Re: how antennas work

[Guest J-Dogg]

Hi. I was wondering if anyone could give me a brief explanation of how transmission antennas work. I understand that continuously changing the direction of electron flow in the antenna causes a magnetic field to surround it, which creates an electric field, which again creates a magnetic field... etc. onward out into space, which is the so-called electromagnetic wave. But my question specifically is how the current is sent through the antenna. I understand that electric current flows from a negative charge to a positive charge, so is an antenna set up like a circuit? For example, my cell phone has a plastic antenna on top... underneath the plastic, is there a little wire like the following?

 

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- +

 

Changing the direction of electron flow would then just involve switching the leads.

 

I'd also like to know, if an antenna is indeed just a circuit, how is it shaped? This would define the shape of the resulting magnetic field, ie. the shape of the electromagnetic wave's path. Are antennas set up like the above illustration, in which case the magnetic field would be a ring around the antenna horizontally; or are they set up like a coil, in which case the magnetic field would be like that of an electromagnet?

 

Any help with this would be appreciated.

[alt_f13]

I'm pretty sure your antenna does not have a circuit in it. Mine doesn't and no others I know do either.

 

I think those types of antennas merely conduct the EM waves better than air/plastic/etc. so the waves are collected and directed to the reciever inside the phone.

 

What you described is how AM antennas on radios work though.

[YT2095]

a regular dipole antenna works like this simplified model;

 

imagine your transmition frequency has the wave length of 1 meter, and your antenna is 1 meter long, as the electrons pas from + to - in a sine wave, the antenna will see 0 at the base and 0 at the top and in the dead center it will have 0 volts, but at 1 quarter of a meter in it will be at peak + voltage and also at 3 quarters of the way up, thereby setting up your circuit

 

draw a line (your antennat) then overlay a single cycle of a sinewave to fit the length of your line.

where it touches your line that will be 0 volts, where it`s furthest away (peak) that will be your top voltage.

now along the line put a 0 or a + coresponding to these, you`ll see 0 + 0 + 0

as you can see that`s a circuit

a quarter wave antenna will be just 0 +, to make things a little clearer

 

hope that helps somewhat?

[Guest J-Dogg]

ok... but I just mean how do they get the electrons to shoot up and down the antenna... antennas, to me, always just look like a stick of metal (or some conductor)... but to have electrical current, don't you need a circuit, ie. a conductor in some sort of loop, starting and ending at the power source...

 

from what I understand, you can't get current to flow through a conductor unless there's some positive "destination" at the other end...

 

sorry, I don't know much about science...

[Guest J-Dogg]

Actually, I think I figured it out myself... and I'll post my solution in case anyone else read this thread and got to wondering...

 

take for example an RC car remote... it consists of a base (the part you hold) and an antenna sticking out the top of it, and it must send signals to the car as EM waves through the antenna. Initially, the antenna has a neutral charge... a positive charge is created in the base, so the electrons in the antenna all move down towards it, leaving the antenna with a positive charge... then a highly negative charge is created in the base, so electrons flow back up into the positive antenna... which brings us back to step 1, ie. the antenna is again neutral. So we start over, and introduce a positive charge below the antenna... etc, over and over... thus causing electrons to flow up and down the antenna.

 

This is how I figure it must work... but if someone can confirm this, that would great.

[YT2095]

that`s very close indeed

they don`t ever flow Down the antenna though, at least not in transmit mode, and also there will be + volts at one point of the antenna and 0 volts at another point, making a sort of circuit an EM type anyway, that`s what radiates, the photons generated by that "circuit"

of course at low power the antenna length isn`t critical, it simply reduces range, but if the frequency is known and the antenna length (SWR as we call it) is matching that, then even at low power the signal distance can be optimised