1. Jan 10, 2017

### ilyasse

hello people . i don't know if this is the right place to ask , but i'm studding for my cisco ccna and now i'm learning more about radio waves and wifi networks and i fall in love with the subject , what i don't understand is what's the difference between a radio and a antenna , because i asked in another forum they told me that the radio create the radio waves and the antenna only transport the encoded signals of information inside the waves . i just would like to know more i don't like the feeling of something messing because for me the waves are spread using the antenna ,also because antenna's are semi duplex ( can send only to one other antenna at the same time) so how can it works at two different frequencies at the same time ? hope you help me see it better thanks

2. Jan 10, 2017

### Henryk

It is clear that you don't know much about radio communication, it is rather hard to explain, not sure where to start.
Let me try this:
A radio transmitter convert an input signal, for instance, voice, music, digital signal into a modulated high frequency current. The most common ways of modulating is Amplitude Modulation (AM) and Frequency Modulation (FM). AM stations operate in the high frequency range of 500 to 1500 kHz while FM stations operate in 88 to 108 MHz range.

Next thing you need to do is to convert high frequency electrical current in a wire to an electromagnetic wave (EM) This is a role of an antenna. Actually, any piece of wire connected to a high frequency current source will emit electromagnetic waves. An antenna is simply a piece of wire with shape and dimensions optimized for converting high frequency current into electromagnetic wave and vice versa. For instance, most AM station have 1/4 wavelength monopole antenna, that is a mast whose length is equal to 1/4 of the wavelength of the emitted electromagnetic wave. FM stations use 1/2 wavelength dipole antennas.

Antennas are two-way devices, that is if you supply current to the antenna terminal, it will emit EM wave. However, if you subject it to an EM wave, it will generate a proportional voltage at its terminals.

A radio receiver will then contain an antenna to pick up EM wave and convert it into an electrical signal.. An antenna will pick up all the electromagnetic waves. In order to select one radio station, a receiver has to filter out all the signals except the one at the frequency of interest (that is tuning your radio to a particular station). After that, the picked up signal is amplified and de-modulate and send to the speakers so you can enjoy the music.

For WiFi, you need two-way communications, so both devices have to have transmitter and receiver functions. WiFi also operates at much higher frequency than the radio stations (2.4 GHz range). Also, the signal transmitted is digital, and the modulations is more complex than simple AM or FM but the basic idea is the same.

3. Jan 10, 2017

### ilyasse

thanks i was able to understand some informations , but one last question before , if you create a radio wave just by currant passing in the antenna , then how can wifi be backward compatible with 2.4 and 5 ghz , i mean how can we create two different waves at the same time from the same antenna ? , from what i understood is that antenna are passive abjects so we will need two antenna's one for 2.4 ghz and another for 5 ghz so both frequencies are available but at the same time it doesn't mean that antenna 1 and 2 are operating at 2.4 or 5 right ? also i would prefers if you give me a book , i would like to understand how the carrier and fm am modulations works . thanks

4. Jan 10, 2017

### davenn

the radio transmitter generates a RF ( radio frequency) current. This is sent to the antenna via a transmission line, eg., coax or waveguide
the antenna then radiates that signal for receive, the opposite ... the antenna picks up the radiated signal ( electromagnetic wave) a small current is generated in the antenna which flows to the receiver circuitry via the transmission line

radiated as I used above is a much better word

antenna can be and often are used in full duplex in many situations ... mobile phone systems, repeater systems etc

my last response sort of covers that but a little more info ....
Antennas are designed and manufactured for a specific frequency range and depending on the requirements that range can be very narrow
eg. an antenna may be designed for say 100 MHz with only a + or - 5 MHz bandwidth before its losses ,due to being out of tune, become too great

but a different design may allow for a 20 MHz bandwidth before losses become unacceptable
I have an antenna for the microwave bands, because of its design can cover approximately 9 GHz of bandwidth 2 GHz to 11 GHz

see my above comments . the antenna just radiates the signal it gets from the WiFi transmit section
most antennas for WiFi, Bluetooth and similar forms are, in general, very inefficient. This is because that are made small and compact
Yes, they work, but they have their limitations

they are not going to be operating on both bands at the same time ( well unlikely unless they are in a mixed equip system ... but as I described above, dual freq operation is not a problem).
The 2 WiFi transceivers will establish the frequencies and protocols that are compatible for use between the units and will use that freq and protocol

hope that helps
cheers
Dave

5. Jan 11, 2017

### ilyasse

thanks for those info , but i wasn't very precise , i was talking to wifi case only . in wifi each pc demands different informations so you can't broadcast all the pc's at once it force you to use only one antenna for each pc at a time . for example the same information like in a tv , my bad , that's why i said semi duplex , also i didn't get where you were going with the explanations about multi frequencies at the same time , i mean i understand that we can have many channels over frequency what i don't understand was how can wifi can be compatible with both 5 and 2.4 ghz at the same time , how is it possible ? only wifi not other type of radios . thanks

6. Jan 11, 2017

### davenn

what I said still applies

again, what I said still applies

all in the electronics of the WiFi module ... it can transmit and receive on whichever band is required

Dave

7. Jan 11, 2017

### ilyasse

i know what i don't understand is from where the 2.4 and 5 ghz came from , where are they produced ? i know from what i learned here that the electromagnetic waves are produced by the currant passing through the entenna the only way to have at the same time two bands is in fact that we have one different types of currant passing through each antenna , right ? that's for one and for two there are many channels in a 2.4 ghz frequency band , ok so the more frequency you have the faster it is right ? , then it is natural that i think that channel number 11 is faster then channel number 1 (even if just a little) right ? , i need to understand something to imagine with me , when we say frequency we imagine multiple waves equal in size , then if we take a 20 mhz channel ok ? , here we are using a interval of what ? because imagine with me i did some research that there is no minimum or maximum frequency , so it's not something comptable not like a cpu that for example if we have 1 ghz so it will make 1 billion operations because we know that each 1hz is a operation , but here i don't get it , if you understand what i'm searching and wondering about and i know i don't know much just give me a book or the title of the thing i will search it on my own thanks

8. Jan 11, 2017

### davenn

to quote my last comment again

the faster what is ?

ohhh ... NO .... EM travels at a constant speed in the air ( in whatever medium it's travelling in ... the speed varies in different mediums)
It is rounded to 300,000 km / sec .... actual speed in a vacuum is 299792458 metres per second, a little slower in the atmosphere
even slower on the outside of a copper conductor ( wire, etc)

20 MHz has a wavelength of 15 metres. 2.4 GHz WiFi has a wavelength of 12.5 cm = 0.125 m. The propagation speed of both is the same

CPU's and clocking speeds is a different subject to what has been getting discussed in this thread. Stick to the core thread topic
and make sure what has been explained to you is understood before delving into other topics

Dave

9. Jan 11, 2017

### Henryk

iliasse,

Please, use the proper capitalization of SI units and the prefixes. Take this for example.
I'm guessing you meant 20 megahertz, in that case you should have written as 20 MHz. The correct abbreviation of the frequency unit is Hz, spelled with capital 'H'. The bigger problem is the prefix. You used small 'm', that stands for 'milli' or one thousands of the basic unit. The prefix for 'mega', or 1000,000 time the basic unit is capital 'M. 20 mHz is 0.02 Hz, 20 MHz is 20,000,000 Hz,

The link I posted in my first reply is to the Wikipedia page. It has a gif that animates AM and FM modulation.
For digital communication a different modulation schemes are used most of them based on modulation of the phase of the carrier. The reason is to make the system immune for interference from, for instance, neighbour's WiFi or microwave oven.

The WiFi standard 802.11 defines 11 channels within 2.4 GHz band of center frequencies ranging from 2.412 to 2.484 GHz. Each channel has also allowed width of 22 MHz. The transmission rates achievable is not related to the channel center frequency but to the width of the channel. Since all the channels in this band have the same width, they can transmit data at the same rate.

Henryk

10. Jan 12, 2017

### 256bits

No. If you are thinking that the rate of data transmission depends directly upon the carrier frequency.

If one uses a carrier frequency to transmit and receive data ( which can be an analog signal such as voice audio, or a digitized data ), what mostly is talked about is bandwidth, which can be thought of as the spread in frequency from the base carrier frequency. More bandwidth and more data can be relayed from sender to receiver. Human voice has a bandwidth of approximately 5 kHz. If your electronics is designed to have a bandwidth of only 1 kHz, then obviously the high notes from a soprano will not be able to be transmitted, in which case less data has been sent and received.

Of course, one could choose to not use a carrier frequency, but transmit and receive the data with no carrier frequency modulation and de-modulation. Your electronics would then have to be able to function then from 0 Hz up to whatever higher frequency you desire for your bandwidth. That becomes more difficult to do as the bandwidth increases.

What Wifi uses is called https://en.wikipedia.org/wiki/Quadrature_amplitude_modulation.
That looks pretty difficult.

AM frequency modulation is probably the easiest to learn how to understand, so if I was you I would learn that type inside out.
Look up carrier frequency, modulation, beat frequency, https://en.wikipedia.org/wiki/Heterodyne, intermediate frequency, and other bunches of terms that you may find along the way.

11. Jan 12, 2017

### davenn

yeah, that's what he was thinking and which I also commented "no" several posts back

you have been given lots of good answers. Radio transmission and reception is a huge subject

a good place to start would be the ARRL -- Amateur Radio Handbook
it will take you through the whole process

Dave

12. Jan 12, 2017

### sophiecentaur

Every piece of metal in the World has currents flowing through it, due to all the radio waves going past it. An antenna (a piece of metal) is designed to 'pick up' waves of a certain range of frequencies, better than others by its shape and size. The currents flowing in the antenna can be filtered off by a number of different receivers, each tuned to a different frequency. An antenna can also be fed by a number of transmitters, each operating on a different channel. The currents flowing in it are quite independent of each other and the antenna ends up radiating each transmitted signal perfectly well. The TV mast that you get your programmes from will have four or five different transmitters, all feeding their signals to you via the same antenna at the top of the mast. The two or three antennae on your wifi router can be transmitting and receiving more than one band at a time.
There is a general principle, called Superposition which says that all the different radio signals can occupy the same bit of space without affecting each other. It obviously applies to the way light behaves. You can look across the room and see something and your friend can look across your line of sight and see something else without either of you affecting the other's vision.
Superposition requires a 'linear' medium and it doesn't apply in some situations - like in some electronic devices - but it works a treat in antennae, cables and through the air. This is really useful to us!

13. Jan 16, 2017

### Phellippe Marques

Is it possible to catch 2 different signals with one single antenna and then decompose them using a fourier transformation? Also didnt take "telekom" in my grad yet

14. Jan 16, 2017

### sophiecentaur

You can 'catch' as many as you like; all the arriving RF signals will induce a current in the down lead which consists of all those signals. A single receiving TV antenna at the top of an apartment block can feed dozens of receivers, all tuned to different stations. Your different signals would need to have spectra that did not overlap or it could be hard to distinguish between the two. Conventional Radio uses channels, separated in frequency. They are usually split between two different receivers, using hardware based filters but it is quite possible to do the operation by digital signal processing (assuming the operating frequencies are suitable). This could involve Fourier Transforms.

15. Jan 16, 2017

### Phellippe Marques

What would be a better word for replacing "catch"? English is not my mother language.

16. Jan 16, 2017

### sophiecentaur

Fourier Transform is just one way of analysing a time varying signal into its various components. The same thing can be achieved using 'signal filters', made with Capacitors, Inductors or even lengths of transmission line. The way it's done usually depends on what's best for the user of the manufacturer. Digital Signal Processing is very clever these days and can work at extremely high rates so it is possible to avoid using too many expensive physical components.

17. Jan 16, 2017

### sophiecentaur

"Catch" is not a bad word for it. I just haven't come across it used that way before. People often say "Capture". "couple with" or just "Receive". The job of an antenna is to couple the waves through space with the feeder cable to the receiver. This is much the same as the external parts of your ear do with sound waves and couple the sound energy to your sensors.
It's the other way round for Transmitting signals, where the antenna radiates the power from the transmitter.

18. Jan 16, 2017