Antenna fundamentals

Thread Starter

_nisha246_

Joined Dec 13, 2020
9
Hello, I'm new to RF design and I've been researching extensively, but I haven't found an answer to my question. I'm curious to know what happens to the energy stored in inductance and capacitance when it comes to the near-field region of an antenna. Is this energy eventually converted into radiation? I would greatly appreciate any assistance with this matter.
 

Ya’akov

Joined Jan 27, 2019
8,960
Hello, I'm new to RF design and I've been researching extensively, but I haven't found an answer to my question. I'm curious to know what happens to the energy stored in inductance and capacitance when it comes to the near-field region of an antenna. Is this energy eventually converted into radiation? I would greatly appreciate any assistance with this matter.
Welcome to AAC.

Is this coursework? Is the question involved in something for which you will get academic credit?
 

Thread Starter

_nisha246_

Joined Dec 13, 2020
9
Welcome to AAC.

Is this coursework? Is the question involved in something for which you will get academic credit?

No. I have recently completed my studies in electronics and communication engineering, and my passion lies in designing RF systems. I am currently seeking clarification on certain fundamental concepts by posing questions.
 

Ya’akov

Joined Jan 27, 2019
8,960
What is your conception about inductive and capacitive “storage” of energy in an antenna?

I ask to get an idea of your starting point.
 

Thread Starter

_nisha246_

Joined Dec 13, 2020
9
if we consider the case of a dipole antenna then, the gap between wires or gap between antenna and air creates a capacitance. capacitive energy storage occurs in the electric field.
meanwhile, when alternating current flows through the antenna, self-inductance of the antenna occurs according to faraday's law. inductive energy storage occurs in magnetic fields.
 

Ya’akov

Joined Jan 27, 2019
8,960
OK.

So yes, there is loss associated with parasitic inductance and capacitance in antennas. Energy stored in the capacitance between the antenna element and free space won’t be radiated. Some of that can be avoided, some can’t.

As is the case with capacitive and inductive reactance in a tank circuit, reducing reactance for the capacitance increases it for the inductance, and vice versa.

This is also true of attempts to minimize capacitive and inductive losses in antennas since the measures to reduce one increases the other. For example, you can reduce capacitance by reducing surface area but that will increase inductance.

The capacitance depends on the permittivity of the material that makes up the storage part of the capacitor which for bare antenna elements is pretty much free space. The dielectric constant will be low anyway since the permittivity difference between free space and air is only 1.00059 (dry). Nonetheless, there will be some loss of radiation albeit quite small.

The case of inductance is a little different to the extent that it tends to equalize itself. When the B field collapses, an E field is induced into the element and so the power loss is small, and there can be radiation.

This is my understanding of the situation but a caveat: antenna theory is almost as bad an economics when it comes to being a ”dismal science”. Most antenna design leans heavily on empirical solutions for obvious practical reasons. Lately, of course, there are simulations that offer more opportunity to leverage theoretical possibilities, but as soon you put an isotropic radiator into real space it acts completely differently than its theoretical self.

I am not an expert on this, so take my explanation as my best understanding and stand by for others to say I am mistaken, wrong, or crazy.

Also, I don‘t know how much emag you took for your degree, but reviewing the MIT 802x Emag lectures by Walter Lewin on YouTube may be very useful not only for the review but also because he does very good demos and is lucid and cogent in his explanations. The first of the lectures is linked, below:

 

Ya’akov

Joined Jan 27, 2019
8,960
I want to clarify something I said vis-à-vis “dismal science”.

Of course unlike economics we have very solid antenna design methodologies with repeatable results, and the underlying theory seems to comport well with the working, practical systems built with it as a foundation.

But even with this contrast there is a good comparison in the area of the complexity of the practical vs. the relative simplicity of the theoretical. Perhaps antenna theory has a leg up because, unlike economics, antenna theory doesn’t have to intersect with psychology… except possibly in antenna marketing theory…
 

Papabravo

Joined Feb 24, 2006
20,982
I have always found it helpful to consider an antenna as part of a complete system with impedance discontinuities. You have a source, a transmission line, an antenna, and free space. At each of those "coupling" points there is an abrupt change in the characteristic impedance and what happens to an RF signal at a discontinuity is that part of the signal is transmitted, and part of the signal is reflected back toward the source. Fortunately, it is easy to measure reflections so you can determine how well the system is performing its intended purpose. In a well-designed system, the amount of the reflections is usually quite small.
 

nsaspook

Joined Aug 27, 2009
12,732
https://forum.allaboutcircuits.com/threads/propagation-of-a-pure-electric-field.103711/post-785165
https://www.antenna-theory.com/basics/fieldRegions.php

You will see complex answers because this is a complex subject.
It's usually easier to think this way about EM radiation.
"When you take into account the time it takes for "news" to travel about charges and currents, you are able to uncover the origin of EM waves"

You start to get EM radiation when the EM field symmetry of the system (EM energy and antenna) is broken by phase shifts (retarded potentials) across space.
https://en.wikipedia.org/wiki/Retarded_potential
https://www.ruf.rice.edu/~baring/phys532/phys532_2023_lec_040523.pdf

The fields created by changing sources and currents (FARADAY'S LAW OF INDUCTION) propagate to the point where changes (ripples in the EM fields) in the sources and currents don't influence already existing EM ripples (and VICE VERSA) because of the finite speed of light (cause and effect become separated by space and time). At this point the EM fields are self propagating per Maxwell's equations for source free regions.
https://eng.libretexts.org/Bookshelves/Electrical_Engineering/Electro-Optics/Book:_Electromagnetics_I_(Ellingson)/09:_Plane_Waves_in_Loseless_Media/9.02:_Wave_Equations_for_Source-Free_and_Lossless_Regions

This is a very good, older book on the subject.
https://archive.org/details/in.ernet.dli.2015.6364
Transmission Lines Antennas And Wave Guides (1945)
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