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Path Loss
This power loss to the antenna is known as path loss. A properly
designed antenna system must ensure that the path loss from a
transmitter to the nearest antenna in the antenna system, within the
defined telemetry coverage area, is low enough to enable proper
reception.
Path loss is assumed to be approximately equal at every site. This
assumption allows the use of recommended antenna spacing,
measured in feet. Because different types of wall construction will
produce a very different path losses, all antenna systems must
undergo a site survey performed by a trained Spacelabs Healthcare
field service engineer.
A properly designed antenna system uses the minimum number of
antennas necessary to ensure that the path loss between the
transmitter and the antenna system does not exceed defined limits
over the entire coverage area.
Diversity Antenna System
In addition to path loss, multipath nulls are also a cause of poor
reception. These nulls exist anytime there are RF reflective surfaces
in the coverage area. In a Spacelabs Healthcare digital telemetry
system, these nulls are negated by using a diversity antenna system.
In a diversity antenna system, two similar but completely
independent antenna systems are installed. Each antenna system
provides complete coverage by itself, and provides “back-up”
coverage for the other antenna system during instances of multipath
interference. The receiver automatically selects the antenna system
that is receiving the stronger signal. The diversity antenna system
greatly reduces the number of locations in the coverage area that
may have poor RF reception due to multipath nulls.
shows a simplified antenna system, and
page 3-33 shows an example of antenna placement. In most cases
these antenna systems become multifloored and quite large,
creating a complex system that should be verified regularly and on a
scheduled basis.
