Does a antenna have poles like a magnet?

Thread Starter

Beetle_X

Joined Nov 2, 2012
54
I'm trying to understand the exchange of photons in a general accounting sence. Does a antenna just send out photons or is it like a electromagnet and send/recieve them?

Thanks for your expertise!
 

Papabravo

Joined Feb 24, 2006
17,028
The poles of a magnet are fixed. What happens in an antenna can be fixed, but also can vary with time. It is a consequence of Maxwell's equations that a time varying electric field produces a magnetic field, and a time varying magnetic field produces an electric field.
 

nsaspook

Joined Aug 27, 2009
8,786
I'm trying to understand the exchange of photons in a general accounting sence. Does a antenna just send out photons or is it like a electromagnet and send/recieve them?

Thanks for your expertise!
At the level of an RF (below 10's of GHz) antenna the distinction is meaningless. There are huge (zillions ;)) numbers of photon-electron interactions so accounting for photon exchanges makes no scientific sense because we have a macroscopic scale where classical EM RF theory works perfectly well.
 

drc_567

Joined Dec 29, 2008
1,156
Re: Electric field vs. Magnetic field
It is important to note that the antenna Electric field radiates, or travels, much farther than the simultaneous Magnetic field. That is, only the Electric field is capable of being received at any significant or practical distance.
The Magnetic field is strictly local, with respect to the antenna source, and is oriented so as to be perpendicular to the Electric field. One reference (Feynman Lecture Series on Physics), explains that the intensity of the Electric field attenuates as the distance R from the antenna, decreases as 1/R, while the Magnetic field decreases as
\(1/R^2\).
 

drc_567

Joined Dec 29, 2008
1,156
That, of course, is referring to the near-field radiation from the antenna, not the far-field radiated electromagnetic wave.
... Apparently then, the 1/R attenuation of the Electric field would be related to the so-called ground wave Electric field, emanating from the antenna.
 

nsaspook

Joined Aug 27, 2009
8,786
Last edited:

Thread Starter

Beetle_X

Joined Nov 2, 2012
54
Thank you all for all the responces. It will take a bit to be able to relate to it. I thought antennas where mostly magnetic and not electrostatic. `
 

ronsimpson

Joined Oct 7, 2019
1,549
The Magnetic field is strictly local,
The Earth has a magnetic field I can measure. I know of experiments that measured flux line that went out in space 1000s of miles and came back to earth. (traveled from pole to pole) Those fields were measured near the poles.

I think, if the magnet was spun so the fields went North, South, North, South, I should be able to see it much like and antenna. I agree that it will not work at a great distance.
 

Janis59

Joined Aug 21, 2017
1,355
RE: drc ""intensity of the Electric field attenuates as the distance R from the antenna, decreases as 1/R, while the Magnetic field decreases as 1/R^2."" Yes, however measuring around the large transmitter towers mostly dominates the magnetic component what is most prone to exceed the health standards, but it fast decrease in about kilometer distance, whilst electric component diminishes in about 100 kilometers to practical zero. The same thing around the 50 Hz 110 kV power lines - in the near 50 to 100 meter zone dominates the H-field what most probable will exceed the residential health normative while E-field is exceeded just straight beyound the wires, but something of it may be read even after 300 meters where H-field is sure zero.
 

drc_567

Joined Dec 29, 2008
1,156
RE: drc ""intensity of the Electric field attenuates as the distance R from the antenna, decreases as 1/R, while the Magnetic field decreases as 1/R^2."" Yes, however measuring around the large transmitter towers mostly dominates the magnetic component what is most prone to exceed the health standards, but it fast decrease in about kilometer distance, whilst electric component diminishes in about 100 kilometers to practical zero. The same thing around the 50 Hz 110 kV power lines - in the near 50 to 100 meter zone dominates the H-field what most probable will exceed the residential health normative while E-field is exceeded just straight beyound the wires, but something of it may be read even after 300 meters where H-field is sure zero.
... Say you have a dipole antenna, in a horizontal orientation, then there will be some part of the E field that will radiate at a vertical angle, such that it can reflect off the ionosphere and achieve a greater communication distance. The fraction of the antenna E field which is directed downward towards the ground will possess a greatly attenuated communication distance. ... And the magnetic field generated by the antenna will not be able to achieve any long distance communication, in a practical sense.
...
 

nsaspook

Joined Aug 27, 2009
8,786
... Say you have a dipole antenna, in a horizontal orientation, then there will be some part of the E field that will radiate at a vertical angle, such that it can reflect off the ionosphere and achieve a greater communication distance. The fraction of the antenna E field which is directed downward towards the ground will possess a greatly attenuated communication distance. ... And the magnetic field generated by the antenna will not be able to achieve any long distance communication, in a practical sense.
...
That's the case of near-field reactive fields having usually very limited range because they are usually expressed as separate components of EM energy.

This assumes that the electric and magnetic fields are separate components in the radiated far-field (isolated from sources and currents) EM wave. They are not. The EM energy of the radiated propagating wave is a single entity that can be expressed as simultaneous expressions of electric and/or magnetic field components of that EM energy that depends on the physical geometry of the transmitting and receiving elements.

http://www.bccdc.ca/resource-gallery/Documents/Guidelines and Forms/Guidelines and Manuals/EH/EH/Section2Final06062013.pdf
In the far field, the electric field E and the magnetic field H are orthogonal and the free space impedance is equal to 377 Ω. ... In the far field, the measurement of only one quantity, E or H or S, is enough. The other quantities can be calculated by means of equation (2.23).
 
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