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Old August 8th 03, 08:58 PM
Reg Edwards
 
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I am much more interested in getting the
generated RF into the Æther.
---

Cec, Good, now we're back on track. But what's the 377 ohms of nothingness
to do with a random length of wire which has any impedance you fancy just by
connecting to it. Even without making a connection and just using your
imagination. Could the person who confidently raised this subject from the
dead please give us some clues about calculating the turns ratio. I don't
have ready access to the works of Maxwell and there's no mention of it in my
1992 edition of the ARRL Handbook.

And I think my smiley is better than yours!



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Old August 8th 03, 09:17 PM
Reg Edwards
 
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Tricky Dick Sez -
What you describe is the feedpoint Z not the antenna Z which to all
intents and purposes is not far from the original, single-wire dipole.


Tricky,
After all these years you're catching on. What you really meant to say was
that the feedpoint impedance is not the same thing as the radiation
resistance.

'Antenna' impedance' in the present context is not a phrase known to radio
engineering. Please define.
---
Reg


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Old August 8th 03, 10:01 PM
Dave Platt
 
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In article ,
Reg Edwards wrote:

Cec, Good, now we're back on track. But what's the 377 ohms of nothingness
to do with a random length of wire which has any impedance you fancy just by
connecting to it. Even without making a connection and just using your
imagination. Could the person who confidently raised this subject from the
dead please give us some clues about calculating the turns ratio.


I think you have to measure the diameter of free space first, before
you can calculate the turns ratio.

Do write, when you get to the far side, and let us know how the
weather is, OK? ;-)

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
  #14   Report Post  
Old August 9th 03, 08:36 PM
Roy Lewallen
 
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Only at some distance from the antenna. You can create local E/H ratios
of nearly any value (magnitude and phase).

Roy Lewallen, W7EL

W5DXP wrote:

The ratio of the radiated E-field to H-field has no other choice.
If you stuff EM radiation into free space, the ratio of E-field to
H-field is 376.7 ohms. Zero energy is lost from the EM spectrum when
an electron throws off a photon (until that photon is annihilated).


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Old August 9th 03, 09:32 PM
William E. Sabin
 
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Roy Lewallen wrote:
Only at some distance from the antenna. You can create local E/H ratios
of nearly any value (magnitude and phase).

Roy Lewallen, W7EL

W5DXP wrote:


The ratio of the radiated E-field to H-field has no other choice.
If you stuff EM radiation into free space, the ratio of E-field to
H-field is 376.7 ohms. Zero energy is lost from the EM spectrum when
an electron throws off a photon (until that photon is annihilated).




The near field has a reactive component to the
impedance. But is it true that the real part of
that complex impedance must be 376.7 ohms resistive?

Bill W0IYH



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Old August 9th 03, 09:57 PM
Roy Lewallen
 
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William E. Sabin wrote:
Roy Lewallen wrote:

Only at some distance from the antenna. You can create local E/H
ratios of nearly any value (magnitude and phase).

Roy Lewallen, W7EL

W5DXP wrote:


The ratio of the radiated E-field to H-field has no other choice.
If you stuff EM radiation into free space, the ratio of E-field to
H-field is 376.7 ohms. Zero energy is lost from the EM spectrum when
an electron throws off a photon (until that photon is annihilated).





The near field has a reactive component to the impedance. But is it true
that the real part of that complex impedance must be 376.7 ohms resistive?

Bill W0IYH


Not at all. For example, the magnitude of of the wave impedance E/H is
much lower than 377 ohms very close to a small loop, and much higher
than 377 ohms very close to a short dipole. Interestingly, as you move
away from a small loop, the magnitude of E/H actually increases to a
value greater than 377 ohms, then slowly approaches 377 ohms from the
high side as you move even farther away. The opposite happens for a
short dipole -- the E/H ratio drops below 377 ohms some distance away (a
fraction of a wavelength), then increases to 377 ohms as you go farther yet.

Roy Lewallen, W7EL



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Old August 10th 03, 05:00 PM
Richard Clark
 
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On Sun, 10 Aug 2003 08:54:56 -0500, "William E. Sabin"
[email protected] wrote:

It seems that very close to (but slightly removed
from) the antenna the real part of the resistive
space impedance is nearly the same as the real
part of the driving point impedance of the
antenna. This real part is then transformed to 377
ohms (real) within the near field, suggesting that
the open space adjacent to the antenna performs an
impedance transformation. The near-field reactive
fields perform this function in some manner.


The figures at:
http://home.comcast.net/~kb7qhc/ante...pole/index.htm
illustrate just how the dipole's near-field reactance maps out
(without respect for phase, and expressed in SWR relative to free
space Z).

Note that employing the term transform and antenna within the same
context is not de rigueur. ;-)

73's
Richard Clark, KB7QHC
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Old August 10th 03, 08:28 PM
William E. Sabin
 
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Richard Clark wrote:


Note that employing the term transform and antenna within the same
context is not de rigueur.


It is 100 percent correct and appropriate.

Bill W0IYH

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Old August 10th 03, 09:28 PM
Roy Lewallen
 
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In the fourth paragraph, you say that "real power is in the real part of
the impedance", and in the last, that it's "found by integrating the
Poynting vector slightly outside the surface of the antenna". The
impedance is E/H, the Poynting vector E X H. Clearly these aren't
equivalent.

The radiated power is, as you say, the integral of the Poynting vector
over a surface. (And the average, or "real", radiated power is the
average of this.) The integral doesn't need to be taken slightly outside
the surface of the antenna, but can be any closed surface enclosing the
antenna. There's no necessity for E/H, or the real part of E/H, to be
constant in order to have the integral of E X H be constant.

The driving point impedance of the antenna depends on where you drive
it, and it bears no relationship I know of to the wave impedance (which
is, I assume, what you mean by "resistive space impedance") close to the
antenna. If you find any published, modeled, measured, or calculated
support for that contention, I'd be very interested in it.

Roy Lewallen, W7EL

William E. Sabin wrote:

There seems to more explanation needed.

If a lossless dipole is loaded with 100 W of *real* power, that is the
real power in the far field, and it is also the real power very close to
the antenna, regardless of the type of antenna.

The value of real power is the same everywhere.

Since real power is in the real part of the impedance, then how does the
value of real impedance (not the magnitude of impedance) vary with
distance from the antenna?

It seems that very close to (but slightly removed from) the antenna the
real part of the resistive space impedance is nearly the same as the
real part of the driving point impedance of the antenna. This real part
is then transformed to 377 ohms (real) within the near field, suggesting
that the open space adjacent to the antenna performs an impedance
transformation. The near-field reactive fields perform this function in
some manner.

The real power radiated is found by integrating the Poynting vector
slightly outside the surface of the antenna, and is equal to the real
power into the (lossless) antenna. This value is constant everywhere
beyond the antenna.

Bill W0IYH


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Old August 10th 03, 10:42 PM
David Robbins
 
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actually i would expect that a change in E/H would change the driving point
impedance and also the performance of the antenna. some possible examples
that show this effect are the changes in element sizes when modeling an
antenna printed on a dielectric circuit board material or sandwiched in a
dielectric media. the change in wire length due to insulation is another
example, the dielectric properties of the insulation change the E/H ratio
near the wire. some examples may be found in many electromagnetics texts,
look at things like dielectric waveguides, or dielectrics in waveguides,
wires in dielectric media. even the detailed calculation of fields within a
dielectric filled coaxial cable should show this effect, change the
dielectric and you change the characteristic impedance... a measurable
effect from changing the 'space impedence' between the wires.

"Roy Lewallen" wrote in message
...
In the fourth paragraph, you say that "real power is in the real part of
the impedance", and in the last, that it's "found by integrating the
Poynting vector slightly outside the surface of the antenna". The
impedance is E/H, the Poynting vector E X H. Clearly these aren't
equivalent.

The radiated power is, as you say, the integral of the Poynting vector
over a surface. (And the average, or "real", radiated power is the
average of this.) The integral doesn't need to be taken slightly outside
the surface of the antenna, but can be any closed surface enclosing the
antenna. There's no necessity for E/H, or the real part of E/H, to be
constant in order to have the integral of E X H be constant.

The driving point impedance of the antenna depends on where you drive
it, and it bears no relationship I know of to the wave impedance (which
is, I assume, what you mean by "resistive space impedance") close to the
antenna. If you find any published, modeled, measured, or calculated
support for that contention, I'd be very interested in it.

Roy Lewallen, W7EL

William E. Sabin wrote:

There seems to more explanation needed.

If a lossless dipole is loaded with 100 W of *real* power, that is the
real power in the far field, and it is also the real power very close to
the antenna, regardless of the type of antenna.

The value of real power is the same everywhere.

Since real power is in the real part of the impedance, then how does the
value of real impedance (not the magnitude of impedance) vary with
distance from the antenna?

It seems that very close to (but slightly removed from) the antenna the
real part of the resistive space impedance is nearly the same as the
real part of the driving point impedance of the antenna. This real part
is then transformed to 377 ohms (real) within the near field, suggesting
that the open space adjacent to the antenna performs an impedance
transformation. The near-field reactive fields perform this function in
some manner.

The real power radiated is found by integrating the Poynting vector
slightly outside the surface of the antenna, and is equal to the real
power into the (lossless) antenna. This value is constant everywhere
beyond the antenna.

Bill W0IYH






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