Understanding Antenna Specifications and Operation, Part 2

Editors’ Note: Part 1 of this two-part series looked at basic electromagnetic field and antenna theory, as well as critical performance parameters. Part 2, examines the performance characteristics of some actual antennas, including newer designs.

The antenna can contribute enormously to a system’s performance, but only if it is given due consideration early in the design process. Otherwise a designer can run into space and form factor limitations that will prevent an antenna from achieving its designed potential.

Also, while it’s critical that designers understand antenna theory and principles, theory can sometimes clash with harsh reality. For example, it should be recognized that specifications on an antenna’s data sheet might not be a clear indicator of its performance in the final product. This is a result of design-specific factors, as well as differing references, methods of test, and presentation formats among antenna suppliers.

This article will introduce popular antenna configurations such as the monopole and dipole, and discuss their performance characteristics and how to work with them in the real world. It will also discuss newer antenna designs such as patch and chip antennas, and how they can help designers meet more demanding application requirements.

The monopole antenna

The performance of a monopole antenna is critically dependent upon the counterpoise used as the other half of the antenna [see part 1]. 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, is also referenced (connected) to ground. The size of the ground plane counterpoise, as well as its location with reference to the antenna, will have a significant impact upon its voltage standing wave ratio (VSWR) and gain.

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 makes for great specifications, but in the real world a cordless phone will not have a one foot radius ground plane for its antenna. What to do?

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 input impedance are compromised, significantly impacting performance.

The VSWR graphs for the ANT-916-CW-RH-ND monopole antenna from Linx Technologies show its bandwidth related performance when tuned to a 4 × 4 inch ground-plane counterpoise (Figure 1a and 1b). As VSWR rises versus frequency, the useful bandwidth of the antenna decreases; of course, some applications benefit from narrower bandwidth while it is a negative attribute for others. The general guideline is that an antenna is useful as long as the VSWR is below 2:1, but this is not an inviolate guideline.

Figure 1a: Antenna measured on the 4 x 4-inch ground plane

Figure 1b: Antenna measured on a 26.5 × 26.5 inch full-wave ground plane.

Figure 1a and b: The VSWR graphs show that the size of the ground plane as counterpoise, measured with respect to the wavelength at the frequency of operation, has a significant impact on real-world antennae performance. (Image source: Linx Technologies)

Looking at the VSWR graphs, it can be seen that a larger ground plane will lower the resonant frequency and widen the bandwidth. In this case, the wider bandwidth offsets the drop in frequency so that the VSWR at the intended center frequency is still less than 2.0:1.

Conversely, if the antenna had been tuned to the larger plane, then placed on the smaller one, the center frequency would have shifted higher, and the bandwidth would be smaller. This could result in a VSWR that is out of specification. This effect would be magnified with helical antennas.

Helical antennas are coiled to reduce their size, but this also has the effect of narrowing the bandwidth. A ground plane that is too small could narrow the bandwidth to a point where it would be difficult to maintain the antenna’s performance over production tolerances and in the presence of external influences.

Regardless of the antenna style chosen, the size of the implemented ground plane should be considered in comparison to the antenna manufacturer’s reference plane and calculated ideals. Whenever possible, actual antenna performance should be measured with tools such as a network analyzer and spectrum analyzer, since shifts such as those described above can affect the efficiency of the system and significantly impact the product’s final range. By their nature, antennas are sensitive to and affected by their surroundings, so actual in-product placement, as well as changes due to the way the product is being used, held, or mounted, can greatly affect key specifications – usually to the negative, unfortunately.

If the antenna is mismatched, the transmitter output power could be increased to compensate. The trade-off is higher current consumption and shorter battery life. For most receivers, there is little that can be done to recover the lost sensitivity. In some cases, a low-noise amplifier (LNA) can be placed after the antenna and before the receiver’s front end, but that adds to the cost, current consumption, and size.

Not only does the size of the ground plane dictate performance, but also the location of the antenna upon that ground plane. The plots show the radiation pattern for two 418 MHz antennas on a 4 × 4 inch ground plane (Figure 2a and 2b). Both antennas have the same elements, but one is mounted in the middle of the plane, and one is mounted on the edge with a right-angle connector.

As can be seen from the plots for a typical antenna – in this case, the data associated with the Linx Technologies’ ANT-418-CW-RH 418 MHz unit – when the antenna is mounted in the middle, the pattern is uniform. With the antenna mounted on the edge of the plane, more energy is radiated away from the plane. This will result in the system having a better range in one direction than in another. This may impact the performance and perceived quality of the final product, so it should be considered early in the design stage.

Figure 2a: The placement of the antenna relative to the ground plane also has a major impact on performance and radiation pattern; shown is the pattern for Linx Technologies’ ANT-418-CW-RH 418 MHz antenna centered on a 4 × 4 inch ground plane. (Image source: Linx Technologies)

Figure 2b: Shown is the edge-mounted 418 MHz antenna radiation pattern, also on a 4 × 4 inch ground plane. Compare this to the centered version above. (Image source: Linx Technologies)

All of these examples have shown a quarter-wave monopole that is orthogonal to the ground plane. It is also very common to have the antenna in the same plane as the ground. Once again, the ground plane becomes the other element of the antenna system (Figure 3).

This orientation is very common in handheld products such as cellular phones. The length of the ground plane that points in the opposite direction from the antenna is critical. Ideally, it would be a quarter-wavelength long, but it can be shorter if the sacrifice in performance can be accepted.

Figure 3: Orientation of the ground plane versus wavelength is also critical. Shown is the radiation pattern for Linx Technologies’ ANT-916-CW-RH-ND 916 MHz antenna when in the same plane of a 4 × 4 inch ground plane. (Image source: Linx Technologies)

These measurements are good for illustrating concepts, but they are only valid for that specific antenna, when measured on that specific board. Since anything placed on the board is in the near field (within one wavelength) of the antenna, it will have an impact on the radiation pattern. Any change in the shape of the board, within one wavelength, will also have an impact on the pattern.

Use caution with manufacturer-supplied patterns

While manufacturers’ patterns can give a general idea of the antenna’s performance, they often bear no resemblance to the antenna’s performance in the final product. Polar plots (for these types of antennas) are expensive to make and do not provide much useful information to the customer. This may be why some antenna manufacturers do not list gain specifications or polar plots for monopole antennas.

However, there are many antenna styles for which manufacturers’ gain and radiation patterns are valid. Yagi, parabolic, corner and horn antennas are all types that do not depend on a ground plane provided by the designer. Broadcast antennas for AM/FM radio will often use the Earth as a ground plane at the transmitting towers. Since the Earth is much larger than one wavelength at these frequencies, it acts like an infinite ground plane. However, none of these styles would be considered for use in a portable product.

Dipole antennas are viable alternatives

A dipole antenna can also be affected in a similar way by the ground plane, depending on its construction. Some dipole antennas are in the same form factor as whip antennas (which can be misleading if you assume but do not check the data sheet), but will have a counterpoise as well as the element inside the sleeve. Typically, the counterpoise will be a metal tube with the antenna element positioned on top.

The figure shows a member of Linx Technologies’ WRT Series of dipoles, which are compact and tamper-resistant. They are designed for applications such as vending machines and similar equipment where the physical security of the antenna is important.

Figure 4: A center-fed dipole antenna such as this 2.4 GHz unit from the WRT Series from Linx Technologies includes its own integral counterpoise; this specific family is designed to be tamper resistant for physically exposed applications such as vending machines. (Image source: Linx Technologies)

The WRT series of dipoles is available in a variety of models for operation centered at 418 MHz, 433 MHz, 868 MHz, 916 MHz and 2.45 GHz. The 2.4 GHz antenna is about 1.9 inches long, including its “top hat,” and mounts through a hole in the product case. It is secured with a nut or threaded fastener, and is attached to an 8.5 inch long RG174 cable with an SMA or FCC Part 15-compliant RP-SMA connector on the other end (other connector options are available).

A common misconception about antennas with an internal counterpoise is that their characteristics are unaffected by external factors. While it is true that an external ground plane is not required for the antenna to operate correctly, if you connect one of these antennas to a product that has a ground plane, you will see the same shifts as shown in Figure 3. The product’s external plane will add to the antenna’s internal counterpoise and shift the frequency, gain, and radiation pattern.

The performance shift can be minimal, but it should be recognized that while a dipole does not require a ground plane, it is not immune to external factors. Part of the attractiveness of a dipole is that an external ground plane is not required for the antenna to perform well. The downside is that dipole antennas are usually larger and more expensive since they include the counterpoise internally.

Designing with a quarter-wave monopole antenna

A common pitfall for designers new to the wireless arena is 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. Designers have to get it right.

The ground plane is a solid copper fill on one layer of the circuit board that is connected to the negative terminal of the battery. This fill not only acts as the antenna’s counterpoise, but is also the ground connection for all of the components on the board. The problems arise when other components are added and the traces are routed to connect them.

It is a very rare and simple design that does not need to route a trace on more than one layer. Every trace that gets routed on the same layer as the ground plane can have a significant impact on the RF performance. It is best to look at the board from the perspective of the antenna connection. The goal is to have a low impedance path back to the battery or power connection. This is accomplished with wide, unobstructed paths.

If the ground plane is cut up with traces, through-hole components, or vias, then it will not be able to do its job as an antenna counterpoise. One of the worst things that can happen is for the ground plane to get so cut up that it has to get connected by jumping back and forth between layers through vias. A via is associated with inductance, which increases its impedance at high frequency. This will result in the ground plane floating somewhere above ground at RF frequencies, which will reduce the performance of the antenna and, consequently, the range of the product.

When running traces on the ground plane layer, try to present the smallest profile to the antenna, which is normally the width of the trace. In practice, this means running traces away from the antenna rather than across the board.

Figure 5: This pc board layout, with wide ground plane paths between antenna and battery, results in good performance due to the low RF impedance across the board. (Image source: Linx Technologies)

This board uses a quarter-wave monopole antenna that is mounted in the same orientation as the ground plane. The top layer is in red and the bottom layer is in blue. Almost all of the bottom layer traces are running away from the antenna (up and down) rather than across its resonant path (left and right). The one through-hole component is also "running away" from the antenna. Looking at the board from the perspective of the antenna, there are very wide paths from the antenna to the battery (at bottom). This will mean a good, low-impedance ground connection for all of the RF stage, which will maximize the RF performance.

The ground plane also allows for the implementation of a microstrip line between the RF stage and the antenna. Microstrip refers to a pc board trace running over a ground plane that is designed to serve as a transmission line between the module and the antenna. A transmission line is a medium that allows RF energy to be transferred from one place to another, with minimal loss.

This is a critical factor because the trace leading to the antenna can effectively contribute to the length of the antenna, changing its resonant frequency. The width of the microstrip line is based on the desired characteristic impedance of the line, the thickness of the pc board, and the dielectric constant of the board material. When implemented correctly, the microstrip line will connect the antenna to the RF stage without affecting the antenna’s resonant frequency or the match to the RF stage.

One other thing that frequently seems to catch designers off guard is that standard connectors, such as SMA, BNC, and MCX, are illegal for use as an antenna connection in the United States for devices falling under some sections of CFR Part 15. The FCC does not want the end user to be able to change the antenna from the one that was certified with the product. For this reason, the antenna will need to use a nonstandard, proprietary, or permanent connection. Fortunately, the FCC considers reverse-polarity connectors to be non-standard, so they are commonly used by OEMs for the antenna.

Product trends, needs drive even-smaller antenna implementations

The proliferation of portable devices such as wearables and smartphones clearly requires antennas that are very small and that can also be placed entirely within the end product. It is not just the need to minimize its size that is forcing this situation: it is also driven by the trend toward multiband wireless devices and devices that will need to be compliant with the emerging, next-generation 5G standard.

Here, the driving factors are the need for multiple antennas to serve several independent bands, as well as for antenna configurations that implement MIMO (multiple input/multiple output) antenna arrays for diversity and beamforming, so space is at a premium. For more on MIMO links, see the TechZone article “Understanding and Implementing MIMO RF Links.” For many of these applications, it's also required, or at least desirable, to have the antenna be a component which can be mounted on a pc board along with other active and passive components.

These needs have driven the development and mass-market use of less obtrusive antennas, such as chip devices. Other newer antenna options include independent patch configurations and even antennas that make use of some of the pc board's copper.

Chip antennas are multilayer ceramic components which resonate at specific frequencies and thus act as antennas as a result of their dimensions, etching, and layering. For example, the 2450AT18B100E from Johanson Technology measures just 3.2 × 1.6 mm, and 1.3 mm high, and is designed for the widely used 2.4 to 2.5 GHz band, making it well suited to compact, handheld or wearable devices. The antenna has a peak gain of 0.5 dBi, a typical gain of -0.5 dBi, and can handle up to 2 W of transmit power. Of course, numbers will change with actual placement, so modeling and verification are critical.

Figure 6: A chip antenna, such as the 2450AT18B100E from Johanson Technology, can implement this vital RF function in a tiny, surface-mount package, but does require some pc board as its ground plane. (Image source: Johanson Technology)

They are most commonly fabricated as monopoles, inverted-F antenna (IFA), and planar inverted-F antenna (PIFA) configurations. They are placed on the pc board just as any conventional surface mount component; no special handling or manufacturing considerations are required. However, some amount of the pc board copper must be allocated and reserved as a ground plane.

As its name indicates, the patch antenna is a small adjunct, patch-like component that is connected to the pc board via a short, coaxial lead. This design means that it does not require board area, nor is it affected by nearby board components. It is usually placed within the product enclosure and is often simply taped to the inside of that housing.

The Antenova SRF2W012-100 DROMUS is an example of a patch antenna. It measures just 30.0 × 6.0 × 0.15 mm and supports dual-band operation for the two most used Wi-Fi bands: 2.4 to 2.5 GHz and 4.9 to 5.9 GHz. It is offered with a selection of standard coaxial cable lengths. The cable itself is just 1.13 mm in diameter and comes with an attached RF micro connector. The SRF2W012-100’s RF performance depends on the band of interest. For the low band, peak gain is 3.0 dBi, average gain is -1.5 dBi, and VSWR is 1.5. For the high band, the numbers are 4.0 dBi, -1.8 dBi, and 1.8, respectively.

Figure 7: The patch antenna is mounted away from the pc board, which means there is little or no interaction between the two functions of the RF design. Shown is the SRF2W012-100 from Antenova that comes with an attached coaxial pigtail in a selection of lengths. (Image source: Antenova)

Finally, pc board antennas, also called "trace" antennas, use precisely etched copper on the board itself as the antenna. This approach offers distinct advantages and disadvantages. On the positive side, it obviously has no or minimal direct cost and BOM impact, and needs just a few extra square centimeters of the board. Unique among antennas, it can be designed and fabricated in complex configurations which are not only resonant across multiple bands, but which can also can incorporate filtering or specialized characteristics (such as polarization) without additional manufacturing cost.

On the negative side, it occupies precious board space, is very sensitive to placement of nearby components, and any board redesign will have a significant impact on its performance. It is an attractive solution for some situations, such as a complex phased-array radar system with its hundreds or thousands of identical transmit/receive modules, but it is much less attractive for small consumer products such as wearables or smartphones.

Conclusion

The antenna is a critical component to a system’s performance and should be considered early in the design process. It should be recognized that specifications on an antenna’s data sheet will not necessarily reflect its performance in the final product. This is a result of design specific factors, such as those discussed here, as well as differing references, methods of test, and presentation formats among antenna suppliers.

Bearing this in mind, allowance must be made for testing and optimizing the antenna as an integral part of the overall design process. While it is unlikely an end user will spend much time contemplating the nuances of antenna implementation, they will certainly appreciate the range and reliability of a well-designed product. The field of antenna design and application is complex, but by understanding a few ground rules, it is not necessary to be an antenna designer to get the most out of an antenna design.