Antenna Matching Within an Enclosure Part I: Theory & Principle

Thu, 10/18/2012 - 01:00

Joining us today will be guest blogger, RF Design Engineer, John Lienau. John works at LSR designing antennas and RF circuits for our customers. He is a graduate of Marquette Univeristy in Milwaukee, WI with a Masters degree in Electrical Engineering. His graduate research focused on electromagnetic theory and antenna design. We welcome him to our blog to discuss two parts of Antenna Matching with an Enclosure.

antenna_designDeveloping wireless products can be a daunting task.  There are many pitfalls, traps, and common mistakes people fall into during RF design and development.  It is very common to save the implementation of the antenna design for last.  The fact of the matter is that the antenna makes your product wireless.  It is the most critical component that launches your signal into space.  Often little consideration is given to the antenna location and how the object nearby can affect it.  This can be devastating not only to a product’s performance, but also to schedule and cost. 

An antenna is a function of its environment.  Whether it’s sitting on a desk, on a manufacturer’s development board, or in a product, all three scenarios result in different performance.  Unlike most components in an RF design that can be dropped in with an expected effect on the circuit, an antenna is affected by everything around it.  The radiated electromagnetic fields from an antenna interact with nearby materials, and can change its frequency of operation.  The antenna must be placed in its final environment and impedance matched so that it operates in the desired frequency band.  A poorly matched antenna can degrade your link budget by 10-30 dB and severely reduce range.  All antennas, whether they are off-the-shelf or designed in a lab may require matching.    

To understand why an antenna is affected by objects near it, how an antenna radiates must be reviewed.  First, forget about what is happening to the antenna or what is nearby.  What is important is the input impedance to the antenna.  Examining the dipole in Figure 1, when a potential is applied to the antenna inputs, there is opposite charge buildup on the ends.  Essentially the dipole ends can be viewed as open circuits, with a high voltage and no current.  Due to the charge buildup at either end of the dipole, current begins to flow.  As you move from the end of the dipole inwards towards the feed point, the voltage falls and the current rises.  At the feed point the current reaches its peak along with some now reduced voltage.  The ratio between the voltage and current at the feed point is the input impedance to the antenna.  This is the impedance that drives the antenna performance.  Additionally, since there is current flow on the antenna, electromagnetic fields are radiating. 

dipole_antenna

Figure 1: Dipole Diagram

The antenna impedance acts as a load impedance for the transmission line that carries the signal to the antenna.  The impedance of the antenna determines the voltage and current along the transmission line as well as their phase relationship.  On the transmission line there is a Standing Wave Ratio (also known as VSWR).  This ratio measures the amount of standing wave versus traveling wave.  Roughly speaking, a VSWR = 1 means a pure traveling wave and a VSWR = ∞ means a pure standing wave.  A very low VSWR is desirable because it indicates that most of the energy is being delivered to the antenna.  A high VSWR means most of the energy is being reflected back onto the transmission line and is not being radiated.  When the antenna is matched at the transmission line terminals, there is a low VSWR.  Energy that is not delivered and instead reflected back at the antenna input can affect the RF circuitry, as it must be dissipated elsewhere.

 

Click below to finish reading our report on Antenna Matching.

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