Mobile Wireless Terminal Metal Casing Antenna Impedance Management
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Solution Overview
Problem
Mobile wireless terminals with metal casings face challenges in maintaining good antenna characteristics over time while reducing the risk of electric shock due to insufficient insulation in chargers connected to commercial power supplies, which can apply voltage to the metal casing, potentially causing electric shock.
Innovation Solution
Incorporating a capacitor connected to the metal casing and ground conductor to set high impedance for leakage currents from the commercial power supply and low impedance for antenna currents, along with an overcurrent protective element with an operating voltage higher than the commercial power supply voltage to protect the capacitor from surge voltages, ensuring good antenna performance and reduced electric shock risk.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the metal casing and ground conductor are electrically disconnected, then the risk of electric shock is reduced, but antenna characteristics are significantly deteriorated
Solution Approach 1:
A capacitor is introduced as an intermediary component between the metal casing and ground conductor. This capacitor blocks low-frequency leakage currents (reducing electric shock risk) while allowing high-frequency antenna currents to pass through (maintaining antenna characteristics). The capacitor acts as a frequency-selective mediator that differentiates between harmful and useful currents.
Solution Approach 2:
The impedance characteristics of the connection between metal casing and ground conductor are changed by introducing a capacitor. The capacitor provides high impedance to low-frequency leakage currents and low impedance to high-frequency antenna currents, thereby changing the electrical parameters dynamically based on frequency to resolve the contradiction.
2Reliability
If the metal casing and ground conductor are connected with low impedance, then antenna characteristics are improved, but the risk of electric shock increases
Solution Approach 1:
The capacitor serves as a frequency-selective intermediary that enables low-impedance connection for antenna currents while maintaining high impedance for leakage currents, thus improving antenna characteristics without increasing electric shock risk.
Solution Approach 2:
By changing the impedance parameters based on frequency through the capacitor, the system achieves low impedance at antenna frequencies (improving reliability) while maintaining high impedance at power supply frequencies (preventing electric shock).
3Object-affected harmful factors
If a capacitor is used to block leakage current, then electric shock risk is reduced, but surge voltage degrades capacitance and may cause short-circuit
Solution Approach 1:
An overcurrent protective element is connected in parallel with the capacitor to provide beforehand protection against surge voltages. When surge voltage occurs, the overcurrent protective element activates first to divert the surge current, cushioning the capacitor from damage and preventing short-circuit faults.
Solution Approach 2:
The overcurrent protective element acts as a protective intermediary between the surge voltage source and the capacitor. It intercepts and diverts harmful surge currents before they can reach and damage the capacitor, ensuring long-term reliability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration maintains good antenna characteristics over the long term while minimizing the risk of electric shock by effectively managing antenna and leakage currents, with the overcurrent protective element preventing capacitance degradation and short-circuit faults.
Implementation Method 1
where the metal casing and the ground conductor are connected by the capacitor, an impedance between the metal casing and the ground conductor is able to be set high for a leakage current of low frequency from the commercial power supply, and is able to be set low for an antenna current of high frequency
Implementation Method 2
the overcurrent protective element, which has an operating voltage higher than the maximum instantaneous value of the commercial power supply voltage, does not operate in response to leakage current from the commercial power supply
Implementation Method 3
When the metal casing and the ground conductor are electrically disconnected, an induced current that cancels out radio waves radiated from the ground conductor is produced in the metal casing by an electric field between the metal casing and the ground conductor
Data Source
AI summary
A mobile wireless terminal includes a metal casing, a printed wiring board disposed in the metal casing and including a ground conductor thereon, the ground conductor defining a ground plane for both a power supply circuit and an antenna, a capacitor connected to the metal casing and the ground conductor, and an overcurrent protective element connected to the metal casing and the ground conductor and having an operating voltage higher than a maximum instantaneous value of a commercial power supply voltage. A capacitance of the overcurrent protective element may be smaller than a capacitance of the capacitor. The capacitor may be a multilayer ceramic capacitor, and the overcurrent protective element may be a discharge gap overcurrent protective element.