Antenna Fundamentals and Integration Advanced Antenna Topics Informational

How do I design a dual band antenna that operates at both sub-6 GHz and millimeter wave frequencies?

Designing a dual-band antenna that operates at both sub-6 GHz (e.g., 3.5 GHz 5G n78) and millimeter wave (e.g., 28 GHz 5G n257) frequencies is challenging because the two bands have vastly different wavelengths (approximately 86 mm at 3.5 GHz vs. approximately 10.7 mm at 28 GHz), making it difficult to create a single radiating structure that resonates efficiently at both. Design approaches include: shared aperture (a large sub-6 GHz patch or slot antenna with a mmW array integrated within or around it; the sub-6 GHz element serves as a ground plane or parasitic element for the mmW array; the two bands are isolated by frequency-selective filtering), nested elements (a mmW patch array is placed inside the area of a larger sub-6 GHz patch; the mmW patches are electrically small at sub-6 GHz and do not significantly disturb the low-band operation), stacked elements (the sub-6 GHz antenna is on one PCB layer and the mmW antenna is on another layer; vertical separation and ground planes provide isolation between the bands), and dual-function elements (a single antenna element is designed to radiate at both bands using different modes: a rectangular patch radiating TM10 at 3.5 GHz and TM30 at 10.5 GHz, though this does not reach 28 GHz directly). The key design challenges are: achieving > 20 dB isolation between the two bands (to prevent the strong sub-6 GHz signal from saturating the mmW receiver), maintaining acceptable patterns at both bands simultaneously, and fitting within the available device volume.

Category: Antenna Fundamentals and Integration

Updated: April 2026

Product Tie-In: Antennas, Arrays, Feeds

Dual-Band Sub-6 GHz / mmW Antenna Design

Dual-band sub-6 GHz / mmW antennas are increasingly important for 5G smartphones and small cells that must support both frequency ranges. Co-locating both antennas in a single module reduces overall size and simplifies the RF front-end design.

Design Strategies

Co-designed shared aperture: The most compact approach. A large patch (approximately 20-40 mm) operates at sub-6 GHz, and an array of small patches (approximately 2-3 mm each, in a 4x4 or 8x8 array) is placed within or adjacent to the large patch at mmW. The frequency difference (approximately 8:1) provides inherent isolation. Filtering networks may be added for additional isolation
Phased array integration: At mmW, beam steering is mandatory due to the high path loss. The mmW antenna must be a phased array (typically 4x4 to 16x16 elements). The sub-6 GHz antenna can be a single element or small array. Co-designing these on a single module requires careful electromagnetic isolation
Practical implementations: Qualcomm QTM525 and similar 5G antenna modules integrate sub-6 GHz antennas with mmW phased arrays. These are typically implemented on multi-layer organic substrates (approximately 5-10 layers) with the mmW array on the top layer and the sub-6 GHz antenna on an inner or bottom layer

Dual-Band Antenna Parameters

Sub-6 GHz patch: L ~ lambda/2 = c/(2f sqrt(Er)) ~ 20-40 mm at 3.5 GHz
mmW patch: L ~ lambda/2 ~ 2.5-4 mm at 28 GHz
Wavelength ratio: lambda_low/lambda_high ~ 8:1 (3.5 vs 28 GHz)
mmW array gain: G = N × G_element + 10 log(N) ~ 15-25 dBi for 16-256 elements
Isolation target: > 20 dB between sub-6 and mmW ports

Common Questions

Frequently Asked Questions

Can a single element work at both sub-6 and mmW?

Not practically for 3.5 GHz and 28 GHz (8:1 frequency ratio). A single patch or dipole cannot efficiently resonate at both frequencies. The closest approach: a wideband element like a Vivaldi or tapered slot that covers a very wide bandwidth (3-30 GHz), but such elements are large at the low end and have decreasing gain at the high end. For phased arrays: the low-frequency element is too large for the mmW array lattice spacing. Separate elements with shared aperture are the practical solution.

How do smartphones implement dual-band 5G?

Current 5G smartphones use separate antenna modules: one or more sub-6 GHz antennas (typically slot or IFA antennas in the phone frame, shared with 4G) and three to four mmW antenna modules (Qualcomm QTM series or equivalent) placed at different locations around the phone edges. Each mmW module contains a small phased array (4x2 or 4x4 dual-polarized elements) with integrated phase shifters and amplifiers. The modules are placed to provide coverage in multiple directions regardless of how the phone is held.

What isolation is needed between the bands?

The sub-6 GHz transmitter can output up to +23 dBm (handset) or +46 dBm (base station), while the mmW receiver sensitivity is approximately -90 to -100 dBm. The isolation must ensure the sub-6 GHz signal at the mmW receiver input is below the receiver compression point (typically -30 to -20 dBm). Required isolation: 23 - (-30) = 53 dB for a handset. The 8:1 frequency ratio provides approximately 30-40 dB of inherent isolation from the frequency selectivity of the mmW matching network and antenna. Additional filtering (5-20 dB) may be needed.

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