Ham Radio Exam Preparation

Which layer of ionosphere disappears during nighttime?
a) F
b) E
c) D
d) C

These are the layers of ionosphere, within the atmosphere, by day time and night time. Lower most is the D layer. D layer is the one which absorbs medium waves, so that medium waves will not get reflected by the ionosphere during day time, and propagation is by ground wave propagation. But at night what happens is that, the D layer disappears, due to decreased ionization. Then medium waves can penetrate up to the E layer and get reflected. That is why medium wave stations can be heard at a longer distance at night. E layer is the one which reflects the short waves usually, because short waves will not be absorbed by the D layer and they will be reflected by the E layer. Still higher frequencies in the short wave, will go beyond the E layer, and get reflected from the F1 or F2 layer. The difference between F1 and F2 is that, when there is higher ionization during day time, F layer splits into two, F1 and F2. At night, when the ionization decreases, these two layers fuse together and there is only one F layer at night. Long distance amateur radio communication depends mainly on short waves reflected by the ionosphere.

Line of sight propagation is the mode of communication in
a) Low Frequency
b) High Frequency
c) Medium Frequency
d) Very High Frequency

Low Frequency or LF band is from 30 to 300 kHz
Medium Frequency or MF band is from 300 kHz to 3 MHz
High Frequency or HF band is from 3 to 30 MHz
Very High Frequency or VHF band is from 30 to 300 MHz

Higher bands like VHF/UHF and above have line of sight propagation, meaning that it will not be heard beyond the horizon where the Earth’s curvature sends the signal off to space! But there is some difference between actual radio horizon and visual horizon. Usually radio horizon is a bit beyond the visual horizon. In lower bands like LF and MF, ground wave propagation by waves bouncing off the ground can take signals beyond the horizon. In case of HF band reflection from the ionosphere can take signals very far around the globe. VHF/UHF penetrates the ionosphere and goes into outer space, without getting reflected from the ionosphere. That is why only line of sight communication is possible without the aid of a repeater or satellite. Long distance communication through satellites is possible with VHF/UHF as the satellite receives the uplink signals from Earth stations and retransmits back to Earth on different frequency known as the downlink.

Which of the following is an advantage of a Class C amplifier?

a)High efficiency

b)Linear operation

c)No need for tuned circuits

d)All of these answers are correct

Conduction angle of class C amplifier is less than 180°, meaning that less than half of the input signal is used. This leads to high distortion and requires a tuned circuit as load. It is suitable for radiofrequency transmitters operating at a single fixed frequency. Efficiency is as high as 80% in such radiofrequency applications. Diagram shows the output waveform in the absence of a tuned circuit. When a tuned circuit is used in the output, it restores the waveform to its proper shape. The resistor shown in the diagram is replaced by a parallel-tuned inductor capacitor combination which resonates at the operating frequency. Power transfer from the inductor can be done using a secondary coil wound on the inductor. Average voltage at the collector is equal to the supply voltage and variation in signal voltage can be between zero and twice the supply voltage during the radiofrequency signal cycle. Bias voltage of the transistor is adjusted so that it conducts only for about one third of the radiofrequency cycle (120°) or less.