Want to support both 5G and legacy bands? You need an antenna like this!

Latest update time：2024-06-19

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This article explores broadband antennas that serve low-band 5G spectrum as well as legacy bands, using an illustrative unit from Abracon LLC as a representative. It shows how this type of antenna, either a visible external unit or an internal embedded unit, can be used to simplify design and bill of materials (BOM), and speed up installation of upgrades to 5G when needed.

In addition to the ubiquitous consumer smartphones, 5G-based wireless links can meet the needs of diverse embedded applications such as the Internet of Things (IoT), machine-to-machine (MTM) links, smart grids, vending machines, gateways, routers, security and remote monitoring connections, etc. However, this transition to 5G cannot be achieved overnight. This requires the use of antennas at the front end of the wireless communication link to meet the needs of 5G as well as traditional 2G, 3G and other non-5G links; these other links will continue to coexist with it even as 5G applications increase dramatically in the next few years.

For these reasons, engineers need to consider other frequency bands in addition to those supported by the 5G standard when designing products. Even if the built-in RF front end or power amplifier is different for each band, there are many benefits to using a single broadband antenna to serve both 5G and legacy bands .

Challenges of Power Rail Management

Start with the regulatory band

The antenna is both the last element in the RF transmit signal path and the first element in the complementary receiver path. The antenna functions as a transmitter between two worlds: the circuit world of current and voltage and the RF world of radiated energy and electromagnetic fields.

When selecting an antenna for a target application, an important thing to remember is that the antenna will operate independently of the type of modulation or industry standard to be used. There are parameters used when selecting an antenna, such as center frequency, bandwidth, gain, power rating, and physical size, but none of them depend on whether the antenna will be used for amplitude, frequency, or phase modulated (AM, FM, PM) signals, or 3G, 4G, 5G, or even proprietary signal formats.

Of course, designing for 5G standards currently gets a lot of attention in system design for emerging applications, especially the sub-6 GHz 5G bands where most 5G activity will occur. It is important to distinguish the wireless standards supported by the system from the frequencies and spectrum that dictate antenna selection.

The new 5G standard not only uses previously unavailable spectrum segments, but also leverages portions of spectrum already in use by incorporating higher-level modulation schemes to achieve higher throughput. As a result, while industry and operator support for existing standards may be phased out (or “decline”), such as 3G in 2022, portions of the spectrum used by 3G will remain available for 4G and even 5G standards (Figure 1).

parameter		Specification
working frequency		600 MHz ~ 900 MHz、1,710 MHz ~ 2,690 MHz、3,300 MHz ~ 6,000 MHz
polarization		Linear
impedance		50 Ω
Supported frequency bands	5G NO	n - 1,2,3,5,6,7,12,14,18,20,25,28,29,30,34,38,39,40,41,65,66,70,71,77,78,79,80,81,82,83,84,86,89,90,95
	4G LTE	B - 1,2,3,4,5,7,8,12,13,14,17,18,19,20,25,26,28,29,34,37,38,39,41,42,43,44,48,49,52,65,66,67,68,69,70,71,85
	3G	PCS、DCA、UMTS

Figure 1: 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 an antenna that supports 3G or 4G bands may still work well for 5G , and vice versa. The standard may be dead, but its antennas may not, with forward/backward antenna compatibility possible. In each of these cases, reusing antennas that support multiple standards and bands is a practical and often desirable solution.

Other important standards in the 600 MHz to 6 GHz RF spectrum include:

Citizens Broadband Radio Service (CBRS), a 150 MHz wide, lightly regulated band between 3550 MHz and 3700 MHz (3.5 GHz to 3.7 GHz). In the United States, the Federal Communications Commission (FCC) has designated this service to be shared among three tiers of users: incumbents, priority access license (PAL) users, and general authorized access (GAA) users.
LTE-M , short for LTE Cat-M1 (commonly referred to as CAT M) or Long Term Evolution (4G) Category M1, enables low-duty-cycle, battery-powered IoT devices to connect directly to 4G networks without the use of a gateway.
Narrowband IoT (NB-IoT) is a cellular-grade wireless technology that uses Orthogonal Frequency Division Multiplexing (OFDM) in the 3G range. It was developed by the 3rd Generation Partnership Project (3GPP), the supporting organization for cellular system standardization, to meet the needs of very low data rate devices that need to connect to mobile networks, often on battery power.

Here’s a note about the terms “broadband” and “multiband,” as there can be confusion and ambiguity between the two. “Broadband” means that the bandwidth of the antenna is a significant fraction of its center frequency. While there is no formal definition of this number, informally it usually means a bandwidth of at least 20% to 30% of the center frequency. In contrast, “multiband” means that the antenna is designed to support two or more frequency bands defined by regulatory standards; these bands may be closely spaced or widely separated.

An extreme example of a multi-band antenna is one that can be used for both broadcast AM (550 to 1550 kHz) and broadcast FM (88 to 108 MHz). A multi-band antenna may be, but is not necessarily, a broadband antenna.

Regardless of the number, spacing, and bandwidth of the bands it supports, a multi-band antenna has only one RF connection, even though internally it may be made up of two or more different antenna combinations. Unlike simpler broadband antennas, multi-band antennas may actually be designed with intentional gaps in gain coverage across the bandwidth to minimize co-channel interference.

Internal or external antenna

The wireless connectivity standard used by the antenna is not a question of antenna design, but the frequency and bandwidth make the physical implementation of the antenna an important design decision and therefore an absolute consideration. One of the main design considerations is whether to use an external antenna or an internal antenna embedded in the end product.

The internal antenna has the following properties:

They make for a sleeker package with no external attachments that could break or snag
Embedded antenna is always connected and available
They have inherent limitations in coverage, energy efficiency, radiation pattern, and other performance criteria.
The performance of an embedded antenna will be affected by adjacent circuits, so its placement is closely related to the size, layout, components, and overall arrangement of the board.
The user's hands or body may cause changes in antenna pattern, effectiveness and performance

In contrast, external antennas have the following characteristics:

They have more potential for customization of radiation pattern, bandwidth and gain because they have greater design freedom
They do not have to be connected to the IoT/RF unit and can be optimally positioned by maintaining the appropriate distance using coaxial cables
They are less or not affected at all by electrical aspects of product design and packaging
They are available in a variety of styles and configurations
They require a connector or cable to connect, which can be a point of failure

The decision to choose an external or internal antenna is usually based on a number of factors. These include the end product's application and user preferences, the balance with performance, and whether the antenna will be used in a mobile or fixed scenario. For example, a smartphone with an external antenna might be considered poor design. In contrast, a fixed-location IoT node with an external antenna that may be slightly further away has the potential to provide a better and more stable connection.

Benefits of Multi-Band Antennas

Multi-band antennas can meet existing application needs while also providing a future-proof design for upgrades, including 5G connectivity. But if the installation parameters and specific details are known, why would you consider such an antenna? Here are a few good reasons:

One antenna can be used for a range of products targeting different frequency bands, thus simplifying inventory management and procurement
Internal multi-band antenna enables smaller package, while external antenna reduces the number of antenna connectors on the product housing
Multi-band antennas can serve IoT devices that are likely or expected to upgrade to new frequency bands (e.g., 5G), either for performance reasons or due to the retirement of existing bands and standards.
Single external antenna for multiple bands maintains commonality of installation techniques and tools
For critical fixed applications, especially mobile applications, the RF portion of the device can provide dual-band support, allowing the device to dynamically switch between different frequency bands to achieve the best performance in a given location or environment.
Designers can use a single built-in multi-band antenna in unrelated devices and gain the benefit of their experience with antenna modeling, placement, and possible production issues.

Real-World Multi-Band Antenna Example

Despite the varying broadband performance, multiband antennas are not limited in form factor or termination type , as the following three examples illustrate.

The AEBC1101X-S is a 5G/4G/LTE cellular whip antenna that is 115 mm long, has a maximum diameter of 19 mm, and is designed to operate in the 600 MHz to 6 GHz range (Figure 2). It has a standard male SMA connector that can be rotated 90° for direct mounting on a product housing (it can also be used with a coaxial extension cable); a reverse polarity SMA connector is also available.

Figure 2: The AEBC1101X-S 5G/4G/LTE cellular whip antenna is designed to operate in the 600 MHz to 6 GHz range and features an integral SMA coaxial connector that can be rotated 90°. (Image source: Abracon LLC)

Its voltage standing wave ratio (VSWR) and peak gain performance are fairly constant across the frequency band, but there is a variation in efficiency between the low- and high-frequency ranges (Figure 3).

parameter		Specification
parameter		Minimum	Typical Value	Maximum
working frequency		600 MHz		6,000 MHz
VSWR				3.0
Peak Gain				3.0 dBi
efficiency	(600 MHz to 960 MHz)	30%		50%
efficiency	(1,400 MHz to 6,000 MHz)	45%		60%
impedance			50 Ω
polarization		Linear
Radiation pattern (azimuth)		unidirectional

Figure 3: The AEBC1101X-S 5G/4G/LTE cellular whip antenna has modest performance variation at the low end (600 to 960 MHz) and high end (1400 to 6000 MHz) of its range. (Image source: Abracon LLC)

The radiation pattern is nearly circular across the entire band, with some small lobes appearing at 3600 MHz, which become more pronounced at 5600 MHz (Figure 4).

Figure 4: The AEBC1101X-S XY radiation pattern changes between 3600 MHz and 5600 MHz, with some lobes appearing. (Image source: Abracon LLC)

The AECB1102XS-3000S 5G/4G/LTE/NB-IoT/CAT blade antenna also operates from 600 MHz to 6 GHz and measures 115.6 mm long by 21.7 mm wide, with a slim profile of only 5.8 mm thick (Figure 5). The antenna is designed for easy, convenient mounting on flat surfaces using adhesive tape.

Figure 5: The AECB1102XS-3000S 5G/4G/LTE/NBIOT/CAT blade antenna also operates from 600 MHz to 6 GHz and is a low-profile antenna that is easy to mount on a flat surface using tape. (Image source: Abracon LLC)

The RF performance of this antenna is similar to that of the AEBC1101X-S, with a maximum VSWR of less than 3.5, but the peak gain is 2 dB, slightly lower than that of an omnidirectional radiator (dBi). The radiation patterns in the XY and XZ planes are also more complex (Figure 6).

Figure 6: The AECB1102XS-3000S blade antenna XZ and YZ radiation patterns show a more complex set of lobes than a whip antenna. (Image source: Abracon LLC)

AEBC1101X-S 和AECB1102XS-3000S 之间的一个显著差别是可用的端接。AECB1102XS-3000S 刀片单元标配 1 m 长的 LMR-100 同轴电缆（取代了 RG174 和 RG316 电缆类型），并端接广泛使用的公头 SMA 连接器。不过，几乎任何长度的电缆均可以进行订购，而且 SMA 之外的其他连接器类型也能作为标准选项提供，以实现灵活的连接（图 7）。

Cable Type and Connector Type
Code	Cable Type	Connector Type
S (Standard)	LMR-100	SMA (M)
A		FAKRA-D(F)
B		RP-SMA (M)
C		SMB (M)
D		N type (M)
AND		TNC (M)
F		BNC (M)
G		MCX (M)
H		MMCX (M)
I		FME (M)
J		FME (F)

Figure 7: The AECB1102XS-3000S’s standard coaxial cable is terminated with a male SMA connector, but many other connectors are available. (Image source: Abracon LLC)

The ACR4006X 600 to 6000 MHz broadband ceramic chip antenna is a surface-mount device measuring only 40 × 6 × 5 mm. To operate, it requires a tiny inductor-capacitor (LC) impedance-matching network consisting of an 8.2 nH inductor and a 3.9 pF capacitor (both 0402 size) to achieve the required 50 Ω impedance (Figure 8).

Figure 8: The ACR4006X 600 to 6000 MHz broadband ceramic patch antenna has a footprint of only 40 × 6 mm and requires only two tiny passive components to achieve 50 Ω impedance matching. (Image source: Abracon LLC)

The datasheet for the ACR4006X shows it as a 600 to 6000 MHz device, but note that its efficiency, peak gain, and average gain plots have some gaps (Figure 9). This is intentional, as this multi-band 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 of the ACR4006X in the 600 MHz to 6000 MHz range show that there are gaps, but users do not need to worry about these gaps because they are outside the 3G, 4G, and 5G operating bands. (Image source: Abracon LLC)

Since the ACR4006X is not intended for use as a GPS receiver, its performance at the GPS carrier frequencies of 1575.42 MHz (L1 carrier) and 1227.6 MHz (L2 carrier) is not specified.

The XY radiation pattern of the ACR4006X is also a function of frequency, but it remains roughly circular across the wide bandwidth, with only some modest gain drop-off at 90° and 270° in the low frequency range (Figure 10).

Figure 10: The ACR4006X chip antenna XY radiation pattern is roughly circular, but there is some frequency-dependent gain drop-off at 90° and 270°. (Image source: Abracon LLC)

Evaluating the performance of an antenna starts with a data sheet, followed by confirmation typically using an anechoic chamber, and finally field testing with the final product. Factors that affect the actual performance of an external antenna include the housing, the user's body and hand of the mobile device, and the location and placement of the antenna. It is largely independent of the product's internal circuit board layout.

In contrast, the performance of built-in units like the ACR4006X chip antenna is affected by adjacent components and the pc board. For this reason, Abracon offers the ACR4006X-EVB evaluation board as a means to accelerate engineering evaluation of this chip antenna.

The evaluation board is used in conjunction with a Vector Network Analyzer (VNA). After an initial calibration of the configuration (a standard step for most VNA testing), the antenna performance is evaluated through the calibrated ports of the VNA using the SMA connectors on the board.

The evaluation board has precise dimensions of 120 × 45 mm to facilitate proper placement of the chip antenna. It includes the necessary 45 × 13 mm metal/ground clearance area around the antenna for proper operation (Figure 11).

Figure 11 : The ACR4006X-EVB evaluation board measures only 120 × 45 mm , allowing easy evaluation of chip antennas through its SMA connector; the data sheet shows key layout areas and dimensions. (Image source: Abracon LLC )

Summarize

Multi-band antennas can resolve many of the challenges facing IoT devices , especially those that need to support a single frequency band today while also providing a smoother upgrade path for newer standards such as 5G. These antennas also enable systems to support multiple frequency bands to optimize performance in areas where a single frequency band cannot guarantee connectivity. As shown, Abracon's internal antenna mounted on the circuit board allows for a sleeker package, while its external antenna using an integral RF connector or coaxial cable attachment provides flexible placement for optimal signal path.

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