Listen Technologies 863.050, User Manual

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5
Design Guide
Frequency Modulation (FM) Technology Overview
Frequency modulation or (FM) is a means of transmitting audio using electromagnetic waves.
This same technology is used by local FM radio stations to broadcast music. FM signals can travel through
most barriers – walls, fl oors, and ceilings. The distance a signal travels has many different variables such as
Radio Frequency (RF) output power, the type and placement of the antenna, and the broadcasted
frequency. Unlike infrared, FM transmission are not secure. This enables a receiver to travel further distances
from the source. This section of the manual will help you design a system that will get the best range and least
amount of interference.
Audio Source
FM Transmitter
FM Receiver
When designing an FM system it is important to keep the following in mind:
• Multipath
Multipath interference is a form of RF interference that occurs when radio signals have more than one path
between the receiver and the transmitter causing the two RF signals to add out of phase. This could occur in areas with
RF-refl ective surfaces, such as furniture, walls, or fl oors. The result of multipath is the receiver audio coming in and out of
squelch when the receiver is moved. Portable applications are much more susceptible to multipath than those that are
stationary. Multipath distortion is often worse as the distance between transmitter and receiver increases
(RF power decreases). Multipath can decrease the audio quality of the transmission.
• Antenna placement
The antenna should be placed as high as possible and in line of sight of the receivers. The antenna should not be
mounted near metal objects. Keeping the coaxial cable as short as possible will also limit the loss of RF power.
The LT-800 comes equipped with an articulating right angle dipole antenna (LA-129). The remote antenna kit
(LA-130) is an optional accessory. The antenna on the receivers should always be in the upright position to maximize
the transmission range. Placing the antenna in areas where the RF cannot properly be transmitted will limit the range
of transmission.