Download experiment 1

INTERPRETATION OF RESULTS
Part I. TRANSMISSION LINE DEMONSTRATION
A. Propagation in a Transmission Line
- The output voltage is generally set to zero and switched for ‘step input to A,’ (source-toload) there’s a two units amplitude and propagating it reversibly (‘step input to B’) the pulse
propagates from the load to the source.
- We raised the output voltage of the generator, we observed that the amplitude gradually
increases and decreases while there’s a constant amplitude when we haven’t raised the output
voltage.
- We computed for the wavelength, λ, using the formula λ = v/f wherein the v = 4L and the
f = 0.25s and we computed the wavelength 16L for the given line.
B. Attenuation and Dispersion
- We set the attenuation control to ‘min’ but when we gradually raise it to ‘max’ there’s a
sudden delay in the transmission line or in other words the amplitude decreases as signals
approaches to the load (preferably delay in filling amplitude units).
- The attenuation decreases as we reduce the frequency of the generator, some of the
causes of attenuation are the resistance and skin effect; conductance and dielectric losses.
- The amplitude decreases as it approaches the load because there’s no generator to
supply the frequency on transmission.
- We observed a reflected wave because we transfer the 600R terminator from ‘B’ to ‘A’
end of the line and operate the ‘step input to B’ switch.
C. Terminations, Simple Cases (matched line, open circuit, short circuit)
- When the transmission line is properly terminated, the transmitted pulse propagates from
source and the maximum power will be transferred to the load, there is no reflected wave.
- If the line is not properly terminated, some of the energy will enter the load and some will
be reflected back toward the source. The greater the mismatch between the load and the line, the
greater are the reflections on the line.
- Some of the energy traveling back toward the source will cancel the forward energy or
some will add to the forward energy.
- The standing wave at the termination B is in maxima (antinode) when the function
generator output raised to give a traveling wave of about half-scale amplitude.
- The short-circuit link is in parallel with the termination ‘B’, the standing wave is minimum
because of the 180o phase reversal of the incident and the reflected wave, thus canceling out.
Part II. STANDING WAVE AND PARTIAL REFLECTION
- Our observation on the reflected pulse, the value of our reflection coefficient in 1kOhms
is equal to 0.5 and in 200R is -0.5. It also agree with the calculated value   Z b  Z o / Z b  Z o .
- Maximum amplitude will occur in terms of the reflection coefficient when the reflected
and incident waves sum up. Add to this, if the reflection coefficient is high, the maximum voltage
is also high and maximum amplitude also occurs.
- As we properly terminated the source and the load, we observed a phase shift in the
minimum and maximum points, because one of the output is 180o out of phase.
CONCLUSIONS
- The proper use of a transmission line is to terminate it in a load impedance equal to
its surge impedance. All the power applied to the line will be absorbed by the load.
- Wavelength is the distance between adjacent peaks of an RF wave. It is also the
distance traveled by a signal in one cycle.
- The characteristics of the transmission line are determined by the length of the line,
the size of the wires and the dielectric material used between the wires.
- If a transmission line is not terminated in its characteristic impedance, the load will
not absorb all the power, some of it will be reflected back to the generator.
- If the load on the transmission line is open or short, all the power applied to the line
will be reflected back to the generator.
- The forward or incident power applied to the line combines with the reflected power
to produce a pattern of voltage and current variations along the line known as standing
waves.
- If the load impedance matches the line impedance, there are no standing waves.
- A measure of the mismatch between line and the load impedances or the maximum
and minimum voltage and current variations along the line is the standing wave ratio (SWR)
which is a number always greater than 1.
- In a balanced transmission line, the energy in each wire or line is 180o out-of-phase
with the opposite line and hence the fields of the two lines cancel.
-In an unbalanced transmission line, one line is at ground potential while the other line
conducts RF energy.
- The SWR indicates how much power is delivered to the load and lost in the line. With
SWR =1, all power is delivered to the load.