View previous topic :: View next topic 
Author 
Message 
tensor20 Antenna Theory Regular
Joined: 12 Jul 2011 Posts: 18

Posted: Thu Sep 01, 2011 11:00 pm Post subject: antenna impedance and reactive fieldL are they related? 


Hello Forum,
When we discuss the impedance of the reactive field we mean the ratio E/H (between the magnitudes of the two fields) close to the antenna, in free space and not the impedance of the antenna (composed of the radiation resistance and the reactive impedance), correct?
Is there a relationship between the reactance of the antenna (as the imaginary part of the antenna impedance) and the impedance of the reactive field? The reactance of an antenna is a single number. The reactive field occupies a region of space near the antenna and the ratio E/H could vary from point to point in that region. For instance, even if E/H>377 ohm (capacitive reactive field) the ratio E/H could still be different from point to point.....
We are usually interested in mapping the radiation field (lobes, beamwidth, etc....). Is the reactive field mapped to? I bet its shape depends on the antenna type... Any example?
The field of near field communications may be interested in mapping the pattern of the reactive field...
Tensor20 

Back to top 


Schubert Antenna Wizard
Joined: 08 Apr 2009 Posts: 161

Posted: Thu Sep 01, 2011 11:37 pm Post subject: 


Not much of this is correct.
You write:
When we discuss the impedance of the reactive field we mean the ratio E/H (between the magnitudes of the two fields) close to the antenna, in free space and not the impedance of the antenna (composed of the radiation resistance and the reactive impedance), correct?
For plane waves, the impedance is related as E/H, but that is because they are orthogonal in direction. For the reactive near fields, we don't have this property. So you can talk about E/H, but it doesn't make sense and isn't done.
You go on:
Is there a relationship between the reactance of the antenna (as the imaginary part of the antenna impedance) and the impedance of the reactive field?
There is no relationship. The fields don't change even if you add a stretch of transmission line, which will alter the phase of the antenna and the impedance.
And then:
For instance, even if E/H>377 ohm (capacitive reactive field) the ratio E/H could still be different from point to point.....
That doesn't make sense. If you want to define something as capacitive you have to talk in terms of voltage and current out phase, one leading the other, or the analog, E leading or trailing H. 

Back to top 


tensor20 Antenna Theory Regular
Joined: 12 Jul 2011 Posts: 18

Posted: Fri Sep 02, 2011 1:45 am Post subject: 


Not true
I have some reliable sources.
Within a distance of lambda from an antenna there is the reactive field where the ratio between the magnitude of the E field to the B field is a number different from 377 ohm (the impedance of free space in the radiation field).
The ratio E/H can be larger or smaller than 377 ohm in the reactive field.
If you are interested in the topic I can provide some references...
thanks
tensor20 

Back to top 


Schubert Antenna Wizard
Joined: 08 Apr 2009 Posts: 161

Posted: Fri Sep 02, 2011 2:18 am Post subject: 


Yes, E/H can be different from 377 Ohms in the near field. However, take a look at the fields in the near fields of a dipole antenna, you will note that Er, Ephi, Etheta, Hr, Hphi and Htheta are nonzero. So are you taking the ratio of what component of the Efield to what component of the Hfield? It doesn't mean anything.
377 is sqrt( mu0/eps0 ). This shows up when you simplify maxwell's equations to solve the 1dimensional wave equation. That is, you assume fields only vary in 1dimension, and that there are only 2 components (Ex and Hy for a +zdirected wave). In that case, you get Ex/Hy = 377.
For the near field, we don't have plane waves of course. This is why it doesn't make sense to discuss the "near field wave impedance". You can take the ratio, but as you note, it won't be consistent, and therefore has little meaning. 

Back to top 


tensor20 Antenna Theory Regular
Joined: 12 Jul 2011 Posts: 18


Back to top 


Schubert Antenna Wizard
Joined: 08 Apr 2009 Posts: 161

Posted: Fri Sep 02, 2011 9:22 pm Post subject: 


What are you trying to show me? I understand that E/H does not equal 377 in the near field.
However, how much do you trust the below document? The last paragraph states the farfield is achieved at 0.7 wavelengths. Does that sound reasonable?
Also, you can call the ratio E/H a "wave impedance". However, if the fields aren't propagating (plane waves) it isn't really a wave. 

Back to top 


tensor20 Antenna Theory Regular
Joined: 12 Jul 2011 Posts: 18

Posted: Sat Sep 03, 2011 3:14 pm Post subject: 


hello,
I guess I am not trying to show much. I can show other references supporting the same ideas. Sure the word choice is not optimal.
The ratio of medium parameters epsilon/mu coincides with E/H only in the radiation field. Near the structure the (epsilon/mu) =not (E/H)
Wave impedance: well, there are standing waves, that are waves too but do not propagate.
A plane wave is only a specific type of wave (good local approx of real wavefields)....
I found an "interesting" (to me) article:
http://twiki.cis.rit.edu/twiki/pub/Main/JosephHandfield/Very_NearField_Region_Equiphase_Apertures.pdf
The last pages show how the ratio E/H changes.... 

Back to top 


Schubert Antenna Wizard
Joined: 08 Apr 2009 Posts: 161

Posted: Sat Sep 03, 2011 5:02 pm Post subject: 


Wave impedance: well, there are standing waves, that are waves too but do not propagate.
Standing waves are actually two propagating waves travelling in opposite directions. That's why standing waves occur on unterminated wires, for instance. 

Back to top 


