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I've seen a few diagrams relating to AM demodulation, and I can't for the life of me figure out where the voltage source and ground come from in an antenna. From what I understand, an antenna can just be a piece of wire. How can it have a voltage source and a ground?

Here's an example diagram:

Where does the voltage source and ground come from?

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USER_RESU
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3 Answers3

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Try thinking about a mechanical analogy; a microphone at some distance away can receive a sound produced by a loudspeaker. The sound is pushing the microphone diaphragm in and out and a voltage is produced at the terminals of the microphone. A bigger diaphragm area collects more sound (more power) and produces a larger terminal voltage.

The microphone can be wired to a differential amplifier (no ground connection) or one side can be grounded and a single-ended amplifier can be used. The term “ground” is just a reference voltage and doesn’t need to be physically connected to earth.

A simple wire antenna (when you look into the maths) has what is called an effective aperture. This is measured in square metres i.e. it is a physical area just like the diaphragm of a microphone. The antenna “catches” incident EM waves (real power) and produces a voltage across its terminals.

This voltage can be fed into a differential amplifier or a single-ended amplifier (just like the microphone). However, due to the nature of antennas and EM waves, some antennas are “upset” by a single-ended input whereas some antennas need a single ended input. As it happens, a quarter wave monopole (a piece of wire) needs a “ground” reference to work optimally. As per the microphone, “ground” is not necessarily “earth” but “earth” can be conveniently used.

A magnetic loop antenna can be "grounded" or fed into a differential amplifier - it receives the magnetic content of the EM wave and is less susceptible to what type of amplifier it connects to. As a side note, Voyager II, the space probe has no physical connection to earth and it receives and transmits radio (EM waves) without a problem (well it did until it passed out of range a couple of years ago)

Andy aka
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  • @DrFriedParts oops yes. qtr wave monopoles are often mistakingly called dipoles and I'm not exempt!! – Andy aka Jan 19 '15 at 09:53
  • I really like your microphone analogy. It uses a different medium but it may be just that little bit easier to understand when explaining the concept to others. – captcha Jan 19 '15 at 22:26
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This is to do with the antenna system. The schematic shown will take out the carrier-modulated signal and only leave the baseband signal on the output terminals.

It looks like the author of the schematic assumed that the antenna would be a grounded system, i.e. a quarter-wave vertical over ground (or some kind of ground plane). You do not need ground (as in earthed ground) for an antenna system to work.

The voltage source comes from the diode which is rectifying the AC signal. The 'ground' as shown with an earth ground symbol is merely for referencing the positive signal to.

captcha
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  • How does the voltage travel down the wire if there is no ground? It would be an open circuit. – USER_RESU Jan 19 '15 at 04:58
  • Ground is merely a reference here. You could call it negative but this system is working fine without ground-to-earth. You could have a 'ground' to a chassis or the negative terminal of a battery but the battery is not even required in your schematic. Google 'crystal set' and you may get some more background info. I guess the confusion comes from the term 'Ground' having multiple meanings. – captcha Jan 19 '15 at 05:28
  • Why would the electrons flow if there was not a ground? If I were to simply touch a wire to the positive or negative terminal on a battery, no electrons would flow. Why are antennas exempt? – USER_RESU Jan 19 '15 at 05:44
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An antenna is generally either a bi-pole, where two wires extend into the air in roughly opposite directions and the signal is sensed between the two wires, or a mono-pole, where only a single wire extends into the air, and the signal is sensed between that single wire and either "ground" or some other large mass which is treated as "ground".

For the mono-pole scheme, the "ground" does not need to literally be connected to earth, but simply needs to be a large conducting body (such as the body of a car, or even simply the metallic frame of a "boombox" radio).

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  • What about a loop antenna, this is a single wire and not referenced to ground as you write. – captcha Jan 19 '15 at 04:24
  • @captcha - Right -- forgot about the loop (which you treat sort of like a bi-pole). And of course there are more exotic antennas with ferrite cores, etc. – Hot Licks Jan 19 '15 at 12:42
  • Yeah, when I wrote my comment I realised that there are probably many more designs that could act as an antenna that I'm not even closely aware of. Very interesting stuff this rf world. – captcha Jan 19 '15 at 21:50
  • @captcha - It gets really weird when you get into phased arrays and the like. – Hot Licks Jan 19 '15 at 22:10
  • and fractal slot antennas, and the list goes on.. Anyways, I hope the OP has a better understanding in that ground can mean many things. There's also an Amateur Radio SE which could provide more background on the matter. – captcha Jan 19 '15 at 22:22
  • Yeah, basically "ground" is anything whose voltage level is not fluctuating as fast or as far as voltages elsewhere in the circuit. – Hot Licks Jan 19 '15 at 22:23