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 remain dark.
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
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
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
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 appear:
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 adds 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
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
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
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 than 1?
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
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
amplitude 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
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 billion 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
You may now stop reading this text if you want, the rest of it
are technical details.
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 as 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
Imagine we have a receiver that makes sums of, say, 1000 periods.
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 that.)
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 clock
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
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 other.
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,
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
- 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.)
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 that way.
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
||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 the emitter.
||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.
- 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
7. The super-heterodyne receiver
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 of two
other sine waves:
The two sine waves inside the result have frequencies that are higher
and lower than the frequencies of the sine waves that have been
The lowest frequency is equal to the difference between the
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
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:
Here you have a short program in BASIC that draws two sine waves and
the result of their multiplication:
- 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
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
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 100,000 Hz.
- 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.
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. Common ways to enhance communications
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
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 length.
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 noise.
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 waves...
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 impedance).
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
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
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
is not transformed into heat. Instead, it is transformed into radio
waves that will travel away. Just like a loudspeaker produces sound
- 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 a
thick metalic antenna with a length of one half the wavelength
(wavelength = 300,000,000 / frequency)
has an impedance of 75.
In other words: you may consider the signal is send trough a resistance
connected to the ground.
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
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 idealy have an impedance close to that of the output
impedance of the generator and the impedance of the antenna. In fact,
the three impedances should be the same (don't bother too much,
practicaly it works often very well with fairly different impedances).
That's why when you buy TV wire
it is written 75 on
it. Other common impedances 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.
Symetricaly, 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 impedances is not the same, then you will get ghosts
and signals bouncing back partialy. 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 polute around and weaken the signal inside the
cable) and they are insensible 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.
- Increase the electric tension of the signal send into the
antenna. Just like a lamp would glow lighter or a loudspeaker would
give louder sound, an antenna will broadcast more powerful radio waves.
- Increase the length of the antenna. The more the length is
increased, the littler becomes the impedance. Thus, the more current
goes trough the antenna and the more power is consumed an emitted as
radio waves. If you double the length of the antenna, you half the
resistance and thus you emit two times more power. Attention:once the
antenna becomes longer than half the wave length you get a disturbing
directivity. That's why half the wave length is the common length for
an antenna. Longer Antennas may emit no more power in the direction of
the receiver and thus give the impression they emit less energy which
is false (hence the illusion an antenna is a resonating device, which
is false too).
8.5 Increase of the received power
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 powerfull
will be the signal it delivers. (In the case of an omnidirectional
antenna, noise and useful signal are both increased.)
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
In the case of a basic straight antenna, you increase the received
by making the antenna longer. You may consider that a thick metalic
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
75. If you
double the length of the antenna, you half 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.)
Attention: once the antenna becomes longer than half the wave length
you get a certain form of disturbing directivity.
A receiving antenna transforms radio waves into an electric signal just
like a microphone transforms sound waves into an electric signal.
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
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:
Thanks to Chris Price for pointing out an error.
- 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 being summed.
- 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 extremely little.)
January 1 1997