Avionics News July 2023 - 49

FIGURE 2:
A generic block diagram of a glideslope system.
VHF Navigation control head
Antenna or coupler
Connection to VHF Navigation system
Aircraft Voltage
Glideslope Receiver
Connection with
VOR/LOC
converter
Course Directing Indicator
Horizontal Needle
the approach runway toward the final approach fix
where aircraft start a straight pathway inbound to the
airport. Figure 1 depicts an ILS approach featuring the
glideslope.
Alongside the runway, the glideslope transmitting
equipment will be housed in a small building, typically
painted red and white in a high-visibility striped or
checkerboard pattern. Next to the building will be
a short tower with two or three directional antennas
mounted. The antennas radiate the glideslope signal
on a channel within the band stretching from 329.15
megahertz to 335 MHz. Each glideslope frequency
within this band is assigned, or collocated, with a
localizer frequency. For example, when the VHF nav
is tuned to 108.95 MHz (a localizer frequency), the
glideslope frequency will be 329.15 MHz. The channel
at the center of the glideslope frequency range, 332
MHz, is collocated with 109.3 MHz. The highest
glideslope frequency, 335 MHz, corresponds with the
localizer frequency of 110.3 MHz. The channels are
spaced 150 kilohertz apart, which yields 40 channels
that correspond to the 40 localizer channels.
Since localizer operates in VHF and glideslope
operates in UHF, the system will have a separate
glideslope radio receiver. This receiver can be located
separately from the navigation receiver, situated within
the navigation receiver housing, or be within the course
directing indicator.
The antenna requirements differ from that of
localizer as well. The glideslope receiver requires a
much shorter antenna than localizer. In some cases, a
separate antenna is mounted on the airframe. In many
cases, a special transformer is used to couple glideslope
signals from the navigation antenna into the glideslope
receiver. Commonly, a single antenna will be used
for dual VHF navigation systems and the glideslope
receivers. The special transformer can be referred to
as a coupler, triplexer or quadraplexer, depending on
the number and types of receivers connected. It is also
possible to split the localizer and glideslope signals
inside the radio. Radios that split internally will have
a single nav antenna port, whereas radios requiring a
coupler will have separate antenna ports for glideslope
and nav.
Often, large airplanes will have multiple glideslope
antennas. One antenna will be mounted in the nose
behind the radome. This antenna will be used for initial
reception of the glideslope signal. The second antenna
will be located on or near the landing gear doors and
will be used during final approach. The two-antenna
placement is necessary because the glideslope locates
the position of the antenna, not the entire aircraft. On
large aircraft, the position of the antenna is important,
not only for visibility of the signal from the ground,
but also to keep the wheels at the appropriate altitude.
A good comparison of the need for a lower antenna
avionics news * july 2023 49
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Avionics News July 2023

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