Question About Antennas For Transmitting Electromagnetic Waves

Thread Starter

Glenn Holland

Joined Dec 26, 2014
705
In a casual discussion about the methods for transmitting RF waves in free space, someone asked why the antenna produces an electric field instead of using a loop to produce a magnetic field.

My response was that the permeability of free space (an empty vacuum) is optimum for the electric field component rather than for the magnetic component which requires permeability similar to iron or an other ferrous material.

Since a vacuum has excellent permeability for the electric field, but poor permeability for the magnetic field, the optimum antenna for electromagnetic waves would be some kind of an air gap rather than a loop of wire.
 

Thread Starter

Glenn Holland

Joined Dec 26, 2014
705
In order to generate EMWs (essentially an electromagnetic field), the antenna has to initially produce either a time varying electric or magnetic field.

However, it seems that a loop antenna for producing a magnetic field would be terribly inefficient for generating waves in free space for a long distance such as RF communications. Therefore what type of antenna is used in portable radio transmitters such as a phone?

Pardon my ignorance on this subject, but I'm not versed in RF or antenna theory so my analysis is just based on elementary physics.
 

crutschow

Joined Mar 14, 2008
25,432
the antenna has to initially produce either a time varying electric or magnetic field.
It's not either or, it produces both.
A wire with a moving current produces both an electric and magnetic field.
If the current changes with time then the EM field can detach from the wire and radiate into space as an EM wave.
 

Thread Starter

Glenn Holland

Joined Dec 26, 2014
705
It's not either or, it produces both.
A wire with a moving current produces both an electric and magnetic field.
If the current changes with time then the EM field can detach from the wire and radiate into space as an EM wave.
OK so which arrangement would work best for generating a radiating EM field - A wire loop with a time varying current or an air gap with a time varying voltage and a subsequently varying electric field -IE- "displacement current"?
 
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crutschow

Joined Mar 14, 2008
25,432
OK so which arrangement would work best for generating a radiating EM field - A wire loop with a time varying current or an air gap with a time varying voltage and subsequent electric field -IE- "displacement current"?
Well, I'm certainly no expert, but loop antennas are common so I would assume a loop.
The gap in such an antenna is not a source of radiation.
 

Papabravo

Joined Feb 24, 2006
14,420
Loop antennas are used as receiving antennas where small size relative to one wavelength or the directional characteristics are important.

From the wikipedia article:

Due to their small radiation resistance and consequent electrical inefficiency, small loops are seldom used as transmitting antennas, where one is trying to couple most of the transmitter's power to the electromagnetic field.

https://en.wikipedia.org/wiki/Loop_antenna

Large loops, such the delta loop, are used as transmitting antennas.
 

nsaspook

Joined Aug 27, 2009
7,592
OK so which arrangement would work best for generating a radiating EM field - A wire loop with a time varying current or an air gap with a time varying voltage and a subsequently varying electric field -IE- "displacement current"?
The physical properties of the antenna at the desired frequency mainly determine what works best. Fundamentally you always have accelerating charges but the properties of the electromagnetic energy as more magnetic or electrical in nature is determined by the configuration of matter surrounding the accelerating charges. For some antennas there is an electromagnetic energy path that causes the magnetic field reactive component to dominate at the (near-field) source. For others we have the opposite but symmetrical condition of electric field reactive component dominance near the source. If the Electrical length and physical length of both are the near the same then the main consideration would be losses (resistive, dielectric, etc ...) from transmitter, to the transmission line, to the antenna (typically low radiation resistance for a loop making conductor resistive losses important) and then to free space. (in-phase far field resistive impedance ~377 ohms) So it's much like how it's usually more efficient to send high power on AC utility lines using high voltage potentials instead of very high currents and lower voltages.
https://en.wikipedia.org/wiki/Near_and_far_field
In the far-field region, each part of the EM field (electric and magnetic) is "produced by" (or associated with) a change in the other part, and the ratio of electric and magnetic field intensities is simply the wave impedance. However, in the near-field region, the electric and magnetic fields can exist independently of each other, and one type of field can dominate the other.
Why antennas radiate: http://www.antenna-theory.com/basics/whyantennasradiate.php
In summary, all radiation is caused by accelerating charges which produce changing electric fields. And due to Maxwell's Equations, changing electric fields give rise to changing magnetic fields, and hence we have electromagnetic radiation.
 
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Janis59

Joined Aug 21, 2017
1,227
Once I bought a EMI measurer and tried it (just impatience to see somewhere where EMI for sure exist) near our local TV station tower (the third tallest in Europe and in first dozen tallest in whole World, 368 meters). By the way, I remember when it was built, the workers begun with the top, then made a under-top, and only last the lower floor - all vice versa as normal.
There was rather small E field at the root of tower but large H, then after 100 meters afar the E component was place by place higher than normative and H still strong over normative (I mean ICNIRP-2007), about 300-500 m it became that H is under the normative and E is sure under, the 500m the H component was near to nothing but E still 3/4 of `norm`, and in 1 km the H was microscopic but E still the half of `norm`. Means that H diminishes very fast, but E slowly. This is very general law.
And another effect, don`t know why, - make a step affront, all components are beyond the norm, one my step more affront, and then are 150% of norm and so on cyclically. What is it? Refraction from bridge structures or other words standing waves? Or it is typical near field character for the field itself?
 

Motanache

Joined Mar 2, 2015
481
I was surprised by post #8. With 377ohm:
https://en.wikipedia.org/wiki/Impedance_of_free_space

At school, we learn that there is no different electric or magnetic field. There is only electromagnetic field.
But it is more a theoretical idea.

For example, between capacitor plates we had an electric field.
There was someone to say: the voltage at its terminals is improbable to stay stable and not vary little at least. Variations generate the magnetic field.
Here we had a large electric field and extremely small magnetic field.

----------------------------------------------------------------------
I've unleashed many radio:
- at high frequencies the electrical component is used
FM radio cell phone
- at low frequency AM radio


The antenna for AM is that ferrite core.
It's magnetic.
I know it can be an outdoor antenna. But the wavelength is so great,
that a small antenna for the electrical component of the field becomes ineffective.
 

Motanache

Joined Mar 2, 2015
481
That ferrite core is abnormally long for what it is necessary to get a simple coil, an inductance.
It is part of the oscillating reception circuit LC.
So why is it so long? Because it's an antenna. A magnetic antenna.
 

nsaspook

Joined Aug 27, 2009
7,592
I was surprised by post #8. With 377ohm:
https://en.wikipedia.org/wiki/Impedance_of_free_space

At school, we learn that there is no different electric or magnetic field. There is only electromagnetic field.
But it is more a theoretical idea.

For example, between capacitor plates we had an electric field.
There was someone to say: the voltage at its terminals is improbable to stay stable and not vary little at least. Variations generate the magnetic field.
Here we had a large electric field and extremely small magnetic field.
What was your surprise?

The electromagnetic field is normally expressed as a four vector entity (like our four dimensional universe). The electromagnetic four-potential is much like the spacetime four-vector because we have "electriclike"/"magneticlike" vector frames of reference with (moving) charges like we have with "spacelike"/"timelike" components with matter.

An easy way to visualize a electromagnetic field analogy as a object is to look at a 3D cylinder. In one axis it's a circle, in another it's a rectangle and in-between it's a mixture as we change the frame of reference.


We have an antenna that stores energy in a reactive (phase shifted like a capacitor and/or coil) near field because the antenna has LC components that generate non-canceling (the antenna does not need to be resonant but it can't generate equal and opposite fields over space like a proper transmission line to radiate) fields that radiate into the resistive load of space. Free Space has a LC component (1.257 x 10 -6 Henry per meter, 8.85×10−12 Farads per meter) because of the finite travel speed of light propagation, this means at that space also stores energy but this energy is resistive (space is dispersive) because the "electriclike"/"magneticlike" far field wave components are in phase as they propagate.

Electric fields



By Averse - http://de.wikipedia.org/wiki/Datei:Felder_um_Dipol.jpg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=8715047
 
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Motanache

Joined Mar 2, 2015
481
What was your surprise?
I remembered this things from college. It is a very good post.
For me only those who worked with microwave know that.
You've been working with microwave?
if you wish and it is possible, please tell us about how you worked with these notions. I'm very interested.

Note: the arrow in fig. from post #12 is from + to - for electric field
and from North to South for magnetic field.
Lines unite points in which the field has the same scalar value.

Why are not we talking about the spark-gap transmitter?
Why do we think that just over 30KHz wave detaches from the antenna?
 
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nsaspook

Joined Aug 27, 2009
7,592
I remembered this things from college. It is a very good post.
For me only those who worked with microwave know that.
You've been working with microwave?
if you wish and it is possible, please tell us about how you worked with these notions. I'm very interested.
Anyone who has taken an freshman level electromagnetism class should know that.

Why are not we talking about the spark-gap transmitter?
Why do we think that just over 30KHz wave detaches from the antenna?
spark-gap transmitter
https://ocw.mit.edu/courses/physics/8-02-physics-ii-electricity-and-magnetism-spring-2007/class-activities/presentati_w14d1.pdf

For the wave to 'detach' from a regular physical antenna there needs to be a phase shift over the electrical (things like loading coils/structures may increase the electrical length while keeping the physical length the same) length of the antenna at the excitation frequency. At these low frequencies the length of conductors needed for transmit antennas with good radiation efficiency are huge.
http://www.rcarc.org/presentations/Biggest_Little_Antenna.pdf
https://en.wikipedia.org/wiki/Naval_Communication_Station_Harold_E._Holt#/media/File:Trideco.png
I routinely received communications from stations operating in the 10-30KHz range.

Sure, with microwaves the concepts of wave transmission, propagation modes, reflections at impedance boundaries need to be mastered but the same concepts apply in the HF range when the distances are great and the earth/ionosphere create transmission modes for lower frequency waves that can extend communications ranges by thousands of miles.

how you worked with these notions
If I told you, then I would have to ... you. :D
 

Motanache

Joined Mar 2, 2015
481
Thank you very much for the answer.

https://en.wikipedia.org/wiki/Naval_Communication_Station_Harold_E._Holt#/media/File:Trideco.png
"The frequency is 19.8 kHz. With a transmission power of 1 megawatt"

It's almost an audio frequency ......
So the water strongly reflects the electromagnetic waves.
I suspect that if the frequency is low, it gets better into the water and we manage to communicate with the submarines.



Do you dreaming bad if you sleep nearby?
Kangaroos they fainted?:p:p:p:p:p
If you keep a light bulb in your hand, it lights up?
 
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BR-549

Joined Sep 22, 2013
4,938
Has anyone here been up high on a tower? ......most of us, I think have seen those youtube videos of the high tv tower work in the flat mid-west.

Have you ever been up high? I went out on a job once and was suppose to put a repeater on a smoke stack.

I arrived at the site..........and that damn thing was 512 ft high. They had sorta a miniature phone booth compartment that road a rail up this stack.....by cable and pulley. The whole assembly was covered in rust....car and rail....and cable looked to be 30 years old. They assured me it was used just two weeks earlier. Looked home brew to me.

I had been in these one man elevators before.........taking me over 100 ft to the bottom of some reservoirs for valve calibration. I did not mind it.

They had a small platform at a little over 200 feet. I told them that's where the repeater was going period. Made them station emergency crew at bottom.......took me 45 minutes to install solar panel...cabinet...and antennas.

Pretty fast because that damn stack sways. That never occurred to me....til I was there.

Ever since then.....if I go up 10 foot or so.......I start swaying.
 

Janis59

Joined Aug 21, 2017
1,227
RE:""At school, we learn that there is no different electric or magnetic field. There is only electromagnetic field""
Then how one may explain the empirical fact that walking with EM meter at the hands, magnetic component (A/m) diminishes very rapidly with the distance to antenna tower (hundreds of meters), whilst E component (V/m) diminishes very much slower, tens of km or hundreds.
 

nsaspook

Joined Aug 27, 2009
7,592
RE:""At school, we learn that there is no different electric or magnetic field. There is only electromagnetic field""
Then how one may explain the empirical fact that walking with EM meter at the hands, magnetic component (A/m) diminishes very rapidly with the distance to antenna tower (hundreds of meters), whilst E component (V/m) diminishes very much slower, tens of km or hundreds.
You are mainly looking at near-field reactive (usually magnetic) effects near the antenna. This is a property of the electromagnetic field interaction with current sources. In the far-field you don't have that interaction.
Small loop receive antennas like in AM radio are normally sensitive to the magnetic component of the far EM field.
 
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