Question about radio waves

[CSL126]

I know that a radio wave is an electical signal. I also know that an antenna pushes electrons in and out at a certain frequency, but is that the frequency that you would put in your reciever? If the signal leaving the antenna is at 158.73 Mhz, is that the frequency that the signal is on? Also, how is voice transmitted over a signal? I know that it has something to do with modulation, but could someone please explain this? Thanks!

 

[N_Jay]

I know that a radio wave is an electical signal. I also know that an antenna pushes electrons in and out at a certain frequency, but is that the frequency that you would put in your reciever? If the signal leaving the antenna is at 158.73 Mhz, is that the frequency that the signal is on? Also, how is voice transmitted over a signal? I know that it has something to do with modulation, but could someone please explain this? Thanks!

The knowledge you are looking for will come much better from a book than an on-line discussion.

Try www.arrl.org

 

[Bill2k]

Understanding FM Transmitters

Understanding FM Receivers

This should get you started...... I'm looking for the Understanding AM Transmitters pdf's on the arrl page.

 

[RayK]

Here is a good place to start, the answers your looking would take too long to explaine here.
Try Here for a start http://electronics.howstuffworks.com/radio.htm

 

[ofd8001]

Let me try a short explanation, and not too detailed. You high tech guys, please bear with me.

A radio signal is electrical energy, in the form of a wave. The size of that wave relates to the frequency.

Radio waves have highs and lows (like peaks and valleys, up and down). The height of a radio wave is called "amplitude".

Radio waves also have a certain amount of time they take to go through one cycle or width. If you know what a sine wave is, that's the way a radio wave is. How rapidly the cycle is repeated is known as the "frequency". Your radio frequency of 158.73 MHz is repeated itself 158.78 million times per second. MHz is "mega-cycle" in terms of seconds.

A radio transmitter does some slight things to the radio waves. An AM radio, or Amplitude Modulation, makes slight changes to the height of the wave based upon the sound it is transmitting.

An FM radio, or Frequency Modulation, slightly changes the speed of that wave, based on the sound it is transmitting.

As you may have guessed, modulation is the changing of the radio wave.

Emphasis on "slight" on both of these.

A radio receiver decodes these changes in a radio wave's Amplitude or Frequency and translates them back into sounds a human ear can make out.

Again, this is a down and dirty explanation and a while lot of real technical details are left out.

 

[CSL126]

Thanks for the replies guys, I did read a few of those resources. Some of this stuff is sort of hard to understand. Like bandwidth, I know that modulated information travels along a carrier wave, but what is bandwidth? I understand that it is the amount of data transferred, but why is it that higher frequencies can handle more data? Also, what happens if too much data is transmitted on a low frequency?

 

[ofd8001]

As far as bandwidth goes, here's this. Remember how the FM radio signal was transmitted by changing the frequency, the process of which is called modulation.

There is a limit imposed by the FCC on the amount of change. That limit is called "bandwidth". The frequency you are listening to is the "center point" of that given bandwidth.

Using your frequency of 158.730 MegaHertz, it's bandwidth ranges from 158,717,500 hertz to 158,742,500 hertz for a bandwidth allowed to be 25 KC (KC is kilo or 1,000, cycles per second). Hertz is cycles per second.

The FCC says that newer radios must limit their bandwidth to 12.5 KC. You can do the math by subtracting half of the 12.5 KC (6.25 KC) from 158.730 Mhz and adding 6.25 KC to 158.730 Mhz.

As far as data goes, I ain't got a clue about that stuff. I do know that if you try to put a gallon of water in a quart jar, you make a mess of things. I suspect that may also be true if you tried to exceed the capacity of your data frequency.

 

[kb2vxa]

Hi guys,

Cummon now, simple questions deserve simple answers especially since he said it's confusing.

"...but what is bandwidth?"

Bandwidth is how "broad" a signal is, how much frequency spread it occupies.

"I understand that it is the amount of data transferred..."

Not exactly, but with digital the higher the data rate the more bandwidth it occupies. This is true no matter if it's over wire or over the air. That's why broadband Internet connections are faster than dial-up and called "broadband".

For you techies out there, the telco line carries the usual 300-3,000Hz "in band" signal for analog voice but above it "out of band" is the data. That makes it easy to design an audio filter, a diplexer if you will to separate the two and send them to the proper device. Dial-up of course is "in band", that is the modem tones are in the analog voice range. The highest data rate is determined by the highest audio frequency the line will pass so with broadband an extended high frequency response is needed. Cable doesn't depend on audio, rather it's RF so even faster rates can be achieved. Fiber optic laser flies but for the time being it's limited to trunk lines, the Internet itself is faster than you can imagine.

"...but why is it that higher frequencies can handle more data?"

Not so, it's a matter of radio spectrum allocation by the FCC. The frequency bands are bigger the higher you go so a higher data rate and more occupied bandwidth is allowed.

"Also, what happens if too much data is transmitted on a low frequency?"

Broadband on HF would take up an entire band, only one station could use it at a time so only narrow band is allowed.

The amount of occupied bandwidth depends on the type of modulation, the subject is far to complex to fully explain here. Just to give a few simple examples, CW and phase shift keying (PSK) are extremely narrow, only a few Hz. SSB is broader, about 3KHz, AM and FM voice twice more and WFM music broadcast broader still, about 75KHz. (The channel is 100KHz to include the guard bands, don't get confused.) Basically the mode and bandwidth depend on how much information is being transmitted and at what rate.

Now crack the books, we'll make a communications engineer out of you yet!

 

[nd5y]

Broadcast FM bandwidth is approximately 180 kHz, the deviaion is 75 kHz and channel spacing is 200 kHz.

 

[PhilJSmith67]

FYI, the speed of a radio wave never changes based on modulation, not with AM, not with FM. It is always equal to 186,282 miles per second (in a vacuum) regardless of the frequency. This speed is only less than 186,282 mi/sec when the signal travels through another medium, such as a metal wire, water, etc.

 

[kb2vxa]

Hi Phil and all,

The rest of the world says it travels at 300M/s, only the US is unable to calculate wavelength and the reciprocal, frequency. Then too we misspell aluminium and pronounce it like we spell it. (;->)

Thanks Tom, I got my deviation and bandwidth mixed up again. Right, deviation is +/- 75KHz.

 

[Bill2k]

300,000,000M/s

 

[Al42]

"...but why is it that higher frequencies can handle more data?"

Not so, it's a matter of radio spectrum allocation by the FCC. The frequency bands are bigger the higher you go so a higher data rate and more occupied bandwidth is allowed.

But it's not an arbitrary decision (one of their few that isn't). The higher the frequency, the smaller percentage a given bandwidth is.

IOW, a 200 KHz FM radio channel takes about .2% of the frequency. That same bandwidth on the AM broadcast band (let's take 1,000 KHz as representative) would take about 20% of the frequency - probably impossible to design a decent receiver for. (Probably impossible to design a transmitter for, as well.)

As far as data vs. bandwidth, the faster the data the higher its frequency so, for reasons far beyond the scope of this discussion, the more bandwidth it needs. (High speed Morse code takes more bandwidth than low speed Morse code, for instance.)

BTW, ofd8001, your explanation was just simple enough for the non-technical and not so short that this technical type cringed when he read it.

 

[kb2vxa]

Hi Al and readers,

"The higher the frequency, the smaller percentage a given bandwidth is."
Isn't that another way of saying;
"The frequency bands are bigger the higher you go so a higher data rate and more occupied bandwidth is allowed.
(;->)

"As far as data vs. bandwidth, the faster the data the higher its frequency so, for reasons far beyond the scope of this discussion, the more bandwidth it needs."

That can be confusing, you meant the modulating frequency, not the carrier frequency. The faster the data the higher the maximum modulating frequency, the very same thing as in any form of modulation which results in sidebands spaced at greater distances from the carrier, thus greater occupied bandwidth. I believe that covers it, it's only the math that's beyond the scope of this discussion. (;->)

I like that definition of modulation, "change" is pretty good but there is another. The phrase "please modulate your voice" is a sophisticated way of saying "please don't shout".

modulate (`m„dj…•le€t) vb.
1. (tr.) to change the tone, pitch, or volume of.
2. (tr.) to adjust or regulate the degree of.
3. Music.a. to subject to or undergo modulation in music.
b. (often foll. by to) to make or become in tune (with a pitch, key, etc.).
4. (tr.)Physics, electronics.to cause to vary by a process of modulation.
[C16: from Latin modul?tus in due measure, melodious, from modul?r“ to regulate, from modus measure]

modulation (•m„dj…`le€‰?n) n.
1. the act of modulating or the condition of being modulated.
2. Music. the transition from one key to another.
3. Grammar.a. another word for intonation (sense 1).b. the grammatical expression of modality.
4. Electrical engineering.a. the act or process of superimposing the amplitude, frequency, phase, etc., of a wave or signal onto another wave or signal or onto an electron beam. See also amplitude modulation, frequency modulation, phase modulation, velocity modulation.b. the variation of the modulated signal.

Ain't I such a pedant? (That's one for YOU to look up.)
(;->)

After market add-on;
Here's a question for the Geek Squad. If you modulate a 1MHz carrier with a 2Mb/s data stream, what wold be the occupied bandwidth?

 

[N_Jay]

Like I said earlier:

The knowledge you are looking for will come much better from a book than an on-line discussion.

You can read book several time to gain deeper understanding.

You can refer back to a book when you learn something new that clarifies a misconception the first time through.

(Sorry for the minor rant, but I think lots of people today want to gain the knowledge that it takes work to get but don't want to do that work to gain it.)

 

[kb2vxa]

Hi N Jay and all,

We know you're a book worm and I have often echoed you, but don't you think that some links and a bit of discussion whets the appetite? Ain't no way, no way Jose a newbie can learn much from this forum or any other, we provide a taste of honey so he'll go looking for the hive.

 

[N_Jay]

Hi N Jay and all,

We know you're a book worm and I have often echoed you, but don't you think that some links and a bit of discussion whets the appetite? Ain't no way, no way Jose a newbie can learn much from this forum or any other, we provide a taste of honey so he'll go looking for the hive.

If lucky,:twisted:

but I find more times than not he will think he has learned "enough" when he gets his fill of 1/2 right, 1/2 wrong and 1/2 misunderstood information.:roll: :roll: :evil:

 

[DickH]

If lucky,:twisted:

but I find more times than not he will think he has learned "enough" when he gets his fill of 1/2 right, 1/2 wrong and 1/2 misunderstood information.:roll: :roll: :evil:

We used to say, "He knows just enough to be dangerous"

 

[RISC777]

modulation n.
4. Electrical engineering.a. the act or process of superimposing the amplitude, frequency, phase, etc., of a wave or signal onto another wave or signal or onto an electron beam. See also amplitude modulation, frequency modulation, phase modulation, velocity modulation.b. the variation of the modulated signal.

Ain't I such a pedant?

And, my pedantic friend :wink:, don't forget about pulse code modulation. Maybe we'll make a pendant out of you one of these days, when you're older perhaps. lol

So, for the pedantry, how many ways can one modulate?

 

[kb2vxa]

Hi dragons,

Yeah, this is dragon on fer sheeer.

"...but I find more times than not he will think he has learned "enough" when he gets his fill of 1/2 right, 1/2 wrong and 1/2 misunderstood information."

Yeah, he'll find plenty here. (;->)

"...don't forget about pulse code modulation."

This is P AM and FM in New York, your DAB station.

"...how many ways can one modulate?"

I haven't checked my wobulator lately. (Do you know what one is?)

Now let's go back over the lesson, you haven't answered my (trick) question. Once again, if you modulate a 1MHz carrier with a 2Mb/s data stream what is the occupied bandwidth? Now here's another, they just keep piling up. What is velocity modulation and how is it used? Hint; there's a clue in the definition.

Oh, just to rephrase your question a bit there Risc, How do I modulate thee? Let me count the ways...

See what happens when I stay up 'till 3:00am? Oh that's nothing, you should see me leaving the Field Day site Sunday morning in a straight jacket! Pray for me, I'm a sinner...