Chapter 2 Principles of radio
transmission
2.1. AM Transmitter
2.2. FM Transmitter
2.3. Wavebands
Transfer of information (speech, music, image, computer data etc.) by
radio can be presented in its simplest form with block - diagram as on
Pic.2.1. That is a transmission realized by amplitude - modulated signal.
Since, in our example, the information being transferred is the sound, the
first step of such transmission is converting the sound into electrical
signal, this being accomplished by a microphone. The low - frequency (LF)
voltage at microphone output (Pic.2.1-a), that represents the electrical
"image" of the sound being transferred, is being taken into the transmitter.
There, under the effect of LF signal, the procedure called amplitude
modulation is being carried out, and on its output high - frequency (HF)
voltage is generated, its amplitude changing according to the current LF
signal value. HF voltage creates HF current in the antenna, thus generating
electromagnetic field around it. This field spreads through the ambient
space, being symbolically shown on Pic.2.1 with dashed circles. Traveling at
the speed of light (c=300 000 km/s), the electromagnetic field gets to the
reception place, inducing the voltage in the reception antenna, as shown on
Pic.2.1-c. This voltage has the same profile as the one on Pic.2.1-b, except
it has much smaller amplitude. In the receiver, the amplification and
detection are carried out first, resulting with the LF voltage on its
output, that has the same profile as the one on Pic.2.1-a. This voltage is
then transformed into sound by loudspeaker, that sound being exactly the
same as the sound that acted upon the microphone. This, naturally, is the
way it would be in ideal case. Back to reality, due to device imperfection
as well as the influence of various disturbances, the sound being generated
by the loudspeaker differs from the one that acts upon the microphone
membrane. The block - diagram on Pic.2.1 (excluding the HF signal shape) is
also applicable in case of radio transmission being carried out by frequency
modulation. In that case frequency modulation is being carried out in the
transmitter, under the effect of LF signal coming from the microphone,
therefore HF signals on Pics.2.1-b and 2.1-c having constant amplitude, and
their frequency being changed in accordance with the actual value of LF
signal from the microphone. In fact, all types of radio transmission can be
presented with Pic.2.1. First, the information being sent is always
transformed into electrical signal through the appropriate converter. In
telegraphy this converter is the pushbutton, in radiophony it's a
microphone, in television engineering an image analysis cathode ray tube
(CRT) etc. Then, with this "electrical image" of information, the modulation
is being done. The modulated HF signal is being transferred into antenna and
transmitted. On the reception place, the modulated signal from the reception
antenna is being amplified and detected and then, again with the appropriate
converter (pen recorder, loudspeaker, TV CRT etc.), the information is
transformed back into its original form.

2.1 AM Transmitter
In order to better understand the way the
radio transmitter works, block - diagram of a simple AM (amplitude
modulated) signal transmitter is shown on Pic.2.2. The amplitude modulation
is being performed in a stage called the modulator. Two signals are entering
it: high frequency signal called the carrier (or the signal carrier), being
created into the HF oscillator and amplified in the HF amplifier to the
required signal level, and the low frequency (modulating) signal coming from
the microphone or some other LF signal source (cassette player, record
player, CD player etc.), being amplified in the LF amplifier. On modulator's
output the amplitude modulated signal UAM is acquired. This signal is then
amplified in the power amplifier, and then led to the emission antenna.

The shape and characteristics of the AM carrier, being taken from the HF
amplifier into the modulator, are shown on Pic.2.3-a. As you can see, it is
a HF voltage of constant amplitude US and frequency fS. On Pic.2.3-b the LF
signal that appears at the input of the modulator at the moment t0 is shown.
With this signal the modulation of the carrier's amplitude is being
performed, therefore it is being called the modulating signal. The shape of
the AM signal exiting the modulator is shown on Pic.2.3-c. From the point t0
this voltage has the same shape as that on Pic.2.3-a. From the moment t0 the
amplitude of AM signal is being changed in accordance with the current value
of the modulating signal, in such a way that the signal envelope (fictive
line connecting the voltage peaks) has the same shape as the modulating
signal.
Let's take a look at a practical example. Let the LF signal on Pic.2.3-b be,
say, an electrical image of the tone being created by some musical
instrument, and that the time gap between the points t0 and t2 is 1 ms.
Suppose that carrier frequency is fS=1 MHz (approximately the frequency of
radio Kladovo, exact value is 999 kHz). In that case, in period from t0 till
t2 signals us on Pic.2.3-1 and uAM on 2.3-c should make a thousand
oscillations and not just eighteen, as shown in the picture. Then It is
clear that it isn't possible to draw a realistic picture, since all the
lines would connect into a dark spot. The true picture of AM signal from
this example is given on Pic.2.3-d. That is the picture that appears on
screen of the oscilloscope, connected on the output of the modulator: light
coloured lines representing the AM signal have interconnected, since they
are thicker than the gap between them.
Block - diagram on Pic 2.2 is a simplified schematic of an AM transmitter.
In reality there are some additional stages in professional transmitters
that provide the necessary work stability, transmitter power supply, cooling
for certain stages etc. For simple use, however, even simpler block diagrams
exist, making the completion of an ordinary AM transmitter possible with
just a few electronic components.

2.2. FM Transmitter
Block diagram of an FM (frequency
modulated) transmitter is given on Pic.2.4. Information being transferred,
i.e. the modulating signal, is a signal from some LF source. it is being
amplified in LF amplifier and then led into the HF oscillator, where the
carrier signal is being created. The carrier is a HF voltage of constant
amplitude, whose frequency is, in the absence of modulating signal, equal to
the transmitter's carrier frequency fS. In the oscillatory circuit of the HF
oscillator a varicap (capacitive) diode is located. It is a diode whose
capacitance depends upon the voltage between its ends, so when being exposed
to LF voltage, its capacitance is changing in accordance with this voltage.
Due to that frequency of the oscillator is also changing, i.e. the frequency
modulation is being obtained. The FM signal from the HF oscillator is being
proceeded to the power amplifier that provides the necessary output power of
the transmission signal.
Voltage shapes in FM transmitter are given on Pic.2.5. Pic.2.5-a shows the
LF modulating signal. The frequency modulation begins at moment t0 and the
transmission frequency begins to change, as shown on Pic.2.5-b: Whilst
current value of the LF signal is raising so is the trasmitter frequency,
and when it is falling the frequency is also falling. As seen on Pic.2.5-c,
the information (LF signal) is being implied in frequency change of the
carrier.
The carrier frequencies of the radio difusion FM transmitters (that emmit
the program for "broad audience") are placed in the waveband from 88 MHz til
108 MHz, the maximum frequency shift of the transmitter (during the
modulation) being ±75 kHz. Because of that the FM signal should be drawn
much "thicker", but it would result in a black-square-shaped picture.

2.3. Wavebands
While considering problems related to the
realization of the long - distance radio links, significant differences
between the electromagnetic waves of various frequencies must be kept in
mind. For example, low frequency waves (below 500 kHz) can bend themselves
following Earth's curvature, while the HF waves are moving in streamlines,
just as light. Some waves can be reverberated from the ionosphere, others
are passing through it etc. According to characteristics of their outspread,
radio waves can be classified into several groups or ranges: long, mid,
short and ultra-short. Limits between the wavebands are not precise, with
the raise of their frequency the waves are gradually losing some features
while gaining some others. This division is shown in Table 1.

* LF low frequencies, MF mid frequencies, HF high frequencies, VHF very high
frequencies, UHF ultra high frequencies, SHF super high frequencies, EHF
extra high frequencies. Waves with wavelength smaller than 30 cm are also
called the microwaves.
In the third table column the
wavelengths are given. Wavelength (ë) is distance that the wave passes
moving at the speed of light (c=3*103 m/s), during the period that is equal
to its oscillating period (T): ë=c*T. Having in mind that the wave frequency
is f=1/T, one can easily get to the well known expression that gives the
relation between the wavelength and the frequency:

Using this formula one can calculate the wavelength knowing the frequency
and vice versa. For example, wavelength of an FM transmitter emitting at
f=100 MHz frequency is L=3*108/100*106=3 m. Similar to that, wavelength of
Radio Belgrade 1 is L=439 m, which makes its frequency equal to
f=3*108/439=684 kHz.
Radio diffusion is being performed in certain parts of the wavebands given
in Table 1, their boundary frequencies are (rounded values):
LW (long waves) 150 kHz (2km) 300 kHz (1 km)
MW (mid waves) 500 kHz (600 m) 1500 kHz (200 m)
SW (short waves) 6 MHz (50 m) 20 MHz (15 m)
FM (ultra short waves) 88 MHz (3.4 m) 108 MHz (2.78 m)
In LW, MW and SW the amplitude modulation is used, while in FM range it is
the frequency modulation.
Here are the frequencies (in kHz) of some
radio transmitters from the MW range, that can serve for tuning of the radio
receivers being described in this issue: Timisoara 630, Belgrade1 684,
Bucharest 855 . |