الفهرس 
IntroductionOnly 14 pages are availabe for public view
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Abstract
The radio altimeters are considered simple frequency modulated continuous
waves (FMCW) radars that operate in C-band. Radio altimeters’ primary
function is to give aircraft height during the approach, landing, and climb
stages of an aircraft. This precise height is critical in autonomous landing.
Radio altimeters are also used in manual landings to inform the pilot when it
is time to start a ”flare maneuver,” which is executed immediately before
touchdown to minimize the ground impact on the aircraft.
The time difference between transmitting an electromagnetic signal by the
transmit antenna and receiving it back at the aircraft receive antenna after
ground reflection is measured. As long as the wave’s trajectory is vertical,
down, and up, the duration is a function of aircraft altitude.
Two distinct antennas on the aircraft’s bottom are always employed to transmit
and receive signals at the same time and that is to avoid using an internal
circulator which causes misleading altitude data readings due to the internal
leakage resulting from the continuous transmission and reception. Although
using two separate antennas is a typical solution for the FMCW radar, the
separation distance between the two antennas should be decreased as small as
possible to avoid measuring the slant distance instead of measuring the
vertical distance. However, as the separation distance between the
transmitting and receiving antennas decreases, the mutual coupling effect
between the two antennas arises. So, both transmit and receive antennas
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should be close enough while maintaining the low levels of the mutual
coupling.
The radio altimeter system’s link budget is introduced, assuming isolated
transmit and receive antennas. Then the effect of mutual coupling is included
in the radio altimeter link budget. A simple relation between the coupled and
the decoupled signal-to-noise ratio is proposed as a function of the transmitter
and receiver noise figures along with the antenna’s coupling coefficient.
The root-mean-square (rms) measurement error, which is inversely related to
the square root of the signal-to-noise ratio, is often used to estimate the radio
altimeter measurement accuracy. The effect of mutual coupling on the (rms)
measurement inaccuracy has been fully established.
According to the suggested link budget analysis, increasing the port-to-port
isolation between the closely coupled antennas decreases the receiver noise
factor, which improves measurement accuracy. As an example, raising the
isolation from 10dB to 50dB decreases the receiver noise factor by 1dB and
improves measurement accuracy by more than 11%.
The design and fabrication of a decoupling structure based on high impedance
electromagnetic structure (HIES) are introduced in this thesis. The proposed
decoupling element consists of two symmetrical rectangular loops buried into
the dielectric and tuned to provide a stopband corresponding to the operational
bandwidth (4.2 – 4.4 GHz).
The proposed decoupling element is implemented between two stacked patch
antenna arrays each one consisting of one driven patch and four parasitic
patches. This approach enables the implementation of the transmitter and
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receiver antennas in a low-profile compact and tight packing with the high
isolation necessary for accurate measurement. The proposed antenna is
backed with a full ground which is necessary to reduce the interference with
other systems.
To our knowledge this is the first time, the mutual coupling between tightly
stacked patch antennas has been reduced using a high impedance
electromagnetic structure (HIES) that does not require a large area for periodic
structure and does not require a large air gap between the antenna and the
metasurface, which is most often (λ₀/10). The proposed design also has full
grounding without the DGS or partial grounding, which is critical to the
proposed use of radio altimeter antennas as it reduces the back radiation which
can increase interference. The entire ground, on the other hand, limits antenna
bandwidth, although this is adequate for the suggested application.
The suggested structure’s prototype is made with overall dimensions of
(1.06λ₀×1.6λ₀×0.03λ₀). At the top layer, the edge-to-edge separation distance
is (0.15λ₀), and the antenna profile height is just (0.03λ₀).
The achieved isolation ranges lie between 47 and 57dB within the operational
bandwidth. The measured gain is approximately 10 dBi which is adequate for
the proposed application.