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Further objectives are categories IIIb and IIIc with RVRs more than 50M and
less than 50M respectively, operation with no decision height and independent
of external visual reference.
[3] The VOR/ILS antenna system is composed of a VOR/LOC antenna, a
glideslope antenna, and a localizer antenna, The VOR/LOC antenna is designed
to receive very high frequency radio signals between the frequencies of 108.00
and 118.00 megahertz. This antenna is a dual element antenna with a
characteristic impedance of 50 ohms. It is composed of two balanced loops
enclosed in a fiberglass housing with metallic fin tips on the leading and trailing
edges and the whole assembly is located on top of the vertical fin. A lightning
diverter strip is embedded along the top edge of the housing for protection .
Fig.27-2 VOR/LOC antenna
[4] The localizer antenna is designed to receive very high frequency radio
signals between 108 and 112 MHz. This antenna is horizontally polarized and
has a characteristic impedance of 50 ohms. It is a dual-element antenna with two
female connectors. Two RF attenuators are installed in the electronic equipment
compartment. The attenuators are connected between the localizer antennas and
the receivers to reduce electromagnetic
interference. The attenuators are mounted
directly to the ILS transfer relays. The ILS
transfer relays switch the navigation unit
VOR/LOC rf input from the fin tip
VOR/LOC antenna to the nose radome
localizer antenna.
Fig27-3 LOC antenna
[5] The glideslope antenna is designed to receive ultra high frequency radio
signals between 329.0 and 335.0 megahertz. The antenna is a single,
horizontally-polarized unit and has a characteristic impedance of 50 ohms.There
are two coaxial connector for routing the incoming rf signals to the VHF
navigation units .The glide slope director bar
is used to alter the glide slope radiation
antenna patterns such that the VHF NAV
receivers have maximum glide slope
sensitivity.
[6] Figure 27-5 illustrates a localizer
transmission field pattern. The localizer signal
is transmitted on frequencies in the range of
108.10 to 111.95 MHz（odd tenths). (odd
tenths means odd tenth frequency channel,
channel spacing is 50 KHz, such as 110.50,
110.55……111.30,111.35 MHz etc. The first
digit after decimal point is a odd number.)
Fig. 27-4 G/S antenna
Fig. 27-5 Localizer theory
[7] The principle of localizer operation involves a comparison of 2 transmitted
signals: one modulated at 90Hz, and the other modulated at 150Hz. These
signals are transmitted to produce two directional overlapping lobes, one on
either side of the runway centerline. The left lobe is modulated with the 90 Hz
signal and the right lobe with 150Hz. The on-course path is the area where the
two signals are of equal strength. It coincides with the centerline of the runway.
When the airplane position is to the left of the on-course path, the 90Hz signal is
stronger than the 150 Hz signal and the localizer deviation indicators are
deflected to the right, indicating that the runway centerline is to the right. When
the airplane position is to the right of the on-course path, the 150 Hz signal is
stronger than the 90Hz signal and the localizer deviation pointers deflect to the
left. On a flight path one degree left or right of the on-course path, the receiver
output is standardized at 75 microamperes which causes a localizer deviation
bar deflection of one dot.
[8] The glide slope signal is transmitted on UHF channels on frequencies in the
range of 329.3 to 335.0 MHz. Glide slope receiving circuits are tuned
automatically whenever a localizer frequency is selected.
Fig. 27-6 glide slope theory
As illustrated in figure 27-6, the glide slope field pattern (lobes) is very similar
to that of the localizer, the essential difference is that the transmission is rotated
through 90 ,so that the 90Hz and 150Hz signals are displaced from each other
vertically rather than horizontally. The upper lobe is modulated with 90Hz, the
lower lobe with 150Hz. The desired glide path is the line where both signals are
of equal strength.
[9] When the airplane position is above the glide slope, the 90Hz signal is
stronger than the 150Hz signal and the glide slope pointers are deflected down
indicating that the glide slope is below the airplane. When the airplane is below
the glide slope, the 150Hz signal is strongest and the glide slope pointers deflect
upward indicating that the glide slope is above the airplane. On flight paths 0.35
degree above or below the glide slope, the receiver output is standardized at 75
micro amperes which produces one dot of glide slope needle deflcetion.
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