How antennas receive AM radio signals

  • #51
That makes me think, is the E field of an EM wave positive or negatively charged or something else?
Charges will flow in the direction of an electric field.
 
  • #52
davenn
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How can the E field induce the current if the E field is parallel to the antenna? The E field would have to 'touch' the antenna to induce current, I must have read it wrong. .
hold your arms out in front of you they are parallel to each other
you left arm is the antenna, your right arm is the E-field wavefront,
as your right arm (e-field ) moves from right to left it encounters the antenna
at that time it induces a current into the antenna

Dave
 
  • #53
So if the antenna is VP, the line the EM wave is going is best when it is parallel with the ground (ignoring the idea that space-time is curved)? Getting very close to making me understand !
hold your arms out in front of you they are parallel to each other
you left arm is the antenna, your right arm is the E-field wavefront,
as your right arm (e-field ) moves from right to left it encounters the antenna
at that time it induces a current into the antenna

Dave
 
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  • #54
davenn
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So if the antenna is VP, the line the EM wave is going is best when it is parallel with the ground? Getting very close to making me understand !
no

if the antenna is vertically polarised the e-field will also need to be vertically polarised (perpendicular/90deg to the ground) to induce maximum signal into the antenna

think of a pebble thrown into the pond and the waves you see propagating out are the E-field. In this case they are parallel to the ground. IF you have a vertical stick ( the antenna) pushed into the water and into the bottom of the pond then it is vertically polarised but the waves are horizontal and there woudl be minimum interaction between the stick ( antenna) and the waves, as the waves passed by the stick.

now you could lie the stick horizontal in 2 significant ways ....

1) the stick lies parallel to the oncoming wavefront . thats going to produce maximum interaction between the stick and the passing wavefront. or ....

2) the stick could be end on to the wavefront...ie... the incoming wave hits the end of the stick ... this also produces a minimum interaction between the wavefront and the stick ( antenna)

OK got it that time ? :)

Im running out of different ways to describe it ;)

Dave
 
  • #55
The antenna is HP in that situation and the E field seems to be VP but that does not seem good.
1) the stick lies parallel to the oncoming wavefront . thats going to produce maximum interaction between the stick and the passing wavefront.
Dave
 
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  • #56
davenn
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The antenna is HP in that situation and the E field seems to be VP but that does not seem good.
yes the stick is HP and I said that the waves were horizontal

read again what I wrote


D
 
  • #57
I think I get it! I'm not a fast learner by the way.
yes the stick is HP and I said that the waves were horizontal

read again what I wrote


D
 
  • #58
davenn
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have a look at this was the only decent wave pic I could find

attachment.php?attachmentid=51035&stc=1&d=1348143547.jpg



OK its the polarity of the wave front that determines whether its horizontal or vertical
in water we can only have horizontal as in the pic above

The wavefront is the length wise measurement of the wave as depicted by the blue line from lower left to upper right

The height of the wave is NOT the polarity of the wave ... and that may be where you are tripping over ..... the height of the wave is its amplitude

So you can see I have 2 poles/sticks ( antennas) one is vertical ( the left one) the other is horizontal and is parallel to the wave front


Dave
 

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  • #59
sophiecentaur
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How's the EM wave HP? Is it because it's heading in a horizontal direction? That doesn't sound right.
The polarisation and the direction of propagation are two entirely different issues. The E field is at right angles to the direction of propagation. There is a slight problem here in actually defining the direction of polarisation in terms of H and V. If you have a 'vertically polarised' transmitting antenna then the E field will, indeed be vertical for the wave that is travelling horizontally. If an HP wave is travelling in a horizontal direction, the E field is pointing at right angles to the direction of propagation; that is still horizontal. For a nominally VP wave that is travelling upwards (remember, it is spreading in all directions) at an angle of 45°, the E field, being at right angles to this direction, is not actually vertical but tilted back at 45°. If you fire any linearly polarised signal vertically, however the antenna is orientated, the E field will be horizontal (all lines at right angles to vertical are horizontal). One of the reasons for using circular polarisation for satellite transmissions is that there is no problem distinguishing between clockwise and anticlockwise and there can be no misunderstanding.

That makes me think, is the E field of an EM wave positive or negatively charged or something else?[QUOTE/]
The question doesn't make sense, I'm afraid. Field is Field and Charge is Charge. A field doesn't 'have a charge' a field can exist between around a charged object or between two charged objects. To have a field, there must have been two opposite charges somewhere to cause it. In the case of an EM wave, the alternating fields were caused by some movement or changes in a set of charges somewhere (e.g. an antenna or a decaying atom).

I suggest you do a bit more reading a bit less one-to-one questioning now. There is loads of information out there.
 
  • #60
Yes, I suddently got it before that post but that post helped to clarify my understanding. I really shouldn't have asked that question about if if the E field was positively or negatively charged. The main reason is that I could probably have found out a lot about it on google and I worded it really incorrectly. Sorry. Anyway thanks for all the help. (No need to reply to this post.)
The polarisation and the direction of propagation are two entirely different issues. The E field is at right angles to the direction of propagation. There is a slight problem here in actually defining the direction of polarisation in terms of H and V. If you have a 'vertically polarised' transmitting antenna then the E field will, indeed be vertical for the wave that is travelling horizontally. If an HP wave is travelling in a horizontal direction, the E field is pointing at right angles to the direction of propagation; that is still horizontal. For a nominally VP wave that is travelling upwards (remember, it is spreading in all directions) at an angle of 45°, the E field, being at right angles to this direction, is not actually vertical but tilted back at 45°. If you fire any linearly polarised signal vertically, however the antenna is orientated, the E field will be horizontal (all lines at right angles to vertical are horizontal). One of the reasons for using circular polarisation for satellite transmissions is that there is no problem distinguishing between clockwise and anticlockwise and there can be no misunderstanding.

I suggest you do a bit more reading a bit less one-to-one questioning now. There is loads of information out there.
 
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  • #61
Thanks so much! I get it now ! (No need to reply to this post.)
no

if the antenna is vertically polarised the e-field will also need to be vertically polarised (perpendicular/90deg to the ground) to induce maximum signal into the antenna

think of a pebble thrown into the pond and the waves you see propagating out are the E-field. In this case they are parallel to the ground. IF you have a vertical stick ( the antenna) pushed into the water and into the bottom of the pond then it is vertically polarised but the waves are horizontal and there woudl be minimum interaction between the stick ( antenna) and the waves, as the waves passed by the stick.

now you could lie the stick horizontal in 2 significant ways ....

1) the stick lies parallel to the oncoming wavefront . thats going to produce maximum interaction between the stick and the passing wavefront. or ....

2) the stick could be end on to the wavefront...ie... the incoming wave hits the end of the stick ... this also produces a minimum interaction between the wavefront and the stick ( antenna)

OK got it that time ? :)

Im running out of different ways to describe it ;)

Dave
 
  • #62
What's the electricity in an antenna like when a radio wave is near and how can it be decoded? I'm not looking for a broad answer but for a deep understanding of how it works, I've spent days and days looking for the deep understanding but all I see is the common practical side. I'm looking for the very foundation inside the components of a radio? I don't know how to explain what I mean by deep, my words are often misunderstood on this subject. I want to understand every, every part of a radio. I'm starting to doubt there's the information on the web even. I like simple-english answers best, I don't need a personal answer but I would appreciate some resources, guidance, or advise for me follow if have any for me, thanks :)
 
  • #63
sophiecentaur
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"Decoded"? A radio receiver does this. It basically amplifies the tiny currents in the antenna and then uses a 'demodulator' circuit to get the data / programme material from the RF carrier. The demodulator will be specific to the form of modulation used at the transmitter.
The currents in an antena are 'induced' by the varying fields in a passing EM wave - in the same way that the secondary winding of a transformer has volts induced in it by the 50(60)Hz AC of the mains without 'physical contact'. You can treat this at all levels of complexity of course but there isn't an in depth 'verbal' way of discussing it - it can only be the arm waving kind of conversation, using English without Maths terms. (You may be asking the impossible, here; it depends upon your actual level of existing knowledge.)
There is plenty of info at all levels available on the internet if you are prepared to trawl around and find something to suit your level.
 
  • #64
Thanks, I'm now looking for a very very simple radio demodulator circuit diagram that shows the essentials. Also an amplifier diagram and a radio transformer diagram. As I've said in my older posts I'm only beginning to learn about radios. Very simple schematic diagrams seem like a good way of learning about this. I don't really know where to go to learn about this stuff though so I try to get my questions answered on Google with no luck so far. I don't have a clue how people can get the information on Google, there's so much sorting :/. Maybe I need some good keywords or online courses. Thanks. :)
"Decoded"? A radio receiver does this. It basically amplifies the tiny currents in the antenna and then uses a 'demodulator' circuit to get the data / programme material from the RF carrier. The demodulator will be specific to the form of modulation used at the transmitter.
The currents in an antena are 'induced' by the varying fields in a passing EM wave - in the same way that the secondary winding of a transformer has volts induced in it by the 50(60)Hz AC of the mains without 'physical contact'. You can treat this at all levels of complexity of course but there isn't an in depth 'verbal' way of discussing it - it can only be the arm waving kind of conversation, using English without Maths terms. (You may be asking the impossible, here; it depends upon your actual level of existing knowledge.)
There is plenty of info at all levels available on the internet if you are prepared to trawl around and find something to suit your level.
 
  • #65
sophiecentaur
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If you are just starting on this topic then I suggest you find out about Amplitude Modulation (keyword?). This is the first form of modulation used and was very well suited to the simplest "cat's whisker" style of receiver. Look up "Diode Demodulator" too.

It might help if you remember that Modulation is not just simply 'adding' a programme signal to a carrier wave. It involves a more complicated operation -more like multiplication than simple addition.
 
  • #66
Thanks, I think that might help me find what I'm looking for. I appreciate it. :)
If you are just starting on this topic then I suggest you find out about Amplitude Modulation (keyword?). This is the first form of modulation used and was very well suited to the simplest "cat's whisker" style of receiver. Look up "Diode Demodulator" too.

It might help if you remember that Modulation is not just simply 'adding' a programme signal to a carrier wave. It involves a more complicated operation -more like multiplication than simple addition.
 
  • #67
davenn
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..............
It might help if you remember that Modulation is not just simply 'adding' a programme signal to a carrier wave. It involves a more complicated operation -more like multiplication than simple addition.
not usually
AM in it's most commonly used form is JUST modulating a fixed frequency. There is no multiplication ... no change in the carrier frequency.
Even FM or SSB doesnt multiply the carrier freq. They just vary the carrier in different ways... FM by causing a relatively small change in the freq. small for voice comms anything from 5kHz to 25kHz. ~ 150kHz for FM stereo broadcast. up to 8 MHz for FM TV that us amateurs use where we have the freq bandwidth to do so.
AM by varying the amplitude of the carrier
In an AM transmitter, the modulation is usually applied to the final amplifer stage ( excluding an external linear amplifier situation) The power level of the AM audio amplifier will pretty much equal the RF carrier power level. The 25W land mobile transceivers I used to work on had a 25W AF amplifier in them to modulate the final stage.
In a FM transmitter, the modulation is usually applied to the oscillator stage. There may be frequency multiplication stages up to the final TX freq needed, but this has nothing to do with the fact that its a FM modulated transmitter ie... the FM modulation is not causing the multiplication to the required output freq.
The same with an AM TX, there may also be several stages of freq multiplication from the initial oscillator freq up to the final output freq and applying AM modulation to the final stage is not part of the freq multiplication scheme.

cheers
Dave
 
  • #68
sophiecentaur
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not usually
AM in it's most commonly used form is JUST modulating a fixed frequency. There is no multiplication ... no change in the carrier frequency.
Even FM or SSB doesnt multiply the carrier freq.
Amplitude modulation (simplest description with sinewave modulation of a sinewave carrier):
A(t) = A0sin(ωcarriert)(1+Bωmodt)
where (1+Bωmodt) is the modulating signal

B is the modulation index (0>B>=1)
It Multiplies the unmodulated carrier signal by the modulating signal

There is no frequency multiplication involved, of course but that's not what I said. I was making the point that it is not 'Addition' of one signal to another. That wouldn't produce any audio components (sidebands) at the RF frequency.

Yes. Audio AM can be done in the output stage because class C amplification can be used that way (for efficiency). However, analogue TV (AM) uses low power modulation and linear amplification with TWT's, Klystrons or UHF transistors in linear mode. (It also allows VSB to be used to restrict channel occupancy)
 

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