Antenna Design

APPLICATION NOTE 102

ANTENNA BASICS – Basic Antenna Design Information Importance of Antenna Design The purpose of this document is to give some knowledge as well as a guideline to the design with antennas. While the antenna is one of the most complicated aspects of RF design, it is also probably the most overlooked part of an RF design. The range and performance of an RF link are critically dependent upon the antenna. However, it is often neglected until the end of the design and expected to fit into whatever space is left, no matter how unfavorable to performance that location may be. Many of these designs will have to ultimately accept degraded performance or go through multiple redesigns. The two most popular Antenna Types a) ¼ Wave Whip: A whip antenna provides exceptional overall performance and stability, has an isotropic pattern, a wide bandwidth, it is cheap and it is easily designed. Since a full-wave or even a half-wave dipole whip is generally quite long, most whips are 1/4 wave. Note: If one branch of the dipole antenna is replaced by an infinitely (enough) large ground plane, due to the effect of mirroring, the radiation pattern above the ground plane remains unaffected and delivers practically quite the same performance of a whole half-wave dipole. This simple and often the most effective antenna is also called a quarter-wave mono-pole and is the most common antenna on today’s portable devices. Since most devices have a circuit board anyway, using it for half of the antenna can make a lot of sense. Generally, this half of the antenna will be connected to ground and the transmitter or receiver will reference it accordingly.

b) Helical: A helical element is a wire coil usually wound from copper, brass or steel. Compared to the monopole, which is essentially a two-dimensional structure, the helical antenna is a 3dimensional-structure but is nothing else as a “shorter quarter wave”. Its radiation pattern is similar in nature to the monopole. This provides an optimum condition for portable communications. A small helical significantly reduces the physical size of the antenna; however this reduction is not without a price. Because a helical has a higher Q factor, its bandwidth is narrower and its ideal gain is as a matter of principle lower than a “full size” quarterwave whip. In many cases, the helical antenna will perform as well as the elongated ¼ wave antenna. The distributed capacity of the helical ¼ wave antenna acts as an impedance matching section that is not present in the full size ¼ wave antenna and minimizes the effect of the underground.

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Subject to modifications | Christian Bach | September 2008 | Page 1/ 3

APPLICATION NOTE 102

ANTENNA BASICS

What does the Ground Plane have to do with the antenna? All antennas are resonant RLC networks and like any electronic component, have at least two connection points. Quarter wave antennas are, in opposite with ½ antennas all ground dependent, that is, they must have a ground plane to work against. A quarter wave antenna and ground plane combine to form a complete resonant circuit at the op-erational frequency. Since this plane is the other half of the antenna, its size and prox-imity are essential. Often an antenna can appear smaller than its specified wave length. This is due to internal mechanical tricks, such as helical windings, that can dramatically reduce the antenna’s physical size. This does not mean that the same size is appropriate for the ground plane. A compromised ground plane can affect antenna stability, per-formance and operational frequency. Please note that the performance of such antennas is critically dependant upon the counterpoise used as the other half of the antenna. This counterpoise can be a solid copper fill on a circuit board or a metal enclosure. Since the RF stage is referenced to the circuit ground, this plane or the enclosure are also connected to ground. The size and shape of the ground plane counterpoise as well as its location with reference to the antenna will have a significant impact upon its performance. Typically, antennas are designed on a counterpoise that is one wavelength in radius. At one wavelength, the counterpoise will act sufficiently like an infinite plane. This begs the question “what happens when the ground plane is reduced to something that is practical for a portable product?” The answer is “quite a bit.” Generally, if the radius of the counterpoise is longer than one wavelength, the performance is close to that of an infinite counterpoise. If the radius is shorter than one wavelength, the radiation pattern and in-put impedance are compromised. Significant performance reductions occur however when this radius is under a quarter-wave. A common pitfall is also the implementation of the ground plane. As stated earlier, the ground plane is the other half of the antenna, so it is critical to the final performance of the product. This means that it is critical to get it right. If the ground plane is either too small, cut up with traces, through-hole components or Vias, then it is not going to be able to do its job as an antenna counterpoise. Another important practical aspect is, that since in the most practical applications it is not possible to integrate an straight 1/4 wave long whip ideal antenna with ground plane into a housing, the performance of an even correct selected and matched “standalone” antenna will be critically affected upon its later custom positioning, shape and housing. Conclusions: 1. Any piece of conducting material is also an antenna. The antenna efficiency is a critical component to a system’s performance and should be considered early in the design process. It should be recognized that data sheet and specifications of a “standalone” antenna will not necessarily reflect its performance in the final product. This is most a result of design-specific factors, such as those presented here, as well as differing references, methods of test, and specification formats among antenna suppliers. 2. An important factor when designing an antenna is also his gain, shape, placement and orientation. Propagation within buildings is influenced by the following factors: attenuation due walls, reflection and diffraction from walls, ceiling, floor, etc. With this in mind and because mostly the transmitter has to be mounted at a pre determinated location (like a wireless light switch on the wall for example), it is the best to mount for example

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APPLICATION NOTE 102

ANTENNA BASICS

the receiver antenna higher than any obstacle between the transmitter and receiver. Any metal surfaces that fully or partially surround the antenna will disturb the radiation pattern. 3. Quarter wavelength whips are good, but should be used only if the final product has an enough large ground plane and place for the antenna. Small housings and ground planes makes 1/4 wavelength monopole antennas unstable, and any object close to the an-tenna changes its performance. The best quarter wavelength whip is almost useless if it is not straight up or if any object touches it. 4. Other antennas, such as helical or even a 1/8 wavelength monopole may offer a much better overall performance in this case. A 1/8 wavelength antenna for example has approximately half the size of a 1/4 wavelength antenna, which makes it attractive for small designs where it can be easier implemented. Because of the reduced dimensions their efficiency is lower, but it is not as sensitive to proximity effects as a quarter wavelength whip. However their design is not so easy, since they have a dominant reactive element in their impedance that needs to be taken into account in the matching network. A very small antenna cannot be efficient and tolerance-insensitive at the same time. 5. As a general physical rule, the antenna’s efficiency is directly proportional to its volume, while the length of an antenna is directly related to the wavelength. 6. For example, a stretched quarter wavelength whip will be basically better as any small helical, while in a small housing a well designed helical can achieve a better performance as a poor shaped whip. A well matched helical can achieve an excellent overall perform-ance while maintaining a very compact size. The helical is therefore very popular, since well designed it can provide excellent range at very small size. 7. Common problems with antennas usually involve insufficient free space around the antenna. The antenna should not run close to ground plane, shield or any other trace. This includes also traces on the other side of the board, batteries or any other near metallic object.

Disclaimer The information provided in this document describes typical features of the EnOcean radio system and should not be misunderstood as specified operating characteristics. No liability is assumed for errors and / or omissions. We reserve the right to make changes without prior notice. For the latest documentation visit the EnOcean website at www.enocean.com.

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Subject to modifications | Christian Bach | September 2008 | Page 3/ 3