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Why does an antenna work even though only 1 end connects?

 
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cnd
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PostPosted: Fri Feb 06, 2015 12:29 pm    Post subject: Why does an antenna work even though only 1 end connects? Reply with quote

I think I understand how radio waves originate ("vibrating electrons"), but I totally can't get my head around how connecting one end of a wire causes the wire to become an antenna?

Take this for instance ( http://i.stack.imgur.com/vMxPU.png )



Why the heck do the electrons care that there's some wire that "goes nowhere" in that lot?

What's the magic source that's convincing the electrons in the antenna to do their vibrations, even though no current seems to be flowing anywhere in it?
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PostPosted: Fri Feb 06, 2015 10:01 pm    Post subject: Reply with quote

There are no electron vibrations. Current is electron flow.

You are right though about two halves of the antenna required. The opposite half is the ground plane of the product. This is why the ground plane size affects the antenna significantly
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cnd
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PostPosted: Sat Feb 07, 2015 11:49 am    Post subject: Electro MAGNETIC - is that the clue? Reply with quote

Radio waves are caused by electron acceleration (they move back-and-forth, as in, AC current flow) - sorry if I sounded nutty - that's what I meant by "vibrations" - the electrons moving back-and-forth (oscilating) in the wire.

The problem is the "moving" part - I understand how and why they move through a wire that's connected at both ends, but take a look at any FM radio antenna, any radio-control transmitter, a car aerial, or anything like that, and you'll observe that the wire is not connected at both ends: only 1 end.

How does "current" flow, when there's nowhere for it to flow *to* ?

Now - you mentioned "ground plane" - I'm taking a punt here, but I expect a FM radio is going to work just fine in deep space as well? Out there, no "ground" exists, nor does any conductive path between the "negative" portion of a circuit, and the wire antenna soldered only at 1 end.

Or, back on earth, I think FM radio works just as well in the snow (0 humidity) as the warm fog (100% humidity) - lending me the idea that there is no "conducting" taking place between the unattached wire, through to "ground", from the air itself completing the circuit.

Right now, the best guess I can come up with, is the "magnetic" part of all this. If you get 2 magnets really close to one another, then spin one of them 180 degrees, the other one "notices" and reacts. If they were free to turn, but not touch, and you lined many up in a row, turning any one of them will cause all the others to turn as well.

Is this in fact what's going on? The wire "radiates" because all the electrons already in it, are *magnetically* reacting to what's "going on" at one end?
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R. Fry
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PostPosted: Sat Feb 07, 2015 1:01 pm    Post subject: Re: Electro MAGNETIC - is that the clue? Reply with quote

cnd wrote:
...Is this in fact what's going on?...

The schematic included in the OP does not show how those components are constructed.

The antenna shown there radiates because (typically in the VHF spectrum) it is comprised of two separate conductors. RF current flows back and forth between the two conductors. Such a process produces EM radiation.

An example of such is a center-fed, 1/2-wave dipole driven by a 2-wire transmission line connected to the transmitter output connector.
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cnd
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PostPosted: Sun Feb 08, 2015 1:08 am    Post subject: Open ended Reply with quote

I realize some antennas are fed by 2 wires - lets ignore those ones and concentrate only on the simple ones.

Take for example my 27mhz radio-controlled car transmitter, and the corresponding receiver: both of those antennas are literally bits of wire soldered onto the circuitboard at only 1 end.

What makes those wires special? What's going on at the solder-point that's making something special take place in the open-ended wire? Electrons (current) clearly cannot *flow* anywhere, since there's nowhere to flow to.
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R. Fry
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PostPosted: Sun Feb 08, 2015 10:09 am    Post subject: Re: Open ended Reply with quote

cnd wrote:
...Take for example my 27mhz radio-controlled car transmitter, and the corresponding receiver: both of those antennas are literally bits of wire soldered onto the circuitboard at only 1 end. What makes those wires special?

The common "ground" rail (or bus) in the circuits of the tx and rx constitute the 2nd conductor of the antenna system, along with whatever capacitive coupling exists between it and the earth.

Such antenna systems usually are not very efficient, but still useful enough for the application.

OTOH, AM broadcast stations use a monopole radiator driven against a set of buried radial wires, and such systems can radiate about 95% of the available r-f power. In this configuration, r-f current flows back and forth between the buried wires and the monopole, due to the capacitive coupling of the monopole to the earth.

Functionally, both such antenna systems are comprised of two conductors.

cnd wrote:
Electrons (current) clearly cannot *flow* anywhere, since there's nowhere to flow to.

As long as r-f current flows along a conductor, EM radiation will be produced by coupling into space. The r-f current may be (theoretically) zero at the open end of that conductor, but it is not zero along its entire length.
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Last edited by R. Fry on Sun Feb 08, 2015 11:30 am; edited 1 time in total
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cnd
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PostPosted: Sun Feb 08, 2015 11:20 am    Post subject: Reply with quote

Hmm - I'm not sure I can get my head around this still?

Quote:
current

This means "electrons flowing", doesn't it?

Quote:
r-f current

The "r-f" in there - what exactly does that mean? Does it just mean "current, in AC, as some particular frequency" ? This is still electrons flowing somewhere, isn't it?

Quote:
2nd conductor

Do you mean the other end of the aerial here?

Quote:
ground

is this the same as "earth", and do both of these mean "everything connected directly to the negative terminal of the battery" - or is there something else special about it? FM transmitters still work great even if they're not connected literally to any dirt, for example.

Looking back at the FM transmitter - the LC circuit is already connected at both ends: I think you're saying that the antenna somehow encourages electrons in it to move, since they're influenced by "earth" (negative?) - if this is so, how? I presume they're not flying out of the wire, through the air, and going into the battery: what exactly is causing them to move?

I actually have another tricky question; I'm lead to believe that humans do not yet fully understand the nature of light, which is also the same "stuff" as radio waves; do we actually *understand* how radios and antennas really work, or, have we just learned how to use them by observing what they do and making up theories, but without actually knowing *how* they work?

So sorry I sound totally crazy: I'm actually working on a project to use a human body as an antenna, and I've managed to get it working by connecting just 1 wire to the skin, and I'm desperately trying to understand *why* it's working, so I know what I'm supposed to do with the "other wire" (which, if it touches the skin, stops it all working, which has me even more confused).
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R. Fry
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PostPosted: Sun Feb 08, 2015 12:29 pm    Post subject: Reply with quote

cnd wrote:
"current"... This means "electrons flowing", doesn't it?

Yes

Quote:
"r-f current" ... This is still electrons flowing somewhere, isn't it?

Yes

Quote:
"2nd conductor" ...Do you mean the other end of the aerial here?

The 2nd conductor of the antenna system in your R/C gear is formed by the common bus of the circuits in the tx and rx.

Quote:
"ground" ... is this the same as earth, and do both of these mean "everything connected directly to the negative terminal of the battery" - or is there something else special about it?

The circuit bus or rail that connects (usually) to the negative terminal of the d-c power supply may, or may not be connected to the earth by a physical conductor. However the conducting path along that circuit bus/rail can function as the 2nd conductor of an antenna system even if it doesn't have a physical conductor connecting it to the earth.

Quote:
FM transmitters still work great even if they're not connected literally to any dirt, for example.

No connection to the earth is necessary when a balanced antenna is used. In the case of your R/C gear, the functional form of their antenna systems is an off-center-fed-dipole consisting of the wire considered to be "the antenna," and the common rail of the circuit boards in the tx and rx. Dipoles using two equal-length conductors are balanced antennas. Dipoles using unequal length conductors are not truly balanced, but they do radiate.

Quote:
Looking back at the FM transmitter - the LC circuit is already connected at both ends: I think you're saying that the antenna somehow encourages electrons in it to move, since they're influenced by "earth" (negative?) - if this is so, how? I presume they're not flying out of the wire, through the air, and going into the battery: what exactly is causing them to move?

When the r-f current produced by the final amplifier in the transmitter flows OUT of the common rail/bus of the transmitter, it is flowing IN to the end of the antenna conductor connected to the transmitter, and vice-versa. Those two conducting paths enable the antenna system to radiate.

Quote:
... do we actually *understand* how radios and antennas really work, or, have we just learned how to use them by observing what they do and making up theories, but without actually knowing *how* they work?

The operation of antenna systems is well understood, and may be studied in various textbooks by authors such as Kraus, Johnson & Jasik, Terman, Balanis etc, and by referring to other areas of antenna-theory.com.

Quote:
I'm actually working on a project to use a human body as an antenna, ...

The human body is a poor electrical conductor and EM radiator compared to a wire. If the goal is to maximize the radiated fields that can be produced for a given transmitter power/frequency, it would be preferable to use wires as radiators.
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cnd
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PostPosted: Sun Feb 08, 2015 1:31 pm    Post subject: Thanks Reply with quote

Hi (again!). That's for your abundant patience with me!!

I'm understanding your terms a lot better now (thanks again for helping me with that).

I'm now understanding that this FM transmitter does work, and the mechanics of it, but the "why" part still eludes me.

I personally "visualize" electronics by loosely basing it all on an analogy of marbles (electrons) flowing through a hose (wire). "Volts" are their speed, "Amps" is the diameter of the hose. Transistors are valves, capacitors kindoflike buckets, resistors I guess "constrict" the flow etc... like I said - loose analogy. I've "modified" my idea by now having the marbles being "magnetic", because I think electrons really are magnetic. Motion disturbs their fields, which generates rf radiation is my newfound learning, and what brought me here.

I am OK with the idea that "current" (r-f, aka, alternating current, aka, electrons moving back and forth rapidly, rather than actually moving in a (dc) line) somehow flows between antenna and bus, but do you know *why* it does so? There's no *electrically* conductive "path" in that scenario that I can see/understand, but it does obviously work.

As for my project - no - I don't actually want it to radiate; it's a PAN (personal area network) - only for other devices also in contact with it to communicate, and specifically (crucially), for anything *not* in contact, to deliberately not be able to communicate anymore. At the risk of getting OT, if you can think of some other way to do that; eg. have my watch "talk" to my anklet, but not the person next to me, and for comms to instantly be impossible upon disconnection of either, I'm all ears. fyi - that's a simplified example, not the actual setup, as in, ble/2.4g/etc is not a possible solution.
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R. Fry
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PostPosted: Sun Feb 08, 2015 1:53 pm    Post subject: Reply with quote

Maybe another person will respond to your questions better than I did.

I do suggest that you study antenna engineering texts and other areas of antenna-theory.com, where many of your answers can be found.
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