Antenna Bandwidth: -10 dB, -6 dB & What It Means

What Is Antenna Bandwidth?

Antenna bandwidth is the continuous frequency range over which an antenna’s return loss stays below a specified threshold—most commonly -10 dB (VSWR ≤ 2.0) or -6 dB (VSWR ≤ 3.0). A -10 dB threshold means at most 10 % of incident power is reflected; at -6 dB, up to 25 % is reflected. Which number a datasheet uses changes the reported bandwidth dramatically, sometimes by a factor of two, and understanding the difference is essential before comparing any two antennas or signing off on a design.

The Two Thresholds: -10 dB vs -6 dB Return Loss

-10 dB (VSWR ≤ 2.0)

This is the standard in most RF system design, base-station antennas, and IEEE antenna measurement practice. At -10 dB, 90 % of the power reaches the antenna. Nearly every discrete antenna vendor and most PCB antenna designers quote bandwidth at this level. If a datasheet does not state the threshold, assume -10 dB—but verify.

-6 dB (VSWR ≤ 3.0)

The -6 dB convention is common in mobile-phone and IoT antenna design, where the transceiver includes an internal matching network or tuner that can tolerate higher mismatch. Chip-antenna vendors and IFA antenna designers routinely use -6 dB because the electrically small structures they work with simply cannot achieve -10 dB over a wide band without external matching. A -6 dB bandwidth will always be wider than the -10 dB bandwidth for the same antenna, so quoting -6 dB makes a small antenna look better on paper.

Quick Comparison

When you read an antenna datasheet, the very first thing to check is which threshold the S11 plot uses. A “900 MHz antenna with 100 MHz bandwidth” could cover 850–950 MHz at -6 dB but only 870–930 MHz at -10 dB. That 40 MHz difference can mean the antenna misses your target band entirely.

Why Customers Mean Different Things by “Bandwidth”

In practice, “bandwidth” in a product requirement can refer to at least three different things:

• Impedance bandwidth — the return-loss-defined range discussed above.
• Radiation bandwidth — the range over which gain stays within, say, 3 dB of peak. A patch antenna may be impedance-matched over 5 % fractional bandwidth but show usable gain over only 3 %.
• System bandwidth — the channel bandwidth the radio standard needs (e.g., 20/40/80/160 MHz for Wi-Fi 6E, or 200 kHz for NB-IoT). This is what the product manager usually means and may be far narrower than the antenna’s impedance bandwidth.

Always ask: “bandwidth defined how, at what threshold, and over what frequency range?” A mismatch in definitions here is the most common source of late-stage antenna redesigns.

Bandwidth, Q Factor and Physical Limits

An antenna’s bandwidth is inversely related to its quality factor (Q). The antenna Q factor sets a fundamental lower bound on Q for a given electrical size, first described by Chu and later refined by McLean. In simplified form:

Fractional BW ≈ VSWR − 1 / (Q × √VSWR)

For a -10 dB threshold (VSWR = 2), that reduces to roughly FBW ≈ 1/Q. A tiny 0.1λ antenna at 2.4 GHz has a minimum Q around 30, limiting its -10 dB fractional bandwidth to about 3 %, or roughly 72 MHz. Trying to squeeze more bandwidth out of it without increasing volume violates physics—no matching network can fix that. The only escape routes are increasing the antenna’s radiating volume, using multiple resonances (as in a stacked patch antenna), or accepting the -6 dB threshold.

Antenna Gain, Beamwidth and Bandwidth Interaction

Gain and bandwidth are loosely coupled through aperture size. A high-gain aperture antenna (horn, patch array, lens) concentrates energy in a narrow beam; its gain in dBi is approximately:

G ≈ 10 log₁₀(4π A_eff / λ²)

where A_eff is the effective aperture area. As frequency increases within the antenna’s bandwidth, the electrical aperture grows and gain rises—meaning gain is not flat across the band. For wideband designs (>10 % fractional BW), you should check gain at band edges, not just center frequency. Use the calculator below to estimate gain from aperture size and see how beamwidth narrows with increasing gain.

For example, a 60 × 60 mm patch array at 5.8 GHz (λ ≈ 51.7 mm) with 60 % aperture efficiency yields roughly 14.3 dBi gain and a half-power beamwidth near 28°. At 5.2 GHz (the low edge of a Wi-Fi 5 GHz antenna), the same physical aperture drops to about 13.3 dBi. That 1 dB gain slope across the band matters for link-budget compliance and should be captured in your spec.

How to Specify Antenna Bandwidth Correctly

1. State the threshold explicitly. Write “S11 ≤ -10 dB from 2.40 to 2.48 GHz” rather than “80 MHz bandwidth.”
2. Include gain limits. Add “peak gain ≥ 2 dBi across band” so the antenna cannot be impedance-matched but radiatively dead at band edges.
3. Define the measurement condition. An antenna measured in free space will detune when placed near a hand, battery or metal chassis. Specify “mounted in final enclosure” or reference your detuning tolerance budget.
4. Add margin. A 5–10 % frequency guard band on each side covers manufacturing tolerance on PCB dielectric constant (typically ±2–5 % for FR-4, ±1.5 % for Rogers RO4350B) and assembly variation.
5. Reference the standard. For formal work, cite IEEE Std 145-2013 (Definitions of Terms for Antennas) to remove ambiguity.

Common Pitfalls

• Mixing thresholds in a comparison. Comparing a chip antenna’s -6 dB bandwidth to a patch antenna’s -10 dB bandwidth is meaningless. Normalize first.
• Ignoring efficiency. A lossy antenna (low radiation efficiency) will show wide impedance bandwidth because the resistive loss damps the Q. Wide bandwidth plus low efficiency is not a win; it is a loss budget problem. Check total efficiency, not just S11.
• Trusting simulation without tolerance analysis. A simulated -10 dB bandwidth of exactly 80 MHz can shrink to 50 MHz after a ±0.1 mm etch tolerance on a patch antenna width. Always run a parametric sweep.

Fractional Bandwidth Reference by Antenna Type

FAQ

Is -10 dB or -6 dB return loss better?

Neither is universally better. Use -10 dB when the system has no mismatch tolerance (radar, base station, test equipment). Use -6 dB when the transceiver can absorb the extra 0.8 dB mismatch loss and you need every MHz of bandwidth from a small antenna.

Can I convert -6 dB bandwidth to -10 dB bandwidth?

Not by a fixed ratio—it depends on the antenna’s impedance curve shape. For a single-resonance antenna with a roughly symmetric S11 response, the -10 dB bandwidth is typically 55–70 % of the -6 dB bandwidth, but you must check the actual S11 data.

Does wider bandwidth always mean a better antenna?

No. Wider impedance bandwidth can come from resistive loss (lower efficiency), larger physical size, or relaxed matching thresholds. Evaluate bandwidth alongside gain, efficiency and size together.

How does PCB material affect antenna bandwidth?

Lower dielectric constant (Dk) and thicker substrates increase bandwidth for planar antennas. Moving from FR-4 (Dk ≈ 4.4) to Rogers RO4350B (Dk = 3.48) on the same 1.524 mm (60 mil) substrate can widen a patch antenna’s -10 dB bandwidth by 15–25 % while also reducing dielectric loss.

What bandwidth do I need for Bluetooth Low Energy?

BLE uses 2.402–2.480 GHz, an 80 MHz span or about 3.3 % fractional bandwidth. At -10 dB, most PCB antennas cover this comfortably. Add at least 5 % guard band (±4 MHz) for production spread.