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Far Field radiation and Non Resonant Antennas

 
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af4rk
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PostPosted: Sat Jun 04, 2016 12:39 am    Post subject: Far Field radiation and Non Resonant Antennas Reply with quote

With complex conjugate impedance matching, power can be transferred efficiently to a non-resonant antenna. My question is this: How does the reactive component which remains part of the antenna load impedance affect Far Field and Reactive Near Field radiation? In a resonant antenna, the impedance is primarily resistive (from this website, E and H are in phase but orthogonal). That is not the case in the non-resonant antenna. How does this affect Far Field Radiation for a non-resonant antenna versus a resonant antenna?
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R. Fry
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PostPosted: Sun Jun 05, 2016 11:20 am    Post subject: Reply with quote

Properly matching the feedpoint Z of a non-resonant antenna to the transmission line connected there means that the r-f current and voltage at the feedpoint are in phase.

Other things equal, both the far- and near-fields that configuration radiates are the same as if the antenna was naturally resonant.
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af4rk
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PostPosted: Sun Jun 05, 2016 9:02 pm    Post subject: Electromagnetic Compatibility: Principles and Applications Reply with quote

Electromagnetic Compatibility: Principles and Applications,
Second Edition, Revised and Expanded by David Weston (Author)

The radiation resistance is defined as that part of the antenna that
contributes to the power radiated by an antenna. In the case of a
resonant antenna, such as a dipole, the antenna input impedance equals
the radiation resistance. For the tuned dipole, the antenna input
impedance equals 70-73 ohms, and for a tuned monopole it equals 30-36
ohms, depending on the radius of the conductors used and assuming the
conductor radius is small compared to the antenna length. We use the
term tuned to denote that the antenna length has been adjusted so
that the antenna is resonant at the frequency of interest.
For an electrically small dipole or monopole antenna, where l,
which is the length of one arm, is much less that a quarter wavelength,
the input impedance is predominantly capacitive and is higher than
the resonant value. The radiation resistance is then only a small
fraction of the input impedance.


Last edited by af4rk on Mon Jun 06, 2016 11:11 am; edited 1 time in total
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af4rk
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PostPosted: Sun Jun 05, 2016 9:04 pm    Post subject: ISBN of reference Reply with quote

Electromagnetic Compatibility: Principles and Applications, Second Edition, Revised and Expanded
by David Weston (Author)
ISBN-13: 978-0824788896
ISBN-10: 0824788893
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af4rk
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PostPosted: Sun Jun 05, 2016 9:31 pm    Post subject: Antenna Theory: Analysis and Design Reply with quote

From the book which is in the Antenna Theory Book Reviews. I bought the Kindle version.
Antenna Theory: Analysis and Design, Constantine Balanis
Section 4.2.2
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af4rk
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PostPosted: Sun Jun 05, 2016 10:42 pm    Post subject: I did further research after submitting this post Reply with quote

So I found some references that answered some of my questions. Here is a question for those of you who believe complex conjugate impedance matching solves every problem. Why do AM broadcast stations typically use a 1/4 wave vertical if a shorter antenna is just as good?
http://www.radiomagonline.com/misc/0082/tall-am-towers/26334
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R. Fry
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PostPosted: Sun Jun 05, 2016 10:52 pm    Post subject: Re: Electromagnetic Compatibility: Principles and Applicatio Reply with quote

af4rk wrote:
... The radiation resistance is defined as that part of the antenna that contributes to the power radiated by an antenna. ... For an electrically small dipole or monopole antenna, where l, which is the length of one arm, is much less that a quarter wavelength, the input impedance is predominantly capacitive and is higher than the resonant value. The radiation resistance is then only a small fraction of the input impedance.

Radiation resistance is only ONE contributor to the radiation efficiency of a given antenna system. Matching network loss, and in the case of monopoles, also the r-f loss in the system path to r-f ground are other contributors.

When the intrinsic radiation resistance of the antenna itself is low, then much of the available r-f power is dissipated in losses other than radiation resistance -- even though by the use of a matching network, the Z at the antenna feedpoint is purely resistive (has zero reactance).
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af4rk
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PostPosted: Mon Jun 06, 2016 11:13 am    Post subject: Two Points Reply with quote

This question remains unanwered:
Why do AM broadcast stations typically use a 1/4 wave vertical if a shorter antenna is just as good?
http://www.radiomagonline.com/misc/0082/tall-am-towers/26334
and
The reply is to a quote from Electromagnetic Compatibility: Principles and Applications (Electrical and Computer Engineering) by David Weston. That's not my opinion.
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af4rk
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PostPosted: Mon Jun 06, 2016 11:58 am    Post subject: Reference Books Reply with quote

In addition to the previously mentioned books, I also have "Antenna Engineering Handbook", by Johnson & Jasik. All three agree on electrically short ("small" is used by Johnson & JasiK) antennas being less effective.
Antenna Engineering Handbook, 6.1.
My thesis is that reactance does not radiate. The capacitive case has been solved. All three references agree on this subject. I submitted the post before finding the Weston quote, and then found support in the other references. If your disagreement is with a reference, kindly dispute with the author, I am not qualified to defend their work.
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R. Fry
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PostPosted: Mon Jun 06, 2016 1:20 pm    Post subject: Re: Reference Books Reply with quote

af4rk wrote:
... My thesis is that reactance does not radiate.

Agreed.

Quote:
... If your disagreement is with a reference, kindly dispute with the author, I am not qualified to defend their work.

I have been agreeing with you and your referenced authors.

The radiation efficiency of an antenna system is the ratio of its radiation resistance to the sum of the radiation resistance + circuit losses. The greatest circuit loss for typical resonant antennas is that of the matching network, and (for monopoles), the loss in the path to r-f ground.

Examples:

1) A base-fed, unloaded, 1/4-wave, naturally resonant vertical monopole has a radiation resistance of about 35 ohms. The r-f loss in a ground system consisting of 120 x 1/4-wave buried radials used with that monopole typically is about 2 ohms, so the feedpoint Z is about 35+2 = 37 ohms.

The loss in a network needed to match that 37 ohms to 50-ohm transmission line might be a few ohms, say 2 ohms. Total R at the input to the matching network then is 37+2 = 39 ohms.

So the radiation efficiency of that antenna system is 35/39 = 0.897, or 89.7%.

2) Substitute a very short vertical conductor for the 1/4-wave conductor of example 1. Its radiation resistance is very low, say 0.1 ohm. Its capacitive reactance is very high, say 3,000 ohms, so the loss in a matching network would be much higher than in example 1 -- say 15 ohms.

If this short, resonant, antenna system was driven against the same radial ground system as in item 1, then its system radiation efficiency is about 0.1/17.1 = 0.006, or 0.6%.

Most of the available transmitter power is dissipated in circuit losses rather than being radiated as e-m energy.

Quote:
...Why do AM broadcast stations typically use a 1/4 wave vertical if a shorter antenna is just as good?

The shorter antenna is not just as good, as illustrated by the above examples.
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Ted130917
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PostPosted: Wed Jun 08, 2016 11:03 am    Post subject: Reply with quote

Something similar was discussed here:
https://forums.qrz.com/index.php?threads/voltage-and-current-in-dipoles.523870/

Note the comment from G3TXQ:
"There's not a simple relationship between the relative phases of the antenna voltage & current on the one hand, and the relative phases of the E and H fields on the other.

For example, an antenna could have a very reactive feedpoint impedance - voltage and current close to phase quadrature - whilst in the Far Field the E & H fields are in-phase. On the other hand the feedpoint impedance can be resistive and yet the E & H fields be in quadrature in the Near Field.
"
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R. Fry
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PostPosted: Wed Jun 08, 2016 1:37 pm    Post subject: An Elaboration on the Comments of G3TXQ: Reply with quote

For consideration/discussion, below are some clips on this topic from https://en.wikipedia.org/wiki/Electromagnetic_radiation . The bold font attribute was added to call attention to important points.
___________

"According to Maxwell's equations, a spatially varying electric field is always associated with a magnetic field that changes over time. Likewise, a spatially varying magnetic field is associated with specific changes over time in the electric field. In an electromagnetic wave, the changes in the electric field are always accompanied by a wave in the magnetic field in one direction, and vice versa. This relationship between the two occurs without either type field causing the other; rather, they occur together in the same way that time and space changes occur together and are interlinked in special relativity. In fact, magnetic fields may be viewed as relativistic distortions of electric fields, so the close relationship between space and time changes here is more than an analogy. Together, these fields form a propagating electromagnetic wave, which moves out into space and need never again affect the source."

"The electromagnetic waves that compose electromagnetic radiation can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. This diagram" (omitted in this post) "shows a plane linearly polarized EMR wave propagating from left to right. The electric field is in a vertical plane and the magnetic field in a horizontal plane. The electric and magnetic fields in EMR waves are always in phase and at 90 degrees to each other."
__________

Therefore all far-field EM radiation (EMR) has a fixed relationship between its E and H fields. Neither field is independent of the other, as was discovered in experience with E-H and "cross-field" antenna concepts.

When r-f voltage and current are in phase at the feedpoint of an antenna, then power factor is 1; all of that r-f power is dissipated in the radiation resistance of that antenna (less wire and other ohmic losses).

For all antenna feedpoint power factors less than unity and for a given applied power, less of that power is radiated as e-m waves. But whatever far-field power that is radiated as e-m waves will always have the same relationship between its E-field and H-field components as described in the clips above.

R. Fry
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