User's Manual

Interfacing and Controlling the RIM 902MAntenna selection: Introduction to antenna

terminology 47

RIM 902M OEM Radio Modem – Integrator’s Guide

An isotropic radiator a 100% efficient point source radiator with a spherical

radiation pattern. Its field energy density is identical in any direction from the

radiator at each fixed distance from the radiator. An isotropic radiator cannot

exist in practice; it is an unrealisable theoretical reference for measuring

antenna gain and radiation patterns.

The power output of the RIM 902M is 62 mW to 2.0 W at the antenna port, and

the antenna gain (or loss) will result in an increase (or decrease) in this value.

The actual output is called the Effective Radiated Power, or ERP. For example,

if the RIM 902M is putting out 2.0 W of power to a 2.3 dB gain antenna, the

ERP is 2.0

10^(2.3

10) = 3.4 W, the actual power radiated by the antenna in

the direction of maximum gain and polarization.

Impedance matching, return loss, and VSWR

The antenna, cables, and connectors in a radio frequency system must all

possess the same impedance. The impedance required by the RIM 902M is

, which is a widely-available industry standard. Any deviation from this

value may result in impedance mismatch.

Impedance mismatch can be caused by cable connections, cable lengths, and

imperfections in the cables and connectors. The mismatch causes some of the

radio frequency energy to be reflected back from the location of the mismatch.

This interferes with the signal and reduces its amplitude, resulting in a power

loss.

Antenna mismatch can be expressed as a return Loss (RL), which is simply the

ratio of reflected power to the incident power, expressed in decibels.

log

reflected

output

Equation 1: Return Loss

The Voltage Standing Wave Ratio (VSWR) is another way of expressing the

ratio of incident power (from the RIM 902M) to reflected power (into the

RIM 902M).