How does an antenna work?

[vk6zgo]

I repeat my original advice.Look it up in one of books I mentioned,Google for it.
Stay away from highly mathematical texts till you get a feel for things.
Antennas do work,& have been working for many years,so I also suggest you re-read BoredAtWork's answer above,he is giving you good advice.

I wrote a huge screed with some (I thought) pertinent information,then accidentally lost it.  ( I hate touchpads!!!)

VK6ZGO

[insurgent]

Here's a PDF that explains without math, how dipole and monopole antennae work. Apparently the monopole is still two parts, just the second part is a "reference plane" which doesn't have to be ground.
http://www.hottconsultants.com/pdf_files/dipoles-1.pdf

[Zero999]

Hero999, are you stating that antenna has currents in it to create EM field because each wire of dipole is capacitively coupled to the other wire?

It's more about self capacitance than mutual capacitance which only exists between two objects.

What about early satellites? They are not grounded in any way, in near vacuum, monopole stick antennas.

Because the satellite has self capacitance like any large metal object as I explained above.

You need to understand how self capacitance works before gong further. An single conductive object millions of miles away from any other object has self capacitance because electrons can be removed and added to it. This is how electrostatics work and the same principles are important at high frequencies.

Apparently the monopole is still two parts, just the second part is a "reference plane" which doesn't have to be ground.

The "reference plane" just has to have enough self capacitance for its impedance to be low compared to the antenna at the frequency of operation.

Large, low frequency aerials just use the earth as a reference because it has a very large self capacitance. At higher frequencies much smaller metal objects can be used.

[Wartex]

I wrote a huge screed with some (I thought) pertinent information,then accidentally lost it.  ( I hate touchpads!!!)

VK6ZGO

You can always hit CTRL-Z if you selected and deleted a huge chunk of text. Also, http://code.google.com/p/touchfreeze/

[jahonen]

John D. Kraus (RIP) defines the antenna in his "Antennas for all applications" book as

A radio antenna may be defined as the structure associated with the region of transition between guided wave and a free-space wave, or vice versa. Antennas convert electrons to photons or vice versa. Regardless of antenna type, all involve the same basic principle that radiation is produced by accelerated (or decelerated) charge. The basic equation of radiation may be expressed simply as di/dt*l=Q*d²v/dt². Thus, time-changing current radiates and accelerated charge radiates.

Thus, I'm not entirely sure if the capacitance approach is intuitive. It seems that current is the fundamental thing which causes radiation from suitable structure.

For example, a slot antenna has almost identical radiation pattern than dipole, but has the electric and magnetic field components interchanged. Slot antenna can be constructed from a metal sheet by making a slot in it which has appropriate length. Obviously, it is a bit hard to think about capacitance over the slot (I'm not even sure that it is required to produce radiation). And how about a horn antenna, which is a suitably formed waveguide?

Regards,
Janne

[Zero999]

Thus, I'm not entirely sure if the capacitance approach is intuitive. It seems that current is the fundamental thing which causes radiation from suitable structure.

I think what he's failing to understand is how the current flows in a monopole when there's only one wire connected to it, the other is connected to nothing so it's apparently open circuit. He needs to understand the electrons are being moved back and fourth between the ground plane and the antenna.

For example, a slot antenna has almost identical radiation pattern than dipole, but has the electric and magnetic field components interchanged. Slot antenna can be constructed from a metal sheet by making a slot in it which has appropriate length. Obviously, it is a bit hard to think about capacitance over the slot (I'm not even sure that it is required to produce radiation). And how about a horn antenna, which is a suitably formed waveguide?

Of course self-capacitance is not important for all types of antennas.

[Wartex]

I think what he's failing to understand is how the current flows in a monopole when there's only one wire connected to it, the other is connected to nothing so it's apparently open circuit. He needs to understand the electrons are being moved back and fourth between the ground plane and the antenna.

I understand that very well, you haven't read my first posts carefully. My question is this: A satellite in space has N number of free electrons in it, PERIOD (assume it has NO solar panel and just a battery).

Somehow the transmitter circuit will displace a certain amount X of electrons from the body of the satellite (dipole lead #1 aka ground plane) to the whip antenna (dipole lead #2). Electrons will exhibit wave nature and fly off the antenna as photons. So here we have the energy conversion from matter to wave. Correct? Correct.

The question is: HOW do you displace the electrons from ground plane to the antenna? For you to move electrons you need a difference of potential between the antenna and the ground plane. According to your explanation, every transmitter is a static pump and nothing more.

[Time]

The answer is not exactly straight forward for an online forum but if you study maxwell's equations you can see how radiation is produced from accelerating and decelerating charge.  Maxwell's equations are a set of partial differential equations which describe electromagnetics in its entirety.  The changing electric field combined with the changing magnetic field produces radiation in a pattern dictated by the antenna shape.  Its not useful to think of antenna radiation as photonic in nature because the wavelength is so large. 

The electrons themselves do not fly off the antenna.  They simply radiate energy when they experience acceleration and deceleration.  This can effectively be quantified as a resistive loss but the bi-product is not heat.  It is radiation.

Current flows but the electrons only move back and forth over a small distance, they don't actually flow around and around the ground and antenna.  They might move only a few millimeters or centimeters.

[Wartex]

Current flows but the electrons only move back and forth over a small distance, they don't actually flow around and around the ground and antenna.  They might move only a few millimeters or centimeters.

How do you induce current in a wire connected at one end only, if it was stated that antenna is NOT capacitively coupled with the ground plane?

[jahonen]

Wire (or any antenna) "sees" the impedance of the vacuum, about 377 ohms, resistive, thus the current in the antenna. It is the antennas task to match the feedline impedance to the impedance of the vacuum (or whatever it is). I personally think that the vacuum impedance is a difficult concept to grasp intuitively, since 377 ohms is pretty low value for something that has nothing in it Corresponding impedance is about the same for air also.

Regards,
Janne

[johnmx]

How do you induce current in a wire connected at one end only, if it was stated that antenna is NOT capacitively coupled with the ground plane?

That antenna is only an open-circuit for DC, not for RF!

[Mechatrommer]

if you think electrons movement is similar to water ripple or string or slinky spring, things will be much easier. but thats just me.

[Wartex]

How do you induce current in a wire connected at one end only, if it was stated that antenna is NOT capacitively coupled with the ground plane?

That antenna is only an open-circuit for DC, not for RF!

I know. Please explain, on a field level, how that current is induced. Also avoid RF as a magic word because it creates circular logic: "current is induced due to RF, RF is what induces current"

[Wartex]

if you think electrons movement is similar to water ripple or string or slinky spring, things will be much easier. but thats just me.

I think of electrons as orbital clouds experiencing particle-wave duality under certain conditions. Simplifying it the wrong way to get the explanation across is not a scientific solution. Oscillating electrons radiate NOTHING unless they jump energy levels and produce/consume a photon. Also, after reading some physics whitepapers I confirmed my knowledge that there is nothing magic about RF radiation and IT IS photonic in nature, and all equations apply equally to visible light and RF radiation.

[Wartex]

Wire (or any antenna) "sees" the impedance of the vacuum, about 377 ohms, resistive, thus the current in the antenna. It is the antennas task to match the feedline impedance to the impedance of the vacuum (or whatever it is). I personally think that the vacuum impedance is a difficult concept to grasp intuitively, since 377 ohms is pretty low value for something that has nothing in it Corresponding impedance is about the same for air also.

Regards,
Janne

Good explanation.

[Zero999]

Electrons will exhibit wave nature and fly off the antenna as photons. So here we have the energy conversion from matter to wave. Correct? Correct.

I think the quote below answers that pretty well.

The electrons themselves do not fly off the antenna.  They simply radiate energy when they experience acceleration and deceleration.  This can effectively be quantified as a resistive loss but the bi-product is not heat.  It is radiation.

Current flows but the electrons only move back and forth over a small distance, they don't actually flow around and around the ground and antenna.  They might move only a few millimeters or centimeters.

The question is: HOW do you displace the electrons from ground plane to the antenna? For you to move electrons you need a difference of potential between the antenna and the ground plane. According to your explanation, every transmitter is a static pump and nothing more.

A static pump is a good analogy for how the oscillator pumps electrons to and from the ground plane in and out of the antenna, although static probably isn't the right word as the electrons are always moving.

How do you induce current in a wire connected at one end only, if it was stated that antenna is NOT capacitively coupled with the ground plane?

Because the ground plane has self capacitance which is not reliant on being coupled to the antenna or anything else.

If you run a Van de Graaff generator until it charges to say 100kV, stand on a non-conductive plastic box and touch the charge dome, a spark will leap towards your finger and you'll receive a shock.

Why can thsi happen you're not grounded?

The answer is the electrons flow from the charged dome (assuming its negatively charged) into your body until the charged is balance.

Where do all those electrons come from?

The earth. The Van de Graaff generator pumps electrons from the earth into the metal dome making it negatively charged. If you now touch the earth, you'll receive another shock as the electrons flow from your body back into the earth.

If you replaced the Van de Graaff generator with a high frequency AC source such as a Tesla coil, a sustained arc will form between you and the dome.

Why? You're not earthed, as you're standing on a plastic box.

The Tesla coil pumps electrons back and forth between you and the earth at a high frequency so the charges never balance.

The same thing happens with a transmitter, antenna and the ground plane, it just shifts electrons between the antenna and the ground plane, no electrons whiz off into space. The electrons don't get converted to radiation. Energy is required to overcome the resistance to moving the elections back and fourth which is mostly converted to radiation.

[Time]

If you excite a single wire with a step type voltage the voltage is not going to appear through the whole conductor instantaneously.  It will take time to propagate down the length of the antenna.  Do this quickly and you have rapidly fluctuating charge ditributions thus producing radiation.  It doesn't matter what the impedance of the of the environment is, that just dictates how well this radiation couples into it.  You need to read about electromagnetic boundary conditions and other EM theory if you don't know the basics. 

If you want to understand fundamental antenna theory you need to goto school and study electromagnetics and maxwell's equations.  Everything that an EE does is just watered down Maxwell's equations with assumptions thrown in. You certainly don't need to know the ins and outs of EM to be a good engineer but if you want to really grasp the fundamental physics there is no other choice.  When you start getting into the raw fundamentals of something like this EM theory becomes unavoidable.  No answer on here will ever be thorough.  Its simply going to be one persons conceptualization of the phenomenon vs. anothers.  To truely understand you have to view it in its elegant mathematical beauty and let your mind manifest its own concepts for what is going on.