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Old July 17th 03, 08:13 AM
Richard Harrison
 
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Dr. Slick wrote:
"I don`t have that book."

I don`t have Kraus either and I miss it.

Space has a magnetic permeability and a dielectric constant. The square
root of their ratio is the characteristic resistance of space. It is
376.7 ohms = 120 pi ohms.

The reciprocal of the square root of the product of the permeability and
dielectric consrtant of space is the velocity of EM radiation
propagation. It is 300 million m/sec.

The above is courtesy of King, Mimno, and Wing, in "Transmission Lines,
Antennas, and Wave Guides", on page 73.

The authors must have considered the information important as they
repeated it on page 117. They followed the repetition with a discussion
of the radiation resistance and input resistance of an antenna. They
note that radiation resistance can`t be measured between two terminals
in a circuit. The I squared R of the antenna power does not conveniently
compute as might be expected with circuit terminals, as current is a
variable along an antenna in most cases.

Best regards, Richard Harrison, KB5WZI


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Old July 17th 03, 08:17 AM
Ian White, G3SEK
 
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Dr. Slick wrote:

But an antenna must be performing some sort of transformer action.


Not quite - but there is a word for what it does: it's a transducer.

A transducer is any gadget that converts energy from one form into a
*different* form. Examples include a loudspeaker (electrical energy to
sound/mechanical energy), a microphone (the reverse), a light bulb and a
photocell.

From that point of view, a resistor is a transducer that converts
electrical energy into heat energy... but it also has some useful
electrical properties :-)

An antenna is a transducer that converts electrical energy into E and H
fields, and the reverse.

You'll also notice that all practical transducers convert some of their
input energy into heat energy.

It's a useful word for a useful idea.

(Cecil - can your IEEE Dictionary help us with a formal definition?)


On the other hand, if you insist on using the word "transformer", you'll
keep on believing you can work out new facts about antennas from what
you already know about transformers:
If an antenna is not a transformer of some type, then why is it
affected by it's surroundings so much? They obviously are, just like
the primary's impedance is affected by what the secondary sees in a
transformer.


That's a perfect example of the trap, because in reality it's not "just
like". An antenna also has E-field interactions with its environment
that a transformer doesn't have, so any resemblance will literally be
only half-true.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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Old July 17th 03, 09:57 AM
Richard Harrison
 
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Dr. Slick wrote:
"What does it say?"

I don`t have Kraus, unfortunately.

I do have Arnold B. Bailey`s "TV and Other Receiving Antennas". Bailey
covers more antenna territory than most, and does an excellent job of
it. Bailey also includes a catalog of antenna types, all sized for 200
MHz for easy comparison.

Bailey says the surge impedance of an antenna is inversely proportional
to the capacitance per unit length. Reminds one of a transmission line.
This is non-uniform, so Bailey has an empirical equation which says the
larger the periphery of the rod, ther smaller the average surge
impedance.

The ratio of the electric field to the magnetic field surrounding an
antenna must be related to the ratio of volts to amps in the antenna
wire (the surge impedance).

The surge impedance of a thin-wire 1/2-wave dipole from page 500 is 610
ohms (average).

The surge impedance of a fat-cylinder 1/2-wave dipole from page 502 is
240 ohms (average).

Pattern and gain are identical for both antennas. But, Dr. Slick may be
on to something after all. The bandwidth of the fat antenna is about 3X
that that of the thin. In antennas, bandwidth is often an indicator of
match.

Best regards, Richard Harrison, KB5WZI

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Old July 17th 03, 11:47 AM
Richard Harrison
 
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Ian, G3SEK wrote:
"Examples include a loudspeaker---."

Good transducer example. Its problem is abysmal efficiency, even if
better than the usual incandescent lamp.

The loudspeaker`s efficiency can be improved by a better match to its
medium. The usual loudspeaker is small in terms of wavelength. A result
is that it is capable of exerting much force on a small area of a very
compliant medium, air. Air could better accept power exerted over a much
larger area, especially at low frequencies, with less force required to
make the air move..

We have a high-Z source and a low-Z sink in the loudspeaker and air.
Conversion from electric power to mechanical power can be more efficient
through better impedance matching. Two solutions are often used for a
better match, a larger loudspeaker or a horn between the loudspeaker and
its air load. The larger speaker is directly a better match. The horn is
an acoustic transformer. They both improve energy conversion efficiency.

Best regards, Richard Harrison, KB5WZI



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Old July 17th 03, 11:51 AM
Dr. Slick
 
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W5DXP wrote in message ...
Dr. Slick wrote:
But a Black Box to me implies you have limited
information from it.


Black boxes radiate heat very well. :-)


Only if they're dummy loads!


Slick
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Old July 17th 03, 12:03 PM
Dr. Slick
 
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Roy Lewallen wrote in message ...
I'd be one of the people arguing. Radiation resistance fits every
definition of resistance. There's no rule that a resistance has to
dissipate power. The late Mr. Carr was quite apparently confusing
resistance with a resistor, a common mistake.


Your point has been well taken, Roy. But you have to admit that
radiation resistance is not a easily understood concept (which is why
it may be a common mistake), so for someone to call it a "fictitious"
resistance can make sense, in the sense that it is not a dissipated
resistance. After all, "Imaginary" numbers are well accepted. And
from an arguing sematics point of view (which is unfortunately
necessary sometimes), even you call it "radiation resistance", which
means that it is obviously not the same thing as a dissipative
resistance like a 50 Ohm resistor.

That being said, rest assured, Roy, that you have convinced me!



Slick
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Old July 17th 03, 12:05 PM
Dr. Slick
 
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Roy Lewallen wrote in message ...
I'd be one of the people arguing. Radiation resistance fits every
definition of resistance. There's no rule that a resistance has to
dissipate power. The late Mr. Carr was quite apparently confusing
resistance with a resistor, a common mistake.


BTW, did Joseph Carr really pass away? Sad, his book is very practical.


Slick
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Old July 17th 03, 12:49 PM
Jack Smith
 
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On Wed, 16 Jul 2003 18:20:22 -0700, Roy Lewallen
wrote:

An antenna can reasonably be viewed as a transducer. It converts the
electrical energy entering it into electromagnetic energy -- fields. As
is the case for any transducer, the stuff coming out is different than
the stuff going in. Think in terms of an audio speaker, which converts
electrical energy into sound waves, and you'll be on the right track.


Roy:


Great analogy!

The characteristic acoustic impedance of air (standard temp &
pressure) is about 413 Rayleighs (or Pascal-Seconds/cubic meter).

Do we worry about matching 8 ohms of electrical speaker impedance to
413 Rayleighs? C.f. Paul Klipsch and the Horn speaker.

I wonder if much of the antenna radiation resitance/Tline
impedance/reflection/intrisnic impedance of free space confusion stems
from use of the same words to describe things that may be modeled
mathmetically identically, but have different physical modalities?

In heat sink calculations, for example, we use "thermal resistance"
and an Ohm's law model but few would confuse ohms of resistance with
degrees C/watt.


Jack K8ZOA
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Old July 17th 03, 03:06 PM
William E. Sabin
 
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Dilon Earl wrote:


Where does the loss occur? If you have 3 db of mismatch loss, is it
in the coax, tank circuit?


The loss in "mismatch loss" refers only to the
fact that the power delivered by the generator to
the load is less than it would be if the load
resistance were the same value as the generator
resistance, in other words if the load and
generator were "matched".

The best way to get a handle on this subject is to
draw a diagram of a generator with voltage V=10,
an internal resistance of 50 ohms, and a load
resistor of R ohms. Let R vary from 1 ohm to 100
ohms and calculate the power dissipated in the
generator resistance (50 ohms), the power in the
load resistance (R), and the total power. Plot a
graph of the three quantities. The load power goes
through a maximum when R=50 ohms.

The maximum power dissipated in the generator
resistance is 10^2/50=2 W, which occurs when R=0
ohms. The minimum power dissipated in the
generator resistance is 3.33^2/50=0.22 W which
occurs when R=100 ohms. When R=50 ohms, the load
power is 5^2/50=0.5 W (the maximum value), the
dissipation in the generator resistance is
5^2/50=0.5 W and the total power is 10^2/100=1 W.

Bill W0IYH



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