Ham Radio Exam Preparation 4

What is the power of a device running at a voltage of 13.8 volts DC using a current of 10 amperes?
a) 138 watts
b) 0.7 watts
c) 23.8 watts
d) 3.8 watts

In DC circuits, power is the product of voltage or potential difference and the current flowing through the circuit.

In case of alternating current circuit, difference will be there depending on whether the load is purely resistive or having a reactive component like capacitance or inductance. That is because when there is a reactive component in the circuit, current and voltage are not in phase, meaning that they do not rise or fall in the same pattern. Capacitance blocks direct current once the capacitor is charged. Inductance opposes the rise in current in the initial phase after the circuit is switched on. In AC circuits, power consumed by a device may not be equal to the product of current and voltage across it. For example if a transformer is connected to an AC voltage, and the output is connected to a load, power consumed by the transformer will be usually very low compared to that consumed by the load.

What type of capacitor is often used in power-supply circuits to filter the rectified AC?
a) Ceramic
b) Tantalum
c) Mica
d) Electrolytic

Filtration of ripples in the rectified direct current output of rectifiers need high value capacitors, typically 1000, 2000 or more microfarad electrolytic capacitors. Larger the value of capacitor, better the smoothing of the waveform to near perfect DC level. If filtering is not good, it will produce a hum in audio amplifier circuits. Frequency of hum will depend on the frequency of the alternating current and the type of rectifier circuit used. Half wave rectifier produces a hum at the frequency of the input AC, while full wave rectifier produces a hum at double the frequency. In addition to capacitors, high value inductors may also be used in power supplies to give better filtering of AC ripples.

The core of power supply transformer is laminated to
a) Decrease impedance
b) Increase impedance
c) Decrease eddy current losses
d) None of these

Lamination decreases eddy currents and decrease loss of energy and efficiency due to heating up the core.

When frequency of a carrier is varied according to modulation the result is
a) Frequency modulation
b) Amplitude modulation
c) Product detection
d) None of these

Here is an animated diagram from Wikipedia illustrating the difference between radio waves modulated by amplitude modulation and by frequency modulation. Vertical axis represents the amplitude of the signal, which could be voltage or current. Horizontal axis is time. Top graph is the modulating signal, a simple sine wave in this case. For an audio signal, there will be multiple such waves at different frequencies in the audio frequency range. Middle graph shows amplitude modulation in which amplitude of the radiofrequency wave changes according to the signal frequency. In the last tracing, the frequency of the radio wave varies according to the modulating signal, in frequency modulation. Handheld radios in VHF/UHF range usually use frequency modulation.

Amplitude modulation is the usual mode used by broadcast stations. Amateur radio HF transceivers usually use amplitude modulation and then filter out the carrier and one side band to produce single side band or SSB signals.

Which among the following is NOT a directional antenna?

a) Whip antenna
b) Yagi-Uda antenna
c) Loop antenna
d) Dish antenna

Whip antenna is an omnidirectional antenna which radiates all around. Yagi-Uda antenna radiates towards the direction of the directors. Loop antenna has maximum signals along the plane of the loop and a null perpendicular to it. Dish antenna radiates and receives in the direction of its focal point.