In addition to highly visible consumer smartphones, 5G-based wireless links are addressing diverse embedded applications such as the Internet of Things (IoT), machine-to-machine (MTM) links, the smart grid, vending machines, gateways, routers, security, and remote-monitoring connectivity. However, this shift to 5G won’t happen overnight. This creates a need for antennas at the front-end of the wireless communications link that can cater to 5G, as well as legacy 2G, 3G and other non-5G links that will remain in place for years to come, even as 5G proliferates.
For these reasons, engineers need to design products for bands in addition to those supporting 5G standards. Even if the internal RF front-end or power amplifier differs for each band, there are benefits to having a single wideband antenna to serve both 5G and legacy bands.
This article looks at wideband antennas that serve the lower-band 5G spectrum as well as legacy bands, as represented by illustrative units from Abracon LLC. The article shows how the use of this type of antenna—whether as visible external units or internal embedded ones—can ease design, simplify the bill of materials (BOM), and facilitate installation of an upgrade to 5G if needed.
Start with regulatory bands
Antennas are the last element of the RF transmit signal path and the first in the complementary receiver path. The function of the antenna is to be a transducer between the circuit world of current and voltage and the RF world of radiated energy and electromagnetic fields.
When selecting an antenna for the target application, it’s important to keep in mind that the antenna functions without regard to the type of modulation or industry standard for which it is being used. None of the parameters used for antenna selection—such as center frequency, bandwidth, gain, power rating, or physical size—are a function of whether the antenna is being used for amplitude, frequency, or phase modulation (AM, FM, PM) signals, or 3G, 4G, 5G, or even proprietary signal formats.
Of course, system designs for emerging applications that support 5G standards are getting a significant amount of design attention, especially for the 5G bands below 6 gigahertz (GHz) where most 5G activity resides. It’s important to distinguish between the wireless standard which the system supports, versus the frequency and spectrum used which determines antenna selection.
The new 5G standards make use of previously unavailable spectrum segments, while also leveraging parts of the spectrum already in use by incorporating higher-level modulation schemes for higher throughput. Thus, while industry and carrier support for an existing standard may be phased out (or “sunsetted”) such as 3G in 2022, some parts of the spectrum used by 3G will still be used for 4G and even for 5G (Figure 1).
Figure 1: The frequencies between 600 and 6000 MHz support multiple standards such as 3G, 4G and 5G, with some spectrum overlap. (Image source: Abracon LLC)
This means that antennas supporting 3G or 4G bands may still be viable for 5G as well, and vice versa. The standard may be sunsetted but its antenna is not, and forward/backward antenna compatibility is possible. In each of these cases, antenna re-use that supports multiple standards and bands is a practical and often desirable solution.
Other important standards in the RF spectrum from 600 megahertz (MHz) to 6 GHz include:
- Citizens Broadband Radio Service (CBRS), a lightly regulated 150 MHz wide segment in the 3550 MHz to 3700 MHz range (3.5 GHz to 3.7 GHz). In the United States, the Federal Communications Commission (FCC) has designated this service for sharing among three tiers of users: incumbent users, priority access license (PAL) users, and general authorized access (GAA) users.
- LTE-M, the abbreviation for LTE Cat-M1 (often called CAT M) or Long-Term Evolution (4G), category M1. This technology enables low duty cycle, battery-powered IoT devices to connect directly to a 4G network without a gateway.
- Narrowband-IoT (NB-IoT), is a cellular-grade wireless technology that uses orthogonal frequency division multiplexing (OFDM) within the 3G umbrella. It is an initiative by the Third Generation Partnership Project (3GPP)—the organization behind the standardization of cellular systems—to address the needs of very-low-data-rate devices that need to connect to mobile networks, also often powered by batteries.
A note about wideband and multiband terminology, as there is the possibility of confusion and ambiguity. “Wideband” refers to an antenna with a bandwidth that is a significant fraction of its center frequency. While there is no formal definition of this number, informally it usually means a bandwidth that is at least 20 to 30 percent of the center frequency. In contrast, “multiband” means an antenna that is designed to support two or more bands as defined by regulatory standards; these bands can be closely spaced or widely separated.
An extreme example of a multiband antenna would be one that simultaneously works for broadcast AM (550 to 1550 kilohertz (kHz)) and broadcast FM (88 to 108 MHz). A multiband antenna may be wideband but is not necessarily so.
Regardless of the number, spacing, and bandwidths it supports, a multiband antenna has a single RF connection, even though internally it may comprise two or more distinct combined antennas. Unlike a simpler wideband antenna, a multiband antenna may actually be designed with deliberate gaps in gain coverage across its bandwidth to minimize co-channel interference.
Internal or external antenna
The wireless connectivity standard for which the antenna is being used is not an antenna design issue, but the frequency and bandwidth are definitely considerations that make the physical implementation of the antenna an important decision. One major design consideration is whether to use an external antenna or one embedded within the end product.
Internal antennas have these attributes:
- They enable a sleeker package with no external attachments to get broken or snagged
- The embedded antenna is always connected and available
- They have inherent limitations with respect to coverage, efficiency, radiation patterns, and other performance criteria
- Performance of the embedded antenna will be affected by adjacent circuitry, so its placement is closely correlated to circuit board size, layout, components, and overall arrangement
- The user’s hand or body may induce changes in antenna pattern, efficiency, and performance
In contrast, external antennas possess these characteristics:
- They offer more potential for tailoring radiation patterns, bandwidth, and gain since they have more degrees of design freedom
- They do not have to be attached to the IoT/RF unit and can be optimally sited at a modest distance by using a coaxial cable
- They are less affected, or not affected at all by the electrical aspects of product design and packaging
- They are available in several styles and configurations
- They require a connector or cable for attachment, which can be a point of failure
The choice between an external and an internal antenna is usually decided according to multiple factors. These include the end-product application and user preference, balanced against performance and whether the antenna will be used in a mobile or fixed situation. For example, a smartphone with an external antenna might be considered awkward. In contrast, a fixed-in-place IoT node with an external and perhaps slightly remote antenna might provide better, more consistent connectivity.
Multiband antenna benefits
Multiband antennas can satisfy existing applications while future-proofing designs for upgrades including 5G connectivity. But why consider such an antenna if the installation parameters and specifics are known? There are several good reasons:
- A single antenna can be used across a family of products targeting different bands, thus simplifying inventory management and purchasing
- An internal multiband antenna results in a smaller package, while an external one reduces the number of antenna connectors on the product enclosure
- The multiband antenna can serve an IoT device where an upgrade to a new band such as 5G is possible or anticipated, whether for performance reasons or sunsetting of the existing band and standard
- A single external antenna for multiple bands provides commonality with respect to installation techniques and tools
- For critical fixed and especially mobile applications, the device’s RF section may provide dual-band support, allowing the device to dynamically switch between bands for optimum performance in a given locale or setting
- Designers can use a single internal multiband antenna in unrelated devices, but gain by leveraging their experience with antenna modeling, placement, and possible production issues
Real-world multiband antenna examples
Despite their wideband performance, multiband antennas are not limited in form factor or termination type, as three examples illustrate.
The AEBC1101X-S is a 5G/4G/LTE cellular whip antenna that measures 115 millimeters (mm) in length with a maximum diameter of 19 mm, designed for 600 MHz to 6 GHz operation (Figure 2). It comes with a standard male SMA connector that can rotate through 90° for direct mount on the product enclosure (it could also be used with an extending coaxial cable); a reverse-polarity SMA connector is also available.
Figure 2: The AEBC1101X-S 5G/4G/LTE cellular whip antenna is designed for 600 MHz to 6 GHz operation and comes with an integral SMA coaxial connector with 90° of rotation. (Image source: Abracon LLC)
Its voltage standing wave ratio (VSWR) and peak gain performance are fairly constant across the entire band, although there is a shift in efficiency between the lower and upper frequency ranges (Figure 3).
Figure 3: The AEBC1101X-S 5G/4G/LTE cellular whip antenna has modest changes in performance between its low end (600 to 960 MHz) and high end (1400 to 6000 MHz) ranges. (Image source: Abracon LLC)
The radiation pattern is fairly circular over the entire band, with some small lobes emerging at 3600 MHz that become a little more apparent at 5600 MHz (Figure 4).
Figure 4: The X-Y radiation pattern for the AEBC1101X-S changes between 3600 and 5600 MHz, with the appearance of some lobes. (Image source: Abracon LLC)
The AECB1102XS-3000S 5G/4G/LTE/NB-IoT/CAT blade antenna, also for 600 MHz to 6 GHz operation, measures 115.6 mm long × 21.7 mm wide with a very thin profile of just 5.8 mm (Figure 5). It is designed for easy and convenient installation against a flat surface with adhesive tape.
Figure 5: The AECB1102XS-3000S 5G/4G/LTE/NBIOT/CAT Blade Antenna, also for 600 MHz to 6 GHz, is a low-profile antenna designed to conveniently be mounted against a flat surface by simply using adhesive tape. (Image source: Abracon LLC)
Its RF performance is similar to the AEBC1101X-S with a maximum VSWR below 3.5, but peak gain is a little lower at 2 decibels relative to an isotropic radiator (dBi). The radiation pattern in the X-Y and X-Z plane is also more complex (Figure 6).
Figure 6: The X-Z and Y-Z radiation patterns for the AECB1102XS-3000S blade antenna show a more complex set of lobes than the whip antenna. (Image source: Abracon LLC)
A notable difference between the AEBC1101X-S and the AECB1102XS-3000S is in the available terminations. The AECB1102XS-3000S blade unit comes standard with a 1 meter (m) LMR-100 coaxial cable (this replaces RG174 and RG316 cable types) terminated with the widely used male SMA connector. However, almost any cable length can be ordered, and connector types besides SMA are also offered as standard options for connection flexibility (Figure 7).
Figure 7: The standard coaxial cable for the AECB1102XS-3000S is terminated with an SMA (M) connector, but many other connector choices are offered. (Image source: Abracon LLC)
The ACR4006X 600 to 6000 MHz wideband ceramic chip antenna is a surface-mount device measuring just 40 × 6 × 5 mm high. In operation, it requires a tiny inductor-capacitor (LC) impedance matching network consisting of an 8.2 nanohenry (nH) inductor and a 3.9 picofarad (pF) capacitor (each of 0402 size) to achieve the desired 50 ohm (Ω) impedance (Figure 8).
Figure 8: The ACR4006X 600 to 6000 MHz wideband ceramic chip antenna has a footprint of just 40 × 6 mm, and requires only two tiny passive components for 50 Ω impedance matching. (Image source: Abracon LLC)
The ACR4006X datasheet indicates that it is a 600 to 6000 MHz device, but note that its efficiency, peak gain, and average gain graphs have some gaps (Figure 9). This is deliberate, as this multiband antenna is designed and optimized for performance in three specific bands within that range: 600 to 960 MHz, 1710 to 2690 MHz, and 3300 to 6000 MHz to support 3G, 4G, and 5G allocations, as well as some smaller spectrum allocations.
Figure 9: The efficiency and gain plots for the ACR4006X from 600 to 6000 MHz shows gaps, but these are of little concern to users as they are not within the 3G, 4G, and 5G bands of operation. (Image source: Abracon LLC)
Since the ACR4006X is not intended for GPS receivers, its performance is not specified at GPS carrier frequencies of 1575.42 MHz (L1 carrier) and 1227.6 MHz (L2 carrier).
The ACR4006X’s X-Y radiation pattern is also a function of frequency, but it still maintains a roughly circular shape across its wide band, with only a few modest gain dips at 90° and 270° at its lower frequency range (Figure 10).
Figure 10: The X-Y radiation pattern of the ACR4006X chip antenna is roughly circular but with some frequency-dependent gain dips at 90° and 270°. (Image source: Abracon LLC)
Evaluating an antenna’s performance begins with the datasheet, often followed with confirmation using an anechoic chamber, and eventually field tests with the final product. Factors that affect external antenna actual performance are the enclosure, the user’s body and hands for mobile units, and the antenna location and placement. It is largely decoupled from the internal circuit board layout of the product.
In contrast, the performance of an internal unit such as the ACR4006X chip antenna is affected by adjacent components and the pc board. For this reason, Abracon offers the ACR4006X-EVB evaluation board to provide a means to facilitate engineering evaluation of this chip antenna.
The board is used in conjunction with a vector network analyzer (VNA). After initial calibration of the configuration—a standard step in most VNA tests—antenna performance is assessed via the VNA’s calibrated port using the SMA connector on the board.
The evaluation board measures 120 × 45 mm and is precisely dimensioned for proper placement of the chip antenna. It includes the requisite 45 × 13 mm metal/ground clearance area around the antenna for proper operation (Figure 11).
Figure 11: The ACR4006X-EVB evaluation board measures just 120 × 45 mm and facilitates evaluation of the chip antenna via its SMA connector; the datasheet shows critical layout areas and dimensions. (Image source: Abracon LLC)
Multiband antennas meet the challenges of IoT devices, especially those which need to support a single band now while providing a smoother upgrade path to newer standards such as 5G. They also allow a system to support multiple bands to optimize performance in zones where connectivity is not assured on a single band. As shown, Abracon’s circuit-board mounted internal antennas enable a sleeker package, while its external antennas using either an integral RF connector or a coaxial cable attachment offer flexibility in placement for an optimal signal path.