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AimThe objective of this experiment is to investigate
the behaviour of a coaxial cable connected to three different termination;
short-circuit, open-circuit and matched load. The simulation is done by the aid
of Computer Simulation Technology (CST). It is then analysed the EM behaviour
of the coaxial cable and compared to one another. ProcedureThe coaxial cable is modelled to be a 50? cable by
the help of the equation below.

Component

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Material

Core
(R1)

0.003

Copper
(pure)

Dielectric
(R2)

0.01

Polytetrafluoroethylene
(PTFE)

Outer
Conductor

0.012

Copper (pure)

Table
1: Modelled Coaxial Cable
Specifications

where Z0 is the characteristic impedance
in ?,  is permittivity is vacuum, R2 is the radius of
the dielectric, and R1 is the radius of the core.By substituting Z0 to 50, the
ratio of the R2:R1 is found to be approximately 10:3. Hence, the coaxial cable
is modelled as data shown in Table 1 with the
length of 1 meter. The excitation signal is fed to one end of the coaxial
cable with a pulse of 40ns width and 300ns period. To make it more realistic,
1ns time rise and fall is added to the pulse wave. The experiment is proceeded by using different type
of termination, by changing the boundaries in the Simulation Tab. The results
are observed, recorded and compared to one another. Result &
Discussiona)      Short-circuit
terminationObserving from the ‘Port Signals’, the reflected
wave has the same magnitude with incident wave, but the reflected wave has 180
phase shifts. However, there is a time delay between the signals as the pulse
needed to travel to the end and be reflected before coming back to the input.
The distance travelled by the pulse is double the length of the wire. Combining
this information with relative permittivity of PTFE, the time delay can be
calculated using formula belowwhere l
is the length of the wire, c is the
speed of light,  is the time delay and is the relative
permittivity of PTFE which is 2.1Based on distance travelled by the pulse
which is 2 meters in total, the time constant is found to be approximately 9.7
nanoseconds. Thus, this comply with the graph resulted from the simulation. b)     Open-circuit
terminationObserving from the ‘Port Signals’, the
amplitude of both incident wave and reflected wave are the same. The reflected
wave has time delay for it to return to the input. The same formula as stated
before can be used to validate the length of the wire and validate the
dielectric constant. c)      Matched
based on the physical specification of the coaxial cable, the impedance is not
exactly 50 ?. Hence, when the coaxial cable is simulated with 50 ? load, there
is still slightly reflected wave with a small amplitude that is relatively low since
the load is not fully absorb the signal. Time delay of the signal can be
calculated using the same formula as stated above.  d)     Further
analysisAs
observed with all three terminations, the S-Parameters give more insight about the
characteristics of each termination. The S-Parameters graph of the matched
loaded has values of below 0 dB while for open-circuit and short-circuit
terminations have an approximate straight line at 0 dB. This means that the
matched load termination has barely reflected pulse while open and short
circuit terminations have.  As EM is
induced to the model, the simulation is observed and found that for both
E-field and H-field have standing waves occurs at all frequency in open and
short circuit terminations. However, for short circuit termination, the H-field
is maximum and E-field is minimum at termination. Meanwhile, for open circuit
termination is vice versa.On the other
side, for matched load, there is no standing wave in the propagation through
E-field and H-field simulations. The wave propagates from the input to the
termination port. This makes full power transferred to the load. The
experiment is furthered by changing the length of the cable of 1 meter and 15
meters. Result found that from the S-parameters phase plot, there is 2? phase
difference at 0.1 GHz and occur every 15 oscillations which concludes as length
of the wire increases, the rate of change of phase increases. Lastly, the
experiment is done on changing the physical properties of the cable, changing
the ratio of the radiuses of the cable. The result found that the impedance
produced by the simulated cable is different but has the same properties when
tested with open, short and matched loaded terminations as tested earlier. e)     ApplicationCoaxial
cables are used to transmit radio frequency signals such the Internet as the
cable Ethernet. The design of coaxial cable allow impedance matching gives the
most power transfer of a certain frequency thus allow low transmission loss and
also prevent external interference by having the outer shield layer. Conclusion

Computer Simulation Technology software allows a
better understanding and visualization of the real transmission line works.
From all three coaxial cable terminations, all the parameters can be analysed
to depict the real-world simulation. Even though most of the assumptions are
ideal, the simulation gives a bigger picture to the engineers or manufacturer
for transmission lines, specifically coaxial cable in this experiment.

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