the thing  i dont understand is when radio wave gets to an ant , what happens in the ant ? does it make electrons move and create current , if it does how can it handle alot of diffrent frenq in the same time ?

thanks

bye
eko
Posted on 2005-05-13 06:17:47 by eko
In much the same way that a wire can actually carry many different frequencies at the same time.  The siignals of different frequencies are added together, creating a compound signal.
Posted on 2005-05-18 02:39:55 by AmkG
this is what i cant understand
how can a wire carry diffrent freq at the same time ?
what realy happens in the wire? for 1 freq the electrons move in the wire
but if you have 2 freq or more at the same time , how do the electorns move?

thanks

bye

eko
Posted on 2005-05-28 10:08:13 by eko
Forgive me if I am wrong, since I am not really trained in this field.

What I was taught is that the reason why it is possible is due to reasonance. In a receiver, there's always an oscillator. If the frequency of the oscillator matches that of the radio wave you are seeking for, there would be reasonance...
Posted on 2005-05-30 04:31:26 by roticv

http://www.opamp-electronics.com/tutorials/introduction_2_07_01.htm

Posted on 2005-05-30 07:06:12 by Azrim
Posted on 2005-05-30 08:05:49 by roticv
Been a long time since I've been online to post, but this one has caught my interest:

The question as i hear it is how many firequencies of electronic information can exist on a single wire at once.

Well the answer is because the math says it will (a cop-out answer i know).  Something call the Fourier Series in mathematics explains that any periodic signal can be decomposed into a summation of sinusiodal singals, and each part has a unique amplitude and phase and *frequency* of oscillation.    The problem is you're trying to rationalize this in *time* and as a result, its very hard to see anything that looks like frequency or a sinusoidal signal (but trust me they are there).

At any moment in time, your seing the result of this fourier summation which if it is composed of many different frequencies will look very irregular.  A good example of this is a sound speaker.  If you every seen on TV a very irratic waveform on an oscilloscope while a computer or something is speaking out a loud speaker, your infact seeing the actual amplutide and fourier summation *in that moment in time* that produces the sounds.

If your listing to your favourite music on a radio, you hear many different instruments all making a frequency of sound at varying amplitudes.  However you dont hear any one instrument, you hear them all at once.  This is because they are all being summed up and carried down a wire as one overall sum of frequencies.

So if you believe me that each frequency is a sinusoidal signal at a given frequency, ampliture and phase then i can move on to explain how radios put thier spin on this...

Sound waves are very low frequecy.  Thier *band width* is only a few kHz wide.  This means that the lowest audible sound to the highest is a frequency spread of about 20-30kHz.  Your radio operates in the manitudes of Mhz (1000 times larger than kHz signals).  So between each radio station there is a bandwidth gap in the order of 0.3 Mhz or so == 300 khz.  300khz is still 10x larger than the bandwith of audible sound.  This is a nice safety margine so different stations don't bleed into each other.

How radio signals are them put to gether is by modulation (which means in basic multiplication of frequencies).  A 0.01 -> 30khz singnal range for sound is multiplied by a radio station's frequency to produce a fundemantal frequency (radio staion tuning) in the order of Mhz and about that fundemantal frequency there is a very narrow bandwidth of sound information:  ie) 3Mhz fundemental with 3.001 HMhz to 3.030 Mhz of audible sound information.  You see its really a few minute decimal places that vary as sound is being transmitted over radio waves.  Since the bandwidth required is so narrow many stations are doing the same to a similare 30khz bandwidth sound signal and packing it in on their own unique fundemental frequency.

The overall picture of the radiowaves is like the speaker example earlier, its a sum of all radio stations at once, but the amount of +/- difflection from their fundemental is very small (0.001 - 0.03 Mhz).  So radio specrum appears realatively pure about thier fundemental with very little extra bandwitdth being used for the audible information being modulated into it.

To receive the radio stations signal, a RLC tuning cirucit (like the link above) is used to generate a matching fundemental signal to the radio station's fundemental and then look for differences (electronically).  The differences is then the 0.001 - 0.030 Mhz of extra frequency that is in turn the sound information we want to hear.  A demodulation cirucit filters out the fundemental and then amplifies and sends the result to your speakers.

This little narrative was aimed to keep the math and tech to minimum.  If your realy keen on the topic there is alot of topics in here you can google for on your own and learn more.  (Fourier Series Expansion, Modulation of frequencies, Demodulation of frequencies and Phase Locked Loops).

I hope this helps.
Regards,
:NaN:
Posted on 2005-08-13 16:12:48 by NaN
Nan, is of course correct, but let me add a few things.

Radio waves are electromanetic energy. Another form of th is energy is... who's hand do I see?

Yes?

CORRECT! Light is also electrmagnetic energy.

Asking how a wire can carry many frequencies at the same time is like asking how can you see red and green at the same time. They just come on in together, and its up to the receiver to employ some clever mannor of detecting what is interesting and letting the other information fall away.

Posted on 2005-08-15 14:46:26 by Ernie
Nicely put!

I will have to remember that analogy.  It kinda wraps it all up in a simple statement.

Regards,
:NaN:
Posted on 2005-08-20 12:00:22 by NaN
hiii i haven't been here for a while . but it better later than never,
Nan and  Ernie  i only now seeing your replays .
i understand that any wave doesn't interface any other wave,

what i understand so far
wave gets to the antenna , now there is a AC in the antenna .
each wave does it , so if i put ampermeter i ee the sum of all the current
my question is how can i separate between them

thanks

eko
Posted on 2005-10-14 08:32:39 by eko
Filters discriminate or detect the desired freqs.
Posted on 2005-10-14 14:20:33 by alpha
Posted on 2005-10-14 22:24:05 by ti_mo_n
That's funny I haven't been here in a while either. Hey Nan my sentiments exactly. Anyway here's my two cents. Granted a "long wire" which is a term used for random length of wire will receive all freqs., an antenna will be resonant at some frequency depending on it's length. This is not crucial in receiving a radio signal but is crucial in a transmitter application. The antenna must be cut to resonance or you will have very high SWR (Standing Wave Ratio) which simply put you will have a bad impeadance match and the antenna will reject the RF frequency energy and will send it back to the transmitter amplifier which can quite easily burn up the final amplifier. A resonant antenna will also improve reception at a particular band of frequencies.
Radio waves & light travel at a speed of 300 million meters per second. From this you can calculate the length of the antenna by dividing the frequency in megahertz in to 300 to abtain the length of a full wave antenna. Example 300/7 = approx. 41 which is why the ham radio guys refer to 7MHz as the 40 meter band. The transmissin line also refered to as coax cable "if using an industry standard cable" is always usually 50 ohms. It's been found that a half wave antenna is a very good resonator and if the antenna is fed at the center, this will be it's lowest impeadance point giving a close match to the 50 ohm transmission line. The voltage swing of the rf signal is lowest at center.
A cheap and dirty method of calculating a half wave antenna length also known as a DIPOLE antenna, is to divide the frequeny in megahertz into the magic number of 468 to get the length in feet. The 468 number is the 1/2 wave length inside a wire. The actual 1/2 wave length in free space is obtained by the number 492 for foot calculation. A frequency wave length is actually shorter inside a wire.
Posted on 2006-07-16 09:24:53 by mrgone
eko,

the thing好bsp; i dont understand is when radio wave gets to an ant , what happens in the ant ? does it make electrons move and create current , if it does how can it handle alot of diffrent frenq in the same time ?

Posted on 2006-07-16 11:19:13 by Ratch
eko,

this is what i cant understand
how can a wire carry diffrent freq at the same time ?
what realy happens in the wire? for 1 freq the electrons move in the wire
but if you have 2 freq or more at the same time , how do the electorns move?

The amplititudes of two or more waves are additive.  This can be proved graphically and algebriacally.  The charge in a conductor moves according to the additive sum of the different voltages actiing on the conductor.  Ratch
Posted on 2006-07-16 11:26:43 by Ratch
The way that virtually all radios today select a particular frequency is a process know as Superhetrodyne. What superhet is, is the mixing of two or more signals. So your radio and for simplicity purposes we will suppose a single conversion superhet and the stages will be like this. 1. You will usually have a front end filter for band selection. This is used to attenuate undesired frequency energy, kind of a focusing unit. It is made of capacitors & inductors. 2. You will usually have a small signal rf amplifier. 3. You will have an oscillator usually variable by VFO or PLL and this stage is known as the 1st Local Oscillator. 4. The mixer where you will feed the filtered/amplified signal & the variable oscillator signal which is your tuning mechanism. The output of the mixer will be (for the most part and is a complicate subject) the sum and difference frequencies of the two input signals and the original two signals. These four signals will have the highest signal strength.5. The crystal filter. Well we are only interested in one and it may be your favorite station you listen to. Here's how we obtain that frequency. OK if the desired signal is say 98.6 FM and my oscillator frequency is being read by a frequency counter and it is at 115.6 MHz the difference between these two frequencies is 17 MHz. I'm kinda working backwards here but this example shows you that we have discovered that the crystal filter is at 17 MHz. Crystal filters are used because they have a very high Q resonance and they are very selective on the order of 5 to 10 KHz. That means when we pass this mess of mixed signals from the mixer to the crystal filter it will cut out all the undesirable mixer products leaving you with only the desire frequency. The important thing to note here is that your tuning oscillator is not at the desire frequency you wish to receive but it will be offset by either the sum or difference of the crystal filter frequency. Also notice that the desired frequency has been converted to 17 MHz. This will be true for all frequencies as you tune through the dial. They will all end up being 17 Mhz, this is the job of the mixer. I tried to make this simple and don't think I did too good of a job but yes there are other more modern methods of filtering in place of a crystal filter known as DSP (digital signal processing) but is is simply a digital version of the same filtering stage.
?  ?  After the filter you will detect the signal which means to demodulate depending on the type of modulation sent by the transmitter. Either AM or FM. AM detection is nothing more than a diode and FM detector use eithe a slope detector or a discriminator circuit. From there you send it though your audio amplifiers and to the speaker.
?  ? That's the basic stages for selecting a frequency but the overall system can get quite complicated. Look up dynamic range of a mixer and you will see what I mean. Normally you will require post mixer amplifier and IF amplifiers etc. The IF or intermediate frequency is the signal coming out of the crystal filter. It is known as first IF. If you are doing multiple conversions than you will have 2nd IF and 3rd etc.
Posted on 2006-07-16 21:58:38 by mrgone