Understanding Antenna’s Radiation Pattern

转载：Ellisys应用笔记

Introduction
    A basic understanding of antenna radiation patterns is very useful for correctly positioning your wireless devices, in order
to ensure the most ideal transmission quality. This is even more important when doing protocol analyzer captures. This
document will provide some basic information about radiation patterns of various antenna types.

Radiation Pattern Concept
    When transmitting, an antenna will not radiate power equally in all directions. The shape of this power transmission is
called the radiation pattern. This pattern is primarily the result of the antenna’s construction, and can be determined
either by simulation or by measurement.

     The easiest way to understand this pattern is to visualize it in a 3D space, but unfortunately this representation is not
normally available from typical datasheets of off-the-shelf devices. Luckily there are not that many different antenna
shapes used on Bluetooth devices.

Antenna Types
    We will specifically cover omnidirectional antennas, as this is the most common form of antennas found on Bluetooth
devices.

   In a perfect world, an omnidirectional antenna would transmit the exact same power in all directions. In that case, this
perfect antenna would create a spherical pattern. Unfortunately, such a perfect antenna does not exist. All antennas are
imperfect and some positions are less ideal or even unusable. In addition, with the increasing prevalence of miniaturized
devices, engineers often need to trade antenna quality to satisfy other constraints.

     When transmission quality is more important than size, a detachable antenna with an SMA connector is usually used.
These stick antennas are very common, and are also the most intuitive antennas, as their radiation pattern is more or less
as we would expect. When positioned vertically, this antenna type will transmit the same amount of power all around
(laterally), but not along its vertical axis (not on the top or bottom). This radiation pattern is known as radial, but
visualized in a 3D space it looks more like a donut:

When space is the major constraint, chip antennas are good alternatives. This type of antenna is found on mobile
phones, dongles, and anything that must be compact and flat. The pattern of these antennas usually looks like a 8, as
shown below:

A mobile phone using such an antenna transmits nicely in front and behind its screen, but transmits a very poor signal on
its sides. If you place this mobile phone flat on a table, it will provide the worst possible transmission quality. This is
unfortunately how mobile phones are usually placed when doing quick tests in the lab, resulting in poor transmission
quality and incomplete wireless analyzer captures.
 

For more information on correct placement of Bluetooth devices, please read the “Placing Correctly Your Analyzer” Expert Note.