't block the signal. The antenna requires a ground plane and a top layer for the slot. Components placed around the ground plane of the slot are no problem. On the ground plane side, no component placement is recommended to maintain noise isolation. It is imperative to ensure that the enclosure contains no RF blocking material such as metal based paints or any embedded metal on the plastic material.

Polarization diversity is covered by the same careful layout taken to maximize the gain of the antenna. The system now can detect horizontal multipath as well as the vertical multipath. All the constructive horizontal multipath signals ignored by dipoles are now detected by a system using the Pinyon antenna.

By isolating the ground side of the antenna, any parasitic signals from the printed circuit board are minimized. Since the Pinyon antenna naturally acts as a band pass filter, external noise is also minimized without exaggerated filtering.

Tolerances for implementing on board antennas can deter an OEM from using them on their main board. The beauty of the Pinyon antenna is that manufacturing tolerances are completely compatible with standard manufacturing processes used for FR4. No special tolerances are needed. The Pinyon antenna does not need any tuning or calibration. This characteristic minimizes yield losses due to the antenna on the manufacturing and assembly line.
Meeting the Required Specs
Let's take a look at how 2.4 GHz model Pinyon antenna meets the properties above. It maximizes gain. Its 3.2 GHz gain is much higher than the unity gain for antenna common to antennas placed directly on the printed circuit board.

It maximizes polarization diversity. It has an equal balance in detecting horizontally polarized signals as it detects the vertically polarized signals. Typical F or Inverted F antennas show a 10 dB down in signal between vertical and horizontal polarizations. For the Pinyon antenna, the gain is the same for both the horizontal and vertical polarization.

How about the property minimizing printed circuit board parasitic signals? The 2.4 GHZ antenna operates from 2400 to 2484 MHz when measuring frequencies below or above that band of frequencies are filtered.

The 2.4 GHz antenna meets the last property of manufacturability. A good indicator of the robustness of the design is shown by the different thickness that two layer FR4 printed circuit boards offer. The 2.4 GHz antennas, built on a .032 inch thickness FR4 printed circuit board, has the exact same performance as the antenna built on .064 inch thickness FR4 printed circuit board. There is no change in performance between the two different thicknesses.
Why Integrate the Antenna?
Why would a system designer want to integrate an antenna on the printed circuit board? It is a way to control the RF performance of the system. Most system designers leave the antenna choice to their customers by outfitting their system with external antenna ports. Does it make sense to risk the performance testing of your system up to a low performing dipole antenna? Is it worth leaving the fate of the systems performance to the vagaries of the low quality of dipole antennas? By integrating the antenna, the designer asserts more control of the system performance and maximizes the probability that the system user will have a better experience with the RF portion of the system. Eliminating the quality variability of antennas on the market allows the OEM to maximize and control the performance of the system.

Using the Pinyon antenna is a way for the OEM to control the system RF performance. Leaving the end customer the option of using an antenna of their choosing puts the systems performance, and possibly the OEM's brand reputation, in jeopardy. The system performance might be degraded when subpar antennas are used.

Take control of the end customer's perception and integrate the antenna directly onto the main board of the system. Choose an antenna that will maximize gain, provide polarization diversity and manufacturability.

Gerardo Gonzalez is the managing director of Sales for Pinyon Technologies, Inc., www.pinyontech.com, 775-786-6003.

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