How antennas receive AM radio signals

[webberfolds]

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 would 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 . that's 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

 

[webberfolds]

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 :)

 

[sophiecentaur]

"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.

 

[webberfolds]

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.

 

[sophiecentaur]

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.

 

[webberfolds]

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.

 

[davenn]

.....
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 doesn't 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

 

[sophiecentaur]

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 doesn't multiply the carrier freq.

Amplitude modulation (simplest description with sinewave modulation of a sinewave carrier):
A(t) = A₀sin(ω_carriert)(1+Bω_modt)
where (1+Bω_modt) 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)