KNEC paper: Diploma in Telecommunication Engineering, Communication Systems, paper 2203/306, June/July 2023 (3 hours). The paper says to answer any five of eight questions. This lesson provides independent revision answers to all eight; it is not an official KNEC marking scheme.
Numerical answers show the formula, substitution and result on separate display lines so they remain easy to follow on desktop and mobile.
Question 1 — Demodulation, AM and SSB
(a) Demodulation and receiver circuits
Demodulation recovers the original message or information signal from a modulated carrier at the receiver. Two common circuits are a diode envelope detector for AM and a product (synchronous) detector for suppressed-carrier AM. FM receivers use a discriminator or ratio detector.
(b) AM transmission efficiency
For a single-tone AM wave, the carrier power is Pc. The two sidebands together carry:
Total transmitted power is carrier power plus sideband power:
Therefore the fraction of transmitted power carrying information is:
(c) DSB-AM powers
Unmodulated carrier power is 150 kW and modulated total power is 180 kW.
(d) Phase-shift SSB transmitter and advantages
In the phase-shift method, audio and carrier signals are each split into two paths 90° apart. Two balanced modulators create DSB-SC signals; combining them with the appropriate polarity cancels one sideband and reinforces the other.
| Audio input | → | 0° / 90° audio phase splitter | → | Two balanced modulators |
| Carrier oscillator | → | 0° / 90° carrier splitter | → | Combiner ± → selected USB or LSB → RF power amplifier |
Compared with DSB, SSB uses about half the bandwidth and avoids wasting transmitter power on a carrier and a redundant sideband. It permits more channels in a band and can improve performance where adjacent-channel interference is a concern.
Question 2 — Television transmission
(a) Digital TV advantages
- Digital signals are more resistant to noise and can be regenerated without cumulative picture degradation.
- Compression and multiplexing allow efficient use of spectrum and more services per channel.
- Error correction and digital processing support reliable reception and features such as subtitles, guides and interactive services.
(b) Colour mixing and interactive TV
For additive RGB colour mixing:
| Primary colours | Resulting secondary colour |
|---|---|
| Red + Green | Yellow |
| Green + Blue | Cyan |
| Blue + Red | Magenta |
Interactive television adds a return path so viewers can respond to or control a service, for example choosing on-demand programmes, voting, using menus or requesting information through an internet, cable or telephone connection.
(c) Colour television receiver sections
RF tuner: selects the wanted broadcast channel, rejects unwanted adjacent channels, amplifies the signal and converts it to the receiver’s intermediate frequency.
Video section: demodulates the video IF to recover the composite video signal, separates luminance, chrominance and synchronisation information, then supplies the display processing stages.
(d) 625-line, 25-frame television standard
With two interlaced fields per frame:
Question 3 — Transmission lines and FM
(a) Microstrip transmission-line merits
Microstrip lines are compact and lightweight, inexpensive to manufacture on printed-circuit substrates, and easy to integrate with microwave components and planar circuits.
(b) Direct FM generation
A direct FM generator varies an oscillator’s resonant frequency with the message signal. A reactance modulator or varactor changes the effective capacitance of the LC oscillator, so the instantaneous frequency follows the input. A buffer isolates the oscillator from load changes.
| Message signal | → | Reactance modulator / varactor | ↔ | LC RF oscillator | → | Buffer / amplifier | → | FM output |
(c) Lossless line
Given L=0.5 mH/km, C=0.12 μF/km and f=400 kHz, with R=G=0:
The scan labels part (iii) “phase delay” but gives no line length. The phase constant is β above; the phase delay through a line of length ℓ km is βℓ radians, and the corresponding time delay is 7.746ℓ μs.
(d) Fading
Two common types are flat fading, which affects a signal’s frequency components similarly, and frequency-selective fading, which affects different components differently because of multipath delay spread. Slow and fast fading are another valid pair.
Question 4 — Waveguides and parametric amplifiers
(a) Waveguide applications
Waveguides carry microwave energy in radar and satellite ground stations, feed high-frequency antenna systems, and guide energy in microwave heating equipment such as microwave ovens.
(b) Parametric amplifier
A parametric amplifier uses a nonlinear reactance, commonly a varactor diode, driven by a strong pump signal. The pump varies the reactance and transfers energy to a weaker input signal at a resonant circuit; an idler frequency is also produced. Filters separate the amplified signal from the pump and idler.
| Weak RF input | → | Input / idler tuning network | → | Varactor nonlinear reactance | → | Output filter | → | Amplified RF |
| Pump oscillator → coupling network → varactor (supplies the energy for amplification) | ||||||||
(c) TE10 guide quantities
The scan gives cutoff wavelength λc=6 cm and operating frequency 6 GHz. The first requested quantity repeats the given cutoff wavelength, so it is stated explicitly below.
Question 5 — Satellite communication and noise
(a) Kepler’s laws
- Each satellite moves in an elliptical orbit with the central body at one focus.
- The line joining the satellite to the central body sweeps out equal areas in equal times.
- The square of the orbital period is proportional to the cube of the orbit’s semi-major axis.
(b) Satellite frequency bands
| Satellite band | General frequency range |
|---|---|
| C-band | 4–8 GHz |
| Ka-band | 26–40 GHz |
| Ku-band | 12–18 GHz |
Satellite services use narrower allocations within these general band ranges, and uplink and downlink allocations differ.
(c) Satellite uplink subsystem
| Information source | → | Encoding / multiplexing | → | IF modulator | → | Up-converter | → | High-power amplifier | → | Feed horn / dish | → | Satellite |
The encoder and multiplexer prepare the baseband signal; the modulator places it on an IF carrier; the up-converter translates it to the assigned uplink frequency; the high-power amplifier raises the RF level; and the feed and dish direct the beam toward the satellite.
(d) Thermal and shot noise
Thermal noise is caused by random thermal motion of charge carriers in resistive components. It is approximately white over a useful bandwidth.
Shot noise is caused by the discrete, random arrival of charge carriers across a junction or barrier.
Question 6 — Radar
(a) Radar terms
Blind speed: a target’s radial speed at which the Doppler shift is an integer multiple of the pulse repetition frequency, causing an MTI radar to cancel the target return.
Duty cycle: the fraction of each pulse repetition interval for which the transmitter is on.
Maximum unambiguous range: the greatest range whose echo returns before the next transmitted pulse.
(b) Pulsed radar operation
The printed wording appears to combine “continuous wave” and “pulsed” radar. The sequence below explains the pulsed radar implied by the pulse timing and range measurement.
| Synchroniser | → | Pulse modulator | → | RF transmitter | → | Duplexer / antenna | → | Target |
| Target echo → antenna / duplexer → receiver → detector / processor → range display | ||||||||
The synchroniser establishes the pulse repetition timing. The modulator keys the transmitter into short RF bursts. The duplexer connects the shared antenna to the transmitter during transmission and to the sensitive receiver during the listening interval. Echo delay gives target range.
(c) Minimum received radar power
For a monostatic radar using the same antenna for transmission and reception, with effective capture area Ae=6 m²:
(d) Doppler effect
The Doppler effect is the change in received frequency caused by relative radial motion. A target approaching the radar produces an upward shift; a receding target produces a downward shift. Radar uses the shift to estimate radial velocity and to distinguish moving targets from stationary clutter.
Question 7 — Antennas
(a) Antenna definitions
Isotropic antenna: an ideal reference antenna that radiates equal power in every direction.
Free-space impedance: the ratio of electric-field strength to magnetic-field strength for a uniform plane wave in free space.
Effective length: the ratio of the open-circuit voltage induced at an antenna terminal to the incident electric-field strength.
(b) Dish illumination methods
| Method | Feed arrangement and beam path |
|---|---|
| Front feed | Feed horn at the parabolic reflector’s focus → reflected parallel beam. |
| Cassegrain | Feed near the dish vertex → hyperbolic subreflector near the focus → main parabolic reflector → parallel beam. |
(c) Rhombic antenna construction
A rhombic antenna uses four long wires arranged as a diamond and supported above ground. A balanced feed line drives one end. The opposite end is terminated through a resistor to ground, reducing the backward wave and making the antenna directional along its long axis. It is commonly used for high-frequency long-distance links.
| Balanced feed | → | Two upper wire legs form a diamond | → | Two lower wire legs | → | Termination resistor to ground |
(d) Short monopole radiation
Assume the 25 m vertical monopole has a linear (triangular) current distribution from 100 A RMS at its base to zero at its tip, over an ideal ground plane.
If 100 A is a peak value rather than RMS, the radiated power is half this result and the RMS field is lower by √2.
Question 8 — Sky-wave propagation and receiver noise
(a) Sky-wave terms
Virtual height: the apparent height at which a sky wave would have reflected if it had travelled in a straight line at the same measured delay.
Critical frequency: the highest frequency that a particular ionospheric layer can return to Earth for vertical incidence.
Skip distance: the shortest ground distance from a transmitter at which a sky wave returns to Earth; a coverage gap may exist between it and the ground-wave range.
(b) Tropospheric scatter
Two stations point narrow microwave beams toward a common volume in the lower atmosphere beyond the radio horizon. Small, irregular changes in refractive index scatter a small portion of the energy forward; a high-gain receiving antenna collects that weak scattered signal. The system supports beyond-horizon links, but requires sensitive receivers and high-gain antennas.
| Transmitter / dish | ↗ | Common tropospheric scatter volume | ↘ | Receiver / dish beyond the horizon |
(c) Two cascaded receiver stages
Let F1 and F2 be linear noise factors and G1 the available power gain of stage 1. Refer stage 2’s added noise back to the input of stage 1:
Use linear noise factors and gains in this equation; convert decibel values to linear ratios first. G2 does not appear in the two-stage expression because it amplifies both the signal and noise already present at the second-stage input.
Revision note: The solutions above are independently prepared for study and should be checked against your course notes and lecturer guidance.