How does a receiver work?
How do radio receivers manage to communicate over huge
distances? Here is a description of the fundamental mechanism
used. The principle of the receiver is the same whatever the way
you communicate; radio waves, sound waves, optical
communications... It is udes when the signal the receiver gets is
very weak or when a very high reliability is needed.
- The signal/noise ratio
- The precision of the clocks
- Accumulator atenuation
- The super-heterodyne receiver
- Common ways to enhance communications
- A practical software example
Suppose I have a little device capable of emitting a beep
sound. It's just a little box, with a loudspeaker and an on/off
switch. When you turn the switch on, the device emits a continuous
beep sound. When you turn the switch off, the device becomes silent.
This device is an emitter
Secondly, suppose I have another little device capable of hearing
the beep sound. It is also a little box, with a microphone and a
lamp. When the microphone hears a beep sound, the lamp glows. When
the microphone hears no beep sound, the lamp stays dark. This device
is a receiver
You can play with those two little devices as much as you want:
When you turn the emitter switch on, the lamp of the receiver begins
to glow. When you turn the switch of the emitter off, the lamp of
the receiver darkens. And so on.
If the receiver has been build a basic way, then the distance
over which the communication works will be a few meters or a few
tens of meters:
If you put the receiver, say 50 meters away of the emitter, then
there will be no more communication. When you turn the switch of the
emitter on, the lamp of the receiver will not begin to glow. It will
But, if you build a long distance receiver
, then the
distance may be a lot more than 50 meters:
Suppose you are in a noisy city. You put the emitter somewhere, then
walk away. At a few tens of meters distance your ears no more hear
the sound of the emitter. But, after 1 kilometre walking away, the
long distance receiver still manages to hear the sound of the
emitter. Amazingly, the communication works: when the switch of the
emitter is pushed to on; the lamp of the receiver begins to glow.
And when the switch of the emitter is pulled to off, the lamp of the
receiver darkens. At 1 kilometre distance! That's magic.
There is only one drawback: the communication is now rather slow.
When the switch of the emitter is pulled to on, you have to wait 1
minute until the lamp of the receiver begins to glow. And when you
push the switch of the emitter to off, you have to wait again 1
minute till the lamp of the receiver darkens.
It is thanks to this technology that communication with space probes
going outside the solar system has been possible.
Our purpose is thus now to explain how this little miracle works.
2. The signal/noise ratio
Suppose we take a microphone and connect it to an
oscilloscope tuned appropriately. (If you don't have an oscilloscope
at hand, you may use a PC with a sound card and a sound recording
The emitter is put a few centimetres away from the microphone.
When the emitter is switched off, the oscilloscope will show a
straight line, no signal:
When the emitter is switched on, the oscilloscope will show a sine
So, a deaf person will be able to tell wether the emitter is
switched on or off. Just by looking at the screen of the
Suppose now we put the emitter two times further from the
When the emitter is on, the signal shown by the oscilloscope will be
two times weaker:
So, in order to still see the signal clearly, we will increase the
amplification of the oscilloscope to make it show the sine wave
again at a correct size:
No problem. The more we put the emitter further from the microphone,
the more we ask the oscilloscope to amplify the signal. That way the
signal remains clearly visible, whatever the distance.
Well, in fact this is not true. Once we have put the emitter, say 5
meters away from the microphone and the amplification has become
relatively important, we see the noise
This is what the oscilloscope shows when the emitter is off:
And this is what it shows when the emitter is on:
The emitter may be on or off, it makes no difference. The noise
remains the same. When the emitter is on, the sine wave simply add
itself to the noise.
Noise is something you cannot avoid. Whatever you measure, if you
amplificate it enough, you always get a noise. In fact it was there
since the beginning, but it was so weak that we didn't notice it. It
became visible once we amplificated it enough.
Let's put the emitter again two times further. We have to increase
two times the amplification. Now the noise has the same amplitude as
the sine wave:
From now on, when we will put the emitter further, we will no more
increase the amplification. Because the noise fills the screen of
the oscilloscope. Any increase in amplification would be useless: it
would simply make the noise go outside the boundaries of the screen
of the oscilloscope.
This is what appears when we put the emitter 10 meters away:
This is what appears when we put the emitter 20 meters away:
And this appears when we put the emitter 40 meters away:
We are no more able to see the sine wave. (You may have the
impression to see pieces of sine wave, but it is only an illusion.)
The emitter may be on or off, the oscilloscope will show exactly the
same thing: a noise.
Thus, at a distance of 40 meters our system does not work any more.
A deaf person can no more use the oscilloscope to tell wether the
emitter is on or off.
The key is the intensity of the signal versus the intensity of the
noise. That's why we use the concept of signal/noise ratio.
The signal/noise ratio is a number. You obtain this number by
dividing the number of the measure of the intensity of the signal by
the number of the measure of the intensity of the noise.
When the emitter was at a distance of 5 meters, the intensity of the
signal was 1 and the intensity of the noise was 0.5. The
signal/noise ratio was thus 2.
When the emitter was at a distance of 10 meters, the intensity of
the signal was 1 and the intensity of the noise was 1. The
signal/noise ratio was thus 1.
When the emitter was at a distance of 40 meters, the intensity of
the signal was 0.25 and the intensity of the noise was 1. The
signal/noise ratio was thus 0.25.
When the emitter is very close to the microphone, the intensity of
the signal is 1 and the intensity of the noise is certainly less
than 0.01. The signal/noise ratio is thus greater than 100.
We can say this:
- When the signal/noise ratio is 1, the signal is clearly
disturbed by noise, but is still visible.
- When the signal/noise ratio is a lot greater than 1, the
signal is very clear, there is virtually no noise.
- When the signal/noise ratio is a lot less than 1, the signal
is completely hidden by the noise.
The question is thus: what is the trick to determine the
presence of the signal when the signal/noise ratio is a lot less
Answer: you must cut what you receive in exact pieces and make the
sum of those pieces.
Let's use what we received when the signal/noise ratio was 1. We had
4 periods of sine wave.
We cut these 4 periods away from each other:
Then we put them one above the other and make the sum of them:
Finaly we divide by four the result of that sum (just to scale it):
As you can see, the noise is now two times weaker. (Compare with any
of the four periods we summed.)
We have increased our signal/noise ratio by two!
Here is the explanation:
When you make the sum of four sine periods, the result is a sine
period four times bigger.
That's because when you make the sum of n precise numbers, you get a
result that is precisely n times bigger.
7 + 7 + 7 + 7 = 28
-5 + -5 + -5 + -5 = -20
When you make the sum of four noise "periods", the result is only
two times bigger.
That's because the noise is sometimes positive, sometimes negative,
at random. When you add random positive and negative numbers
together, they "eat" each other up.
8 + 3 + -5 + -10 = -6
1 + -5 + 11 + -8 = -1
The sum of n sine periods of amplitude a is a sine period of
amplitude n . a
The sum of n noise "periods" of amplitude a is a noise "period" of
n . a.
Thus, when we made the sum of four periods, the sine wave grew four
times, but the noise grew only two times. The signal/noise ratio was
thus increased by two.
When we make the sum of n periods,the signal/noise ratio is
increased by a factor n.
A sum of periods is a very important object. Because it tells us if
the sine wave was there or if it was not there. That allows us for
example to transmit morse code. Here are the results of 27
successives results of sums calculated by a receiver:
absent absent there absent there absent there absent absent there
there absent there there absent there there absent absent there
absent there absent there absent absent
Transcribed with more readability, a space for absent and an
underscore for there, it gives us this:
_ _ _ __ __ __ _ _ _
It's the morse code for S.O.S.
The same way, you can transmit modern digital code.
The space probe Galileo is curently in orbit around the planet
Jupiter. The radio signal we received from the probe was cut into one
periods each tenth of a second. All those periods are
carefully summed to generate an information flow of ten bits per
second. That makes one text character per second. Character afther
character, word afther word, sentence afther sentence, the probe
transmited a description of what it saw or measured.
More refined systems do measure the intensity
of the sine
wave. Each time a sum is calculated, the size of the sine wave is
measured and that measure is transmitted to whatever needs it. AM
long wave and short wave receivers you can bye in any store work
that way. By calculating several thousand sums per second and
transmitting the result to a loudspeaker they make that loudspeaker
reproduce a certain sound, voice or music.
You may now stop reading this text if you want, the rest of it are
4. The precision of the clocks
When the sine wave is visible in the signal, it is not
difficult to know where to cut the signal to get successive periods.
A few periods may even be hidden by the noise, you still know where
to cut just by looking at the position of the other periods near
them. (That's what a PLL does.)
But what if the sine wave is completely hidden by the noise? Where
should we cut?
There is only one solution: rely uppon a clock.
If we know that one period takes a milionth of a second, we make a
clock give a tick each milionth of a second. Each time we hear a
tick, we cut a period out of the received signal, blindly. And when
we have accumulated enough periods, the sum of them will tell us
wheter there was a sine wave hidden inside the noise or not.
OK. But that clock must have a certain accuracy. Let's take for
example the following signal:
If the clock runs perfectly, we will get the following sixteen neat
But if the clock runs 5% too fast, and the periods are thus cut each
time 5% earlier, we get this:
Compare the first period and the tenth. They are each others'
opposite. If you make the sum of them, you get zero
result. In fact, the sum of all the periods will give, perhaps not
precisely zero, but in any case something very little. We will not
see a beautiful sine period emerge.
The more periods we want to cut and sum, the more accurate the clock
will have to be.
If we want to sum hundred periods, we need a clock with a precision
better than one hundredth. (This means that afther a time of say 100
seconds it deviates of less than a second.)
Attention: we have spoken about the precision of the clock used by
the receiver. The clock of the emitter must have the same precision.
It wouldn't help that the receiver cuts accurately the signal in
periods, if the signal send by the emitter is unreliable. Both
clocks must be accurate.
Imagine we have a receiver that makes sums of, say, 1000
And the radio frequency he is made to hear is 10,000,000 Hz
(10 MHz). That makes 10,000 sums calculated each second.
It will hear perfectly an emitter emitting at 10,000,000 Hz. Of
It will also hear an emitter emitting at 10,002,000 Hz. Nearly
But il will not hear an emitter emitting at 10,500,000 Hz. For
the obvious reason given in chapter 4. (Well in fact it may
hear it if it emits a very powerful signal, but let's no think about
So, an emitter at 10,500,000 Hz will not disturb our receiver
working at 10,000,000 Hz.
Thus we can use a second receiver, receiving at 10,500,000 Hz,
to hear that emitter at 10,500,000 Hz.
That receiver at 10,500,000 Hz will not be disturbed by the
emitter at 10,000,000 Hz.
That's wonderful. Each emitter receives the signal emitted by the
emitter using the same frequency, but is not disturbed by the other
emitter using another frequency.
If a receiver can be tuned
it will be able to choose to
which emitter it listens. It can be tuned to listen to the emitter
emitting at 10,000,000 Hz or to listen to the emitter emitting
at 10,500,000 Hz. Or any other frequency. It's just a matter of
We work at about 10 MHz and we do 10,000 sums per second. We
have the ability to use several frequencies at the same time to
allow different emitters and receivers to work at the same place
without disturbing each other. But, if we use frequencies between
9 MHz and 11 MHz, how many
different couples of
receivers ans emitters will be able to work simultaneously?
The answer depends on several things. Commonly a difference in
frequency of ten times the transmission rate is taken. We transmit
10,000 informations per second, so we will rely upon a difference of
100,000 Hz between each emitter-receiver couple. Thus:
9,000,000 Hz, 9,100,000 Hz, 9,200,000 Hz... up to
11,000,000 Hz, that makes 20 couples of emitter and receiver
talking to each other at the same time without disturbing each
10,000 is the bandwidth. It is the number of elementary informations
elements that are transmitted each second. That is, the number of
times per second you calculate the sum of the periods received.
The broader the bandwidth,
- the more elementary information you transmit each second.
- the less far that information is transmitted. Because you are
using less periods to make one sum.
- the less emitters can work together inside a certain frequency
- the less accurate the clocks must be inside emitter and
receiver. (It may seem a paradox, but VHF TV modulators are
easier to manufacture than FM audio modulators. A TV image needs
20 million elementary information to be transmitted (one image
is made of 480,000 pixels (600 lines x 800 columns) and 25
images must be transmitted each second). An audio signal, on the
contrary, only needs 40 thousand elementary informations to be
transmitted each second. An audio signal requires a lot less
informations per second! Thus one puts a lot more audio channels
on a given frequency window than TV channels, and therefore one
needs far out more precise clocks for audio signals.)
This method that allows several emitters to emit at the same time,
is called "frequency multiplexing". It is not the only one. Another
is "time multiplexing": all emitters use the same frequency (or do
not use any frequency at all) but emitting time is shared amongst
them. Each his turn. These two methods have their advantages and
disadvantages, which one is choosed for a given application is a
mather of engineering choice.
6. Accumulator atenuation
The description that is given above of a receiver is good
but a bit theoretical. In reality, common receivers work not exactly
The method we described can be sumarized this way:
At the beginning of a series of periods, an accumulator is set to
null. Then, each period received is added to the accumulator. Ones n
periods have been received and added, the content of the accumulator
is looked at. If it draws a sine period, we state the signal was on.
If it draws pure noise, we state there was no signal. (Or we measure
the size of the sine.)
The method that is most commonly used is this one:
The accumulator is never set to null. Each period received is added
to it, then the content of the accumulator is shrinked a little bit
(it is multiplicated by 0.999, say). The content of the accumulator
is looked at continuously. If it draws a sine period, we state the
signal is on. If it draws pure noise, or a too little sine period,
we state there is no signal. (Or we measure the size of the sine.)
This second method is less mathematicaly correct, but it is more
physicaly realistic, smoother, and easier to use.
The first method has three practical drawbacks:
|It requires perfect memories, that are not disturbed
afther n periods. That can only be done with digital
memories or with delay lines.
||It only requires simple components like a guitar string, a
tuning fork or a condensator and a self.
|When you look at how frequencies different from the
perfect frequency are received, you get unregular results: a
frequency slightly different will not be received at all,
but another frequency further away from the perfect
frequency will be heared a little bit.
||You get a smooth behaviour: the further away from the
perfect frequency, the less it is received.
|You must know when a series of periods starts (for digital
transmissions) and when it ends. This requires circuits or
algorithms in order to allow the receiver to be phased with
||Because the accumulator is looked at continuously, you
don't bother to be synchronised with the emitter.
The first method was characterised by the number n of periods
summed. Everything depends on the number n. You may wonder what
characterises the second method. Answer: the number by which the
content of the accumulator is multiplicated each time a period is
added. That is 0.999 in our example above.
Now let's look at some practical aspects of this second method:
Simple electronic receivers that use a rudimentary LC circuit as
their heart work that way naturally. The LC circuit (one capacitor
and one self latched together) works as a resonator: if it receives
pure noise, it will just oscillate a little bit at low amplitude.
But if the noise contains a signal that has the same frequency as
the resonance frequency of the circuit then the circuit will begin
resonating and will thus oscillate at higher and higher amplitude.
Once the amplitude reaches a given threshold, that will trigger a
transistor and "make a lamp glow". The LC circuit acts as a memory
that sums the oscillations.
Mechanical receivers work the same way too. Early radio command
devices used little tuning forks to determine if a given beep sound
was being received: if the beep sound was there the appropriate
tuning fork would begin vibrating so strongly it's end would touch
an electric contact.
If you want to build some mechanical device to visualize what's
happening, here are two suggestions. I tried out none of them so if
you do please mail me your remarks and recomendations.
7. The super-heterodyne
- Use a guitar with metal strings (or wrap some thin electric
wire a few turns around the middle of a string). Latch some
needle very close to the middle of a string. Use some more
electric wire, a battery and a lamp to build a complete electric
loop between the string and the needle. When you push the string
a little bit it touches the needle and the lamp begins to glow.
Then aim at the guitar any source of music that produces the
same music note as the guitar string: another instrument or an
electronic tunable sound generator. The string will begin
resonating, will make wide movements and thus will touch the
needle and thus the lamp will begin to glow... Should you aim at
the guitar a signal with another frequency, or any noise, then
nothing will happen. Should you aim a mixture of any noise and
the right frequency, then the lamp will glow... You may also put
a needle and lamp on all guitar strings and determine what
frequency will make each lamp glow. That way you may choose
which lamp will glow by emitting the right frequency. You may
make several lamps glow at the same time by emitting at the same
time the right frequency for each choosen lamp.
- Build two the same pendulums (especially their rope's length
must be exactly the same). Latch the end of the first pendulum
to any heavy object and make it oscillate. Take the end of the
second pendulum between your fingers. Look at the first pendulum
and make your fingers go back and forth exactly at the same rate
yet with a very little amplitude. The movement of your fingers
should be hardly noticeable. The pendulum between your fingers
will begin oscillating and make wider and wider movements. The
amplitude of it's movement will become very important, for sure
a lot more important than the amplitude of your fingers'
movements... Now do it again but change the first or the second
pendulum's frequency by making its rope be longer or shorter.
This time nothing happens when you move the second pendulum with
your fingers. It moves a little bit together with your fingers
but that's all. You may also try to move the second pendulum
with any random little movements, nothing neither will happen,
unless those random movements contain a little bit of the
movement of the first pendulum (with same rope length as the
second)... You may also latch both pendulums and link them with
a thin elastic rope. If you make the first one oscillate, the
elastic rope will transmit the oscillations to the second
pendulum that will begin oscillating too and make wider and
wider movements... provided both pendulums rope's lengths are
the same. You may even use several "emitter" and "receiver"
pendulums and latch them all together trough crossed thin
elastic ropes. In order to make one given receiver pendulum
oscillate you will have to launch an emitter pendulum with the
same rope length (frequency). (The thin elastic ropes are not
necessary if the object the pendulums are latched to is not too
heavy and is allowed to make little movements.)
The super-hetrodyne receiver is the most widespread type of
radio receiver. It works on a mathematical trick:
When you multiply
a sine wave by a sine wave with a slightly
different frequency, you get a result that is the sum
other sine waves:
The two sine waves inside the result have frequencies that are higher
than the frequencies of the sine waves that have
The lowest frequency is equal to the difference
frequencies of the two initial sine waves. If the first frequency
was of 1,000,000 Hz and the second was of 999,000 Hz then
the sine wave will have a frequency of 1,000 Hz.
That sine wave with the lowest frequency is the one we are going to
use. The sine wave with the highest frequency is filtered away.
What is that low frequency good for? A lot of things:
- Suppose you receive a signal that is the sum of two
frequencies. Say 10,000,000 Hz and 10,010,000 Hz. But
you only want to measure the intensity of the 10,000,000 Hz
signal. The problem is these two frequencies are close to each
other: there is only 0.1% difference between them. Therefore it
is very difficult to filter away the 10,010,000 Hz and keep
the 10,000,000 Hz. The solution is to multiply the received
signal by a frequency of 9,999,000 Hz. The frequency at
10,000,000 Hz will yield a low frequency of 1,000 Hz,
the frequency at 10,010,000 Hz will yield a low frequency
of 11,000 Hz. Between these two low frequencies you now
have a difference of 1,000%! It is very easy to filter the
11,000 Hz away and keep the 1,000 Hz, even with a
rudimentary filter. By measuring the intensity of that
1,000 Hz signal you get the intensity of the
10,000,000 Hz signal.
- Good filtering and amplification systems are delicate to
build. If besides they must be tunable for different
frequencies, it becomes an impossible task. With the
super-heterodyne system you get a simple solution: you build the
filtering system for one frequency, say 100,000 Hz, and you
multiply the received signal by a tunable frequency before
sending it into that filter. Want to receive a signal at
100,000,000 Hz? Multiply the signal from the antena by a
sine wave at 99,900,000 Hz... Want to receive a signal at
98,000,000 Hz? Multiply the signal from the antena by a
sine wave at 97,900,000 Hz... And so on. The filtering
system will always have to deal with a signal at
- Suppose you want to use frequency modulation, FM. At first
hand, you cannot use the sum of periods, like mentioned in
chapter 3, to make the signal emerge out of the noise. That
looks impossible because the frequency must be stable. FM is, of
course, not stable. The solution is the super-heterodyne system.
If for example you want to receive a signal that varies between
100,100,000 Hz and 99,900,000 Hz, just multiply the
antena signal by a stable sine wave at 99,800,000 Hz and
you will get a low signal between 300,000 Hz and
100,000 Hz. Like pointed above, unwanted noise and signals
will be easy to filter away, then the frequency of the low
signal will be measurable by standard methods.
Here you have a short program in BASIC that draws two sine waves and
the result of their multiplication:
8. Common ways to enhance
frequency1 = .2
frequency2 = .24
FOR t = 0 TO 254 STEP .1
sine1 = SIN(t * frequency1)
sine2 = SIN(t * frequency2)
sinem = sine1 * sine2
PSET (t, sine1 * 10 + 10)
PSET (t, sine2 * 10 + 50)
PSET (t, sinem * 10 + 100)
8.1 Directivity of the emitter
A device is added to the emitter to make as much as possible
of the signal go towards the receiver. So there is no waste in
useless directions. That's what you do when you put your hands
around your mouth when you whant to shout at somebody far away or in
a noisy environment.
The device best known is the parabolic antenna, but there are a lot
of other ways to achieve directivity. For example sets of common
antennas connected together trough wires of acurately calculted
The bigger the antenna, the more directivity you will get.
The bigger the wavelength of the signal you transmit, the bigger the
antenna you will need to achieve the same directivity.
8.2 Directivity of the receiver
A device is added to the receiver to make him listen as much
as possible only to the signal coming from the direction of the
emitter. That's what you do when you put your hands back your ears
to hear better a weak sound. The device best known is again the
parabolic antenna, but there are a lot of other ways to achieve
directivity. Like using several antennas and adding their signals.
The bigger the device, the more directivity you get.
A parabolic antenna acts for radio waves just like a solar oven acts
for the sunlight, concentrating what it receives on one given point.
The considerations about antenna size, directivity and wavelength
are the same as for point 8.1 just above.
8.3 Reduction of the internal noise of the receiver
You can imagine for sure that making a receiver work in a
noisy environment reduces its performances. But a receiver also
produces it's own "internal noise": every electronic component
inside a receiver produces a noise. That's why those components must
be carefully choosed or manufactured to produce the less possible
Metalfilm resistors are prefered over carbon resistors, FET
transistors are prefered over bipolar transistors, and so on.
To decrease even more the remaining amount of noise, and it can
physically not be done another way, the receiver must be cooled
down. It can be plunged into liquid nitrogen or even liquid helium.
This is true whatever the type of communication system you are
using: radio waves, light, light trough fibre optics, sound,
electric signals trough wires, even interstellar gravitational
Inside a simple component like a resistor, the noise is simply due
to the electrons moving around inside the resistor. The hotter the
resistor, the faster the electrons move thus the more noise. The
higher the impedance, the higher the noise tension (this is
compensated by the fact the noise is limited by the higher
If you want to hear directly such a noise, just put your ear inside
an empty glass. Or both ears (inside two separate glasses, not
inside the same glass).
8.4 Increase of the emitted power
The louder you shout, the farther one may hear you...
Here is some data for amateur electronics. Let's talk about the
basic straight antenna.
You may consider an antenna as behaving like a simple electric
resistance connected to the ground (picture below). But it has two
differences with a common electric resistance:
- When electricity is send trough it, the energy of the electric
signal is not transformed into heat. Instead, it is transformed
into radio waves that will travel away. Just like a loudspeaker
produces sound waves.
- The impedance of the antenna depends on it's length and on the
frequency of the signal you send trough it. You may consider
that if you connect in the middle of a thick metallic antenna
with a length of one half the wavelength
(wavelength = 300,000,000 / frequency),
it will present an impedance of 75. (For other lengths, the
impedance of the antenna will be "complex"; not as simple as a
In other words: sending the signal to the antenna is the same as
sending it to a resistor of 75.
Thus there are two ways to increase the emitted power:
One last reminder: don't forget that if the
frequency generator is to be placed a certain distance away from
the antenna, you must make use of a well defined type of electric
wire to link them. Coaxial wire or twisted pairs. What's more that
coaxial wire or that twisted pair should ideally have an impedance
close to that of the output impedance of the generator and the
impedance of the antenna. The three impedance should be the same
(don't bother too much, practically it works often very well with
fairly different impedance). That's why when you buy TV wire it is
written 75 on it. Other common impedance are 50 and 100. The
impedance of a coaxial cable means that if you send a signal of
any frequency trough a cable of infinite length it will behave for
your generator like a resistance of that impedance. Symmetrical,
if a signal comes trough a coaxial cable or twisted pair, it will
be like if it came trough a resistance of that impedance. If one
of the three impedance is not the same, then you will get ghosts
and signals bouncing back partially. Coaxial cables and twisted
pairs have three main particularities: they do not distort the
signal (it may weaken, but it will keep the same shape), they do
not spill radio waves around (that would pollute around and weaken
the signal inside the cable) and they are insensitive to external
noise (even if your cable went trough a room with radio emitters,
electric sparks or whatever, the signal stays clean). For example,
you may transmit RS/232 signals or even VGA screen signals on
hundreds of meters provided you do that trough such wires.
8.5 Increase of the received power
- Increase the electric tension of the signal send into the
antenna. Just like a LED lamp would glow more or a loudspeaker
would give a louder sound, an antenna will broadcast more
powerful radio waves.
- Increase the length of the antenna. This is not an easy task,
as there will be "interference patterns" and possibly a complex
impedance that must be dealt with.
The electronics inside the receiver work on the basis of a
signal received by the "hearing device": a microphone, an antenna, a
light detector or whatever else. Now, the stronger the power of the
signal delivered by the "hearing device", the easier will be the
work of the electronics. That's trivial.
The quality of the "hearing device" matters, but also it's surface,
a bit like for the directivity. The bigger an antenna, the more
powerful the signal that it delivers will be. (In the case of an
omnidirectional antenna, noise and useful signal are both
The most serious reason why the received signal should be as strong
as possible is to make it be stronger than the internal noise
produced by the electronics of the receiver.
It is not necessary to try to make the received power as big as
possible. You just have to make it be louder than the internal noise
of the electronics.
In the case of a basic straight antenna, you increase the received
power by making the antenna longer. You may consider that a thick
metallic antenna with a length of one half the wavelength of the
frequency received has an impedance of 75
(wavelength = 300,000,000 / frequency).
This is to say you may consider the signal is coming trough a
resistance of 75
. If you double the length of the antenna, you halve
the resistance and thus you get two times more power. (The electric
tension of the signal will stay the same, but you will be able to
rely on a stronger current.) But, once the antenna becomes longer
than half the wavelength, you get interference patterns and lobes.
And an antenna that is not well calculated will have a complex
A receiving antenna transforms radio waves into an electric signal
just like a microphone transforms sound waves into an electric
9. A practical software example
Following program simulates the functioning of both an
emitter and a receiver. A signal is emitted, it arrives at the
receiver weakened and with a lot of noise added, yet the receiver
manages to show wether there was an emitted signal or not.
While the program is running, press the 0 or 1 key on your keyboard
to switch the emitted signal on or off. Then look at the sum result
that appears at the bottom of the screen (wait). If the signal was
on, a sine wave period will be drawn. If it was off, just a weak
noise will be drawn.
In order to run this program you need a PC running (or emulating)
DOS or Windows. They contain a powerfull BASIC language interpreter
capable of running this program. Just select the program with your
mouse, copy it, paste it inside a simple text editor, then save it
under the name you want (with the .BAS extension). Start the BASIC
interpreter (QBASIC.EXE), load the program and run it.
SCREEN 1 'switch to 320 x 200 graphical output screen
LOCATE 1, 1: PRINT "Signal emitted (press 0 or 1):"
LOCATE 8, 1: PRINT "Signal weakened, noise added:"
LOCATE 15, 1: PRINT "Result of last sum of 1000 periods:"
t = 0 'time
x = 0 'horizontal display position on screen
i = 0 'sweep inside receiver memory
p = 0 'number of periods received
s = 0 'signal to transmit
DIM r(16) 'receiver memory: 16 registers
i$ = INKEY$ 'key pressed?
IF i$ = "0" THEN s = 0 'signal to transmit
IF i$ = "1" THEN s = 1
m = s * SIN(t * 2 * 3.1415627# / 16) 'modulated signal
LINE (x, 20)-(x, 40), 0 'erase old pixel
PSET (x, m * 10 + 30) 'display modulated signal
t = t + 1
n = RND - RND 'noise
r = n * .9 + m * .1 'received signal
LINE (x, 80)-(x, 100), 0 'erase old pixel
PSET (x, r * 10 + 90) 'display received signal
x = x + 1: IF x = 320 THEN x = 0 'display sweep
r(i) = r(i) + r 'add to register
i = i + 1: IF i = 17 THEN i = 1: p = p + 1 'registers sweep
IF p = 1000 THEN '1000 periods
FOR a = 1 TO 16
LINE (a + 140, 135)-(a + 140, 165), 0 'erase old pixel
PSET (a + 140, r(a) / 10 + 150) 'display register value
r(a) = 0 'reset register
BEEP 'beep sound
p = 0 'start new 1000 periods
Following program is much simpler. It works the same way as
algorithms I implement inside microcontrollers. You may compare it
with the program above to understand clearly how it works. Note only
two registers are used and only the sign of the signal is used.
LOCATE 1, 1: PRINT "Signal emitted (press 0 or 1): no"
LOCATE 15, 1: PRINT "Result of last sum of 1000 periods: nothing"
t = 0 'time
i = 0 'sweep inside receiver memory
p = 0 'number of periods received
s = 0 'signal to transmit
DIM r(2) 'receiver memory: 2 registers
i$ = INKEY$ 'key pressed?
IF i$ = "0" THEN 'signal to transmit
s = 0
LOCATE 1, 1: PRINT "Signal emitted (press 0 or 1): no "
IF i$ = "1" THEN
s = 1
LOCATE 1, 1: PRINT "Signal emitted (press 0 or 1): yes"
m = s * SIN(t * 2 * 3.1415627# / 16) 'modulated signal
t = t + 1
n = RND - RND 'noise
r = n * .9 + m * .1 'received signal
IF i = 1 OR i = 2 OR i = 3 OR i = 4 THEN r(1) = r(1) + SGN(r)
IF i = 5 OR i = 6 OR i = 7 OR i = 8 THEN r(2) = r(2) + SGN(r)
IF i = 9 OR i = 10 OR i = 11 OR i = 12 THEN r(1) = r(1) - SGN(r)
IF i = 13 OR i = 14 OR i = 15 OR i = 16 THEN r(2) = r(2) - SGN(r)
i = i + 1
IF i = 17 THEN
i = 1
p = p + 1
IF p = 1000 THEN '1000 periods
result = r(1) * r(1) + r(2) * r(2)
IF result > 100000 THEN
LOCATE 15, 1: PRINT "Result of last sum of 1000 periods: signal!"
LOCATE 15, 1: PRINT "Result of last sum of 1000 periods: nothing "
BEEP 'beep sound
p = 0 'start new 1000 periods
r(1) = 0
r(2) = 0
Please note two things about this second program:
- In real circumstances there is always a given phase difference
between the received signal and the receivers' clock. The
receiver algorithm the program uses is not disturbed by that
fact. That's thanks to the fact the square of the registers is
- Sometimes this program may tell you it receives a signal
although there is no signal being emitted. Just let it run a
sufficiently long time period and that strange phenomenon will
happen. In fact every receiver bears that handicap. It's
just a matter of probability. Everything depends on that number
10,000 inside the program. If you use a smaller number the
receiver will become able to detect weaker signals but it will
more often say there is a signal when there is no signal at all.
If you use a bigger number the receiver will make less mistakes
but alas it will only detect strong signals. It's up to you to
choose between receiver sensitivity and reliability. If you want
to increase both then you will be obliged to build a "more
expensive" receiver. (Atom bombs behave that way too: the
military do not pretend those bombs cannot explode
spontaneously. But enough research and money has been invested
in order to make the probability of spontaneous explosion be
Thanks to Chris Price for pointing out an error.
Eric Brasseur - January 1