Understanding Vpeak in Antenna Tuner Selection

This page covers the basics of Vpeak in the context of antenna tuners and explains its significance in selecting an antenna aperture tuner.

Introduction to Antenna Aperture Tuning

Antenna aperture tuning involves modifying the electrical length or effective size of an antenna. This shifts the antenna’s resonant frequency, effectively altering its radiation pattern.

An antenna aperture tuner improves the antenna’s Total Radiated Power (TRP) and Total Isotropic Sensitivity (TIS). TRP relates to transmission performance, while TIS relates to reception performance.

A simple resonant circuit, like the T-type antenna tuner shown in Figure 1, can serve as an antenna tuner. Other common types include π-type and L-type tuners. The primary goal is to provide impedance matching between 50 Ohm and non-50 Ohm components within the wireless system. The Vpeak adjustment is often needed where impedance is higher than the input/output impedances.

A well-designed antenna with proper aperture tuning offers several advantages:

• Wide Bandwidth
• Increased data rate
• Lower power consumption
• Longer battery life
• Smaller footprint

What is Vpeak?

• Vpeak specifies the maximum voltage an antenna tuner can withstand while maintaining acceptable harmonic performance.
• Antenna tuning involves strategically placing a tuning element along the radiating structure of the antenna.
• The Vpeak value depends on the proximity of the tuning element to the antenna’s shorting pin. Vpeak is lower when closer to the shorting pin and higher when further away.

The following formulas relate Power and Vpeak:

• $P = \frac{(V_{rms})^2}{Z}$, where Z = 50 Ohm
• $V_{peak} = V_{rms} * 1.414$

Image source: Application Note from Skyworks Solutions, Inc.

Vpeak’s Role in Antenna Aperture Tuner Selection

An insufficient Vpeak can have detrimental effects:

• Damage: It can damage the circuit.
• Performance Degradation: Even without physical damage, TRP and TIS specifications can degrade, leading to excessive harmonic signal generation.
• Certification Failure: This can cause the mobile phone (or device) to fail certification tests (e.g., Radiated Spurious Emission), delaying product release.
• Reduced Efficiency: Insufficient Vpeak decreases antenna efficiency.

Many vendors provide optimum Vpeak values in their product catalogs, correlated with harmonic performance. For example, Skyworks’ SPST antenna tuner (SKY19250) specifies that a harmonic performance better than -36 dBm can be achieved with a Vpeak of 80 V.

The Vpeak value is influenced by:

• Input power
• Antenna matching circuit topology
• Loaded Q of the resonant circuit.

Figure 1 (shown above) depicts a graph of Vpeak versus frequency, which is dependent on the VSWR at the input of the circuit.