Editor’s note: This article is Part 1 of a two-part series. It will introduce designers to antenna theory, specifications, trends in antenna design, and real-world antenna examples. Part 2 will dig more into the latest antenna designs and how to specify and apply them.
The antenna is probably the most overlooked part of an RF design, yet the range, performance, and legality of an RF link are critically dependent upon it. Regardless, antenna selection is often left until the end of the design, at which point optimal antenna performance may not be achievable within the space provided.
The options at this point may be to go through multiple redesigns or to ultimately accept degraded performance. Neither option is desirable, but with a fundamental understanding of the theory and behavior of the various types of antennas, the situation can be avoided and a designer can get closer to optimal RF performance.
To that end, Part 1 of this two-part series focuses on the basic electromagnetic field principles of antennas. It then illustrates these principles with two styles of antennas that are often used in low-power handheld products: dipoles and monopole whips. These types cover a wide range of available antennas and are among the most common to be implemented incorrectly. (For more on antenna selection, see the TechZone article, “Antenna Selection Depends on Many Factors”.)
Part 2 of the series looks at dipole and monopole antennas in more detail, while also discussing some antenna design trends that are being driven by the needs of today's wearable, handheld, and compact devices. These newer antennas include chip antennas such as the Johanson Technology 2450AT18B100E, 2.4 GHz to 2.5 GHz chip RF antenna, and patch antennas such as the Antenova SRF2W012-100 DROMUS, flexible dual-band Wi-Fi antenna. It will also discuss antennas fabricated as part of the pc board itself.
It’s easy to confuse or misinterpret the meaning of antenna specifications and how to apply them. For instance, the gain of an antenna is very different from the gain of an amplifier. However, the most common misconception may be that the radiation pattern on a monopole antenna’s data sheet will be that of the antenna on the final product.
In practice, the radiation pattern for a quarter-wave monopole antenna is so critically dependent on the design and layout of the product, manufacturers’ gain specifications and radiation pattern plots have little use except to ascertain potential antenna performance. This is why it’s critically important to have a grasp of the fundamentals.
An antenna is a device that converts electric currents into electromagnetic waves and vice versa. It can be considered a complex resistive-inductive-capacitive (RLC) network. At some frequencies, it will appear as an inductive reactance, at others as a capacitive reactance. At a specific frequency, both reactances will be equal in magnitude, but opposite in influence, and thus cancel each other. At this specific frequency, the impedance is purely resistive and the antenna is said to be resonant.
Here is where the physical meets the theoretical. Resonance will occur at whole number multiples or fractions of the frequency of interest. These frequencies correspond to a wavelength. That wavelength is the required antenna length. That length is what must be incorporated into the final product, either embedded inside the enclosure or externally attached to the device.
The frequency of the electromagnetic waves is related to the wavelength by the well-known equation λ = c/f, where f is the frequency in hertz (Hz), λ is the wavelength in meters (m), and c is the speed of light (2.998 × 108 meters/second).
The equation clearly shows that the higher the frequency, the shorter the wavelength, and the smaller the antenna. For example, the wavelength for 433.92 MHz is 0.69 m (~2.3 ft.), and the wavelength for 916 MHz is 0.33 m (~1.0 ft.). 433.92 MHz is a popular frequency for remote keyless entry (RKE) systems such as car key fobs, but obviously, there is no way that a 2.3 foot antenna is going to fit into one.
Fortunately for everyone who wants to carry their keys in their pocket, there are ways to make the antenna smaller. Since resonance will occur at whole number fractions (1/2, 1/3, 1/4, etc.) of the fundamental frequency, shorter antennas can be used to send and recover the signal. As with everything in engineering, there is a trade-off. Reducing the antenna’s size will have some impact on the efficiency and impedance of the antenna, which can affect the final performance of the system.
A half-wave dipole antenna has a length that is one-half of the fundamental wavelength (Figure 1). It is broken into two quarter-wave lengths called elements. The elements are set at 180 degrees from each other and fed from the middle. This type of antenna is called a center-fed half-wave dipole and shortens the antenna length by half.
Figure 1a: The half-wave dipole antenna is widely used as it cuts the antenna size in half while providing good overall performance. (Image source: Linx Technologies)
Figure 1b: Linx Technologies’ ANT-DB1-HDP-SMA is a good example of a half-wave dipole antenna. It is center-fed, covers three frequency bands, and measures 5.2 inches across by 0.8 inches high. (Image source: Linx Technologies)
A good example of a half-wave dipole antenna is the ANT-DB1-HDP-SMA from Linx Technologies (Figure 1b). The antenna simultaneously functions at any or all three frequency bands of 824 – 960 MHz, 1.71 - 1.99 GHz, and 2.401 - 2.483 GHz. The compact, center-fed half-wave dipole is "flat", measures 5.2 inches across, 0.8 inches high, with a thickness of 0.5 inches, making it a good fit for wall mounting. Its integral 9.8 foot connecting cable facilitates this type of installation.
A method for making the dipole antenna even smaller is to use one of the quarter-wave elements of a dipole and allow the ground plane on the product’s pc board to serve as a counterpoise, creating the other quarter-wave element.
Since most devices have a circuit board, using it for half of the antenna is space efficient and can lower cost. Generally, this half of the antenna will be connected to ground and the transmitter or receiver will reference it accordingly. This style is called a quarter-wave monopole and is among the most common antenna in today’s portable devices (Figure 2).
Figure 2: The quarter-wave monopole antenna uses the circuit board or other conducting surface as its ground plane, further reducing overall size. (Image source: Linx Technologies)
Another way to reduce the size of the antenna is to coil the element. This is where the straight wire is coiled or wrapped around a non-conductive substrate to create a helical element (Figure 3). This has the advantage of shortening the apparent length, but it will also reduce the antenna’s bandwidth. Like an inductor, the tighter the coil and the higher the Q, the smaller the bandwidth. Where a straight antenna may have a bandwidth of 100 MHz, a helical may only have a bandwidth of 10 MHz. This becomes more pronounced as the frequency gets lower, since the coils typically get closer together to maintain a specific overall length.
A representative example of a helical antenna is the Linx Technologies ANT-916-CW-RH-ND, centered at 916 MHz, with a 35 MHz bandwidth spanning from 900 – 935 MHz. This quarter-wave antenna is omnidirectional, designed for outdoor use, and measures 2.00 inches long with a diameter of 0.33 inches. It comes with either SMA or Part 15 compliant RP-SMA connectors.
Figure 3: Helical antennas further shorten the antenna length, as shown by these 916 MHz (left, Linx Technologies ANT-916-C W-RH-ND) and 315 MHz (right) units, but with the consequence of greatly reduced bandwidth. This may actually be an advantage in many applications. (Image source: Linx Technologies)
If antennas are the least-understood RF component, then antenna data sheets are the least understood of all RF specifications. For instance, many designers look for radiated test data without really understanding what they are looking at or how it relates to the performance of their product. For this reason, let’s examine the most common antenna specifications.
The impedance of an antenna is the real resistance and imaginary reactance that appears at the terminals of the antenna. Because there are inductive and capacitive characteristics of an antenna, they will change with frequency. Objects that are nearby such as other antennas, the components on a circuit board, and users of the device will also affect the impedance.
An antenna will have two types of resistance associated with it. Radiation resistance converts electrical power into radiation. Ohmic resistance is loss on the antenna’s structure that converts electrical power into heat. The radiation resistance should be much higher than the ohmic resistance, though both are important to the antenna’s efficiency. Generally, the radiation resistance at the terminals of a dipole antenna in free space (isolated from anything conductive) is 73 Ω. A monopole antenna will be half of this, or 36.5 Ω.
The reactance is power that is stored in the near field of the antenna. This reactance, combined with the real resistance, makes up the antenna’s impedance. Both values are affected by objects in the near field and will vary down the antenna’s length. The specifics of this are beyond the scope of this article, but can be found in most antenna literature.
These resistance and reactance values are important because the maximum power transfer will occur when the source and load impedances match. If they are different, called a “mismatch,” then some of the power sent to the antenna will be reflected back into the load or lost as heat. This will lower the efficiency of the system, shorten the range, increase the power required for a given range, and shorten battery life. (For more on antenna matching, see the TechZone article “Antenna Matching Within an Enclosure: Theory and Principle".)
Industry convention for RF is an impedance of 50 Ω. Most IC manufacturers will have matched their products to 50 Ω, or will provide a circuit designed to match their product to a 50 Ω load. Likewise, antenna manufacturers frequently design and characterize antennas at 50 Ω.
Voltage standing wave ratio
The voltage standing wave ratio (VSWR) is a measurement of how well an antenna is matched to a source impedance, typically 50 Ω (Figure 4). It is calculated by measuring the voltage wave that is headed toward the load versus the voltage wave that is reflected back from the load. A perfect match will have a VSWR of 1:1. The higher the first number, the worse the match, and the more inefficient the system. Since a perfect match cannot ever be obtained, a benchmark for performance needs to be set.
In the case of antenna VSWR, this is usually 2:1. At this point, 88.9% of the energy sent to the antenna by the transmitter is radiated into free space and 11.1% is either reflected back into the source or lost as heat on the structure of the antenna. In the other direction, 88.9% of the energy recovered by the antenna is transferred into the receiver. (Note that since the “:1” part is always implied, many data sheets remove it and just display the first number.)
Figure 4: A typical VSWR graph shows that a properly matched antenna has a "sweet spot" of unity, or near-unity, across a finite bandwidth. It also shows that VSWR increases sharply on either side of the zone. (Image source: Linx Technologies)
VSWR is usually displayed graphically versus frequency. The lowest point on the graph is the antenna’s center frequency. The VSWR at that point denotes how close the antenna gets to 50 Ω. The space between the points where the graph crosses the specified VSWR typically defines the antenna’s bandwidth.
Measuring radiated energy
True antenna performance can only be determined by measuring the amount of energy that the antenna radiates into free space. This is difficult given all of the variables associated with radiated measurements. When the radiated power is measured around the antenna, a shape emerges called the radiation pattern (Figure 5). This is the most direct measurement of an antenna’s actual performance.
Figure 5: These examples of radiation patterns for quarter-wave monopole, half-wave dipole, and Yagi antennas, three commonly used types, show the diverse patterns of each. (Image source: Linx Technologies)
Antenna radiation patterns can take on many interesting shapes, particularly when presented graphically in their real world three-dimensional state. The adjoining diagram shows shapes typical of the most popular antenna types. For a dipole antenna, the pattern looks like a doughnut. For a monopole antenna on a ground plane, cut that doughnut in half along the edge and set it on the plane with the antenna sticking up through the middle. The Yagi’s directivity can be clearly seen, although that term and the value of these types of plots will become even more apparent as directivity, efficiency and gain are discussed.
After the radiated energy surrounding an antenna is measured, the data is often turned into a radiation pattern plot. This plot graphically presents the way in which the radio frequency energy is distributed or directed by the antenna into free space. An antenna radiation pattern plot is an important tool since it allows rapid visual assessment and comparison of antennas. The antenna’s radiated performance, and the corresponding plot, will be influenced by the test jig or product on which the antenna is mounted. This makes the comparison of plots coming from different manufacturers difficult.
In addition, the plot for a specific design will likely vary from that of a reference design. Pattern plots typically consist of a polar coordinate graph, though Cartesian coordinate graphs are also used. Polar plots are easier to visualize as they show the radiated power 360 degrees around the antenna under test (Figure 6).
Generally, a logarithmic scale is used, which allows a range of data to be conveniently shown on the same plot. Two plots are created, one along the horizontal axis and one along the vertical axis. Together, these give a three-dimensional picture of the radiation pattern.
Figure 6: Polar plots (rather than Cartesian coordinates) and a logarithmic scale are the most common ways to visualize an antenna's radiation pattern. (Image source: Linx Technologies)
An antenna’s radiation pattern and specifications related to it often need a point of comparison or reference. Most frequently, a theoretical antenna called an isotropic antenna or isotropic radiator is used for this purpose. The term “iso” means equal: “tropic” means direction. Thus, isotropic describes an antenna that radiates electromagnetic energy equally in all directions. The isotropic antenna and its perfect spherical pattern are only theoretical, but the model serves as a useful conceptual standard against which “real world” antennas and their specifications can be compared (Figure 7). Now it is time to take a closer look at some of the most important radiated specifications and what they mean.
Figure 7: The idealized isotropic radiation pattern, although not achievable in practice, is a useful starting point for assessing and comparing antenna performance. (Image source: Linx Technologies)
Efficiency, directivity and gain
There are three radiated specifications that are of primary interest: efficiency, directivity and gain. Often these terms are talked about in the context of an antenna’s transmitted signal. It is somewhat easier to visualize these concepts by thinking of radiated power, but they apply directly to the received signal as well.
Efficiency is a measurement of how much of the energy put into the antenna actually gets radiated into free space, rather than lost as heat on the antenna’s structure or reflected back into the source. The antenna’s impedance and VSWR at the center frequency play a big role in this measurement.
Directivity is a comparison of the shape of the radiation pattern of the antenna under test to a reference radiation pattern. Most commonly, the reference would be the perfect spherical pattern of the isotropic model described earlier. The units of this measurement are decibels relative to isotropic, or dBi. A dipole antenna is also sometimes used as a reference, in which case the units are stated in dBd (meaning decibels relative to dipole). A dipole has a gain of 2.15 dB over isotropic or dBi = dBd + 2.15 dB. When comparing gains, it is important to note whether the gain is being expressed in dBd or dBi, and convert appropriately.
Gain can be a confusing specification. There is a significant difference between an amplifier’s gain and an antenna’s gain. The amplifier puts energy into the system, making it an active device. An antenna cannot put energy into the system, so it is a passive device. Gain is commonly misinterpreted as an increase in output power above unity. Of course, this is impossible since the radiated power would be greater than the original power introduced to the antenna.
Directivity and gain are closely related (Figure 8). Gain is the directivity of the antenna reduced by the losses on the antenna such as dielectric, resistance, and VSWR. In other words, it is the product of directivity and efficiency. Gain is the most direct measurement of an antenna’s real performance. As such, it is one of the most important specifications.
Figure 8: Directivity and gain are two closely related antenna parameters, with the latter being a very important figure of merit. (Image source: Linx Technologies)
A simple way to understand directivity or gain is to think of a focusable light source (Figure 9). Assume the light output is constant and focused over a wide area. If the light is refocused to a spot, it appears brighter because all of the light energy is concentrated into a small area. Even though the overall light output has remained constant, the concentrated source will produce an increase in lux at the focused spot compared to the wide source. In the same way, an antenna that focuses RF energy into a narrow beam can be said to have higher directivity (at the point of focus) than an antenna that radiates equally in all directions. In other words, the higher an antenna’s directivity and the narrower the antenna’s pattern, the better its point performance will be.
Figure 9: Antenna gain is not analogous to electronics amplifier gain; instead, it means a "tightening" of the spread of the radiated beam similar to focusing a beam of light form a flashlight. (Image source: Linx Technologies)
Understanding antenna choices and tradeoffs requires distillation of a complex blend of electromagnetic field theory, physical implementation considerations, and real-world attributes. Many of these real-world attributes are the result of proximity effects, installation, and orientation.
While nearly all antennas are based on fundamental constructs such as the dipole and monopole, increasingly complex antenna designs have been devised that leverage these basic elements. Much of this progress is due to advanced computer based modeling tools which are being used to predict both theoretical characteristics and actual field performance.
While antennas are improving, designers still need to acquire an understanding of the core principles of antenna operation and implementation to fully optimize their RF connection.
Expanding on the principles outlined here, Part 2 explores the basic, widely used monopole and dipole configurations in more detail. It also discusses new antenna constructs such as the chip, patch, and pc board trace, which are well suited to portable devices such as smartphones and wearables.