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  #101   Report Post  
Old July 18th 03, 08:34 PM
Richard Clark
 
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On Fri, 18 Jul 2003 11:54:52 -0700, Roy Lewallen
wrote:

How about light bulbs, solar panels, thermocouples, batteries, fuel
cells, and fireflies?


A light bulb is a transducer? Amusing example of non mechanical
translation until you touch it, then the phonons exhibit that
mechanical transfer in the form of heat. Common experience as your
hand approached that opportunity would have revealed that fate through
radiation and that is neglecting another mechanical form of energy
transmission offered. That other form is convection.

You have not offered counter-examples, you simply ignore positive
examples. I've spent nearly 30 years in the field of Electro-Optics
with contracts as recent as this year. I've yet to see the trade
press or research commonly describe these items as "transducers."
That is not to say I would reject any such reference, but I would say
few are treading that path to attend a philosophical distinction that
is largely semantic.

Stick with antenna as transducer, it makes a fine metaphor.

73's
Richard Clark, KB7QHC

Roy Lewallen, W7EL

Tarmo Tammaru wrote:
Any transducer I can think of offhand converts between mechanical energy and
electrical energy, as for instance a loudspeaker, microphone, mechanical
filter, etc.

As for the impedance of free space, one way to build a stealth aircraft is
to cover it with material that has a resistivity of 377 Ohms/square. Then
there is no reflection

Tam/WB2TT




  #102   Report Post  
Old July 18th 03, 08:41 PM
Tarmo Tammaru
 
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Call it a simple example. I would assume in a real system multiple pulses
are would be highly undesirable and eliminated by discharging the line at
the end of the first pulse. Maybe I should have said 100W pulse generator
with on output impedance of 0 or infinity, and a lousy dummy load. At a
lower power you could hook up a 'scope to the middle of the line and see the
multiple reflections.

Tam/WB2TT
"Richard Clark" wrote in message
...
On Fri, 18 Jul 2003 09:47:32 -0400, "Tarmo Tammaru"
wrote:
Consider a 100W pulse radar transmitter.


Hi Tarmo,

Was this a speculation or borne of actual experience? (I am not
talking about the obvious, exceptionally low power.)

I understand the significance of what you wrote following it,
.0027 W is reflected, etc, etc.

but in my experience with radars (megawatt models that I serviced,
calibrated and offered formal training in), this does not happen.

73's
Richard Clark, KB7QHC



  #103   Report Post  
Old July 18th 03, 08:51 PM
Richard Clark
 
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On Fri, 18 Jul 2003 15:41:21 -0400, "Tarmo Tammaru"
wrote:

Call it a simple example. I would assume in a real system multiple pulses
are would be highly undesirable and eliminated by discharging the line at
the end of the first pulse. Maybe I should have said 100W pulse generator
with on output impedance of 0 or infinity, and a lousy dummy load. At a
lower power you could hook up a 'scope to the middle of the line and see the
multiple reflections.

Tam/WB2TT


Hi Tam,

I would call your Radar a faulty example.

The point of the matter is that real equipment exhibits real
dissipation of reflected power in the conventional expectation. Your
new example above anticipates this by forcing a failure of that
expectation through not matching the load.

Such problems that arise as a consequence were written up by NBS,
Hewlett-Packard and Steven Adams in the discussion of Mismatch
Uncertainty.

The fact that various pundits and savants have no actual value to
offer in substitution for the oft-repeated refrain "it a'in't 50Ohms"
is that any value offered would be immediately demonstrated as being
wrong through standard bench top verification.

As Ian has characterized this as an exercise in futility, I am content
to observe no one stepping up to the bench for validation.

73's
Richard Clark, KB7QHC
  #104   Report Post  
Old July 18th 03, 08:57 PM
Jim Kelley
 
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W5DXP wrote:

William E. Sabin wrote:
If the transmitter output is 100 W and the reflected power is 3 W, then
the 100 W is the difference between 100+3=103 W (forward power) and 3 W
(reflected power).


If the source is a signal generator equipped with a circulator and
load, the generator is putting out 103 watts, and the circulator
load is dissipating 3 watts, is the generator still only putting
out 100 watts by definition?


If the sig gen is putting out 100 watts, with 3 watts reflected and 97
watts going to the load, then 3 watts must be going to the circulator.
If 100 watts is going to the load and 3 watts is reflected back to the
circulator, then the sig gen is putting out 103 watts. But if 97 watts
is getting to the load, 3 watts is reflected and there is no circulator
or other load, then how much do you think the sig gen is actually
putting out?

73, ac6xg
  #105   Report Post  
Old July 18th 03, 09:01 PM
Tarmo Tammaru
 
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"Richard Clark" wrote in message
...
On Fri, 18 Jul 2003 09:28:07 -0400, "Tarmo Tammaru"
wrote:

I've read for years that the common RF rig is NOT a 50Ohm source, and
absolutely none dare commit themselves to just what value it is (much
less offer their own measure). Being a physical reality, the rig must
present some real value, but vacuous theory seems to bar that
discussion.

73's
Richard Clark, KB7QHC


Yeah, seems to be a deep dark secret. If you look at the specs of RF power
transistors, they will give the output impedance vs frequency - BUT you have
to look at the footnote. In virtually all cases what they mean is the
conjugate of the load impedance. It is the jX of the transistor (1/jY), in
parallel with
((VCC-Vsat)**2) /2P.

I have never gotten around to doing this, but I believe the data sheets for
tubes like the 811A and 813 do give the plate resistance, which should make
it possible to calculate the output impedance at the lower frequencies like
160m.

Tam/WB2TT




  #106   Report Post  
Old July 18th 03, 09:37 PM
Richard Clark
 
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On Fri, 18 Jul 2003 16:01:43 -0400, "Tarmo Tammaru"
wrote:

Yeah, seems to be a deep dark secret. If you look at the specs of RF power
transistors, they will give the output impedance vs frequency - BUT you have
to look at the footnote. In virtually all cases what they mean is the
conjugate of the load impedance. It is the jX of the transistor (1/jY), in
parallel with
((VCC-Vsat)**2) /2P.


Hi Tam,

Motorola offers quite specific characteristics across frequency.
Reference MRF421, MRF433, MRF454 for examples of dirt ordinary power
transistors found in more than 20 years of transistorized Ham
transmitters. Take their own data, Z transform them through
transformers (not transducers) and you find 50Ohms without any more
sophisticated math than that required of the standard Technology
Certificate of training. Where does it go through after that? A low
pass filter designed for 50Ohms to an antenna jack specified to
deliver full power to a 50Ohm load.

What technical rebuttal do I hear in response to simple engineering
data? "It is impossible to determine the output Z of this source."
For some I can well imagine they do find it difficult....

73's
Richard Clark, KB7QHC
  #107   Report Post  
Old July 18th 03, 10:47 PM
Dr. Slick
 
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Roy Lewallen wrote in message ...

Y'see, if you really, really want an antenna to be a kind of automobile,
you can cook up a bunch of reasons to convince yourself that it is. The
same method works for astrology and fortune telling, too.



Shall i call this a Straw man argument? Or putting words in
someone's mouth?


Feel free to call it what you want. I believe I've made as valid an
argument for an antenna being an automobile as you did for it being a
transformer, and based on the same criteria.



Well, if you agree that two antennas/transducers in close
proximity will make a transformer (albeit a somewhat inefficient
one!), then i don't think i was that far off base.




The optimization of an antenna depends on many factors, only one of
which is the nature of the medium in which it's immersed. And among the
medium's important properties are its permeability, permittivity, and
the velocity of a wave propagating in it. The phase velocity and
characteristic impedance can both be calculated from the permeability
and permittivity, so you can't really say any one of these is more
important than the other.

It doesn't make any sense to throw out the concept of free space
impedance just because it confuses people who don't know what it means.
It's an extremely useful and well-understood concept. For example,
reflection of a wave from a plane conductor or the ground can easily be
found by calculating a reflection coefficient based on the impedance of
the reflecting surface and the impedance of the impinging wave. (The
impedance of a wave can be quite different close to an antenna than it
is after it's traveled some distance.) If you look in some of those
texts I recommended, you'll find the impedance of free space cropping up
all over the place.

What needs to be thrown away is the belief that all impedances are the
ratio of a voltage to a current, along with the notion that only
resistors can have resistance.

Roy Lewallen, W7EL



You have convinced me that you are correct about both of these
points.

But i don't think that an antennas impedance will not be affected
by the permeability of the medium that surrounds it. An antennas
input impedance will be different in free space as opposed to being
immersed in water, for example.

This indicates to me that the antenna is indeed "matching" 50
Ohms to the impedance of free space, even if it is a different type of
impedance.

Do you think that the characteristics of a transformer of a
specific turns ratio, gauge wire, and core geometry, will NOT depend
on the core material? I would say definitely it WILL depend on the
material.


Slick
  #108   Report Post  
Old July 18th 03, 10:56 PM
Roy Lewallen
 
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Actually, several people (W8JI among them) have measured the output
impedance of common amateur linear amplifiers by at least a couple of
methods. The most credible measurements show, interestingly, a value
very close to 50 ohms when the amplifier is adjusted for normal operation.

Of course, it doesn't really matter, but people continue to make a big
deal out of it.

Roy Lewallen, W7EL

Tarmo Tammaru wrote:
"Richard Clark" wrote in message
...

On Fri, 18 Jul 2003 09:28:07 -0400, "Tarmo Tammaru"
wrote:

I've read for years that the common RF rig is NOT a 50Ohm source, and
absolutely none dare commit themselves to just what value it is (much
less offer their own measure). Being a physical reality, the rig must
present some real value, but vacuous theory seems to bar that
discussion.

73's
Richard Clark, KB7QHC



Yeah, seems to be a deep dark secret. If you look at the specs of RF power
transistors, they will give the output impedance vs frequency - BUT you have
to look at the footnote. In virtually all cases what they mean is the
conjugate of the load impedance. It is the jX of the transistor (1/jY), in
parallel with
((VCC-Vsat)**2) /2P.

I have never gotten around to doing this, but I believe the data sheets for
tubes like the 811A and 813 do give the plate resistance, which should make
it possible to calculate the output impedance at the lower frequencies like
160m.

Tam/WB2TT



  #109   Report Post  
Old July 18th 03, 11:02 PM
Roy Lewallen
 
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Tarmo Tammaru wrote:
Roy,

You are cheating. In the steady state there is no load on your source.
Regardless of what the Bird meter reads. Do one of the following:


Why is this cheating? There is reverse power on the line. The source is
not absorbing the reverse power. You and others have said, without
qualification, that it does. I've shown a case where it doesn't.

1.Short the end of the 1/2 wave line.
2.Use a 1/4 wave open ended line.
3.Get a pulse generator, 0 Ohm output impedance +50 Ohm series resistor. Set
the pulse with to 100ns and 1V, and use an arbitrary length of coax, either
open or shorted, but longer than 100ns. Grab a 'scope and look at the
junction of the coax and the 50 Ohm resistor. You will be able to see the
.5V reflected pulse appear across the 50 ohm resistor. ALL of the reflected
energy was absorbed, and half of the forward power.


When talking of amateurs and transmitters, we're dealing with
sinusoidal, steady state conditions. You've just described a transient
pulse situation. It's different in several ways, one of the most
important being that the source is off when the returning pulse arrives.
I'm fully able to discuss TDR phenomena, but it's not relevant, and only
adds confusion to a discussion of amateur transmitters and transmission
lines.

In sinusoidal, steady state conditions, it's absolutely incorrect to say
that the reflected power is absorbed by the source, whether the source
is matched or not. And it's easy to show it's incorrect, as I've done.

Roy Lewallen, W7EL


Tam/WB2TT
"Roy Lewallen" wrote in message
...

Sigh. I guess one more time. A mouse in the maze.

70.7 volt RMS voltage source, 50 ohm series resistor. Connect to an open
circuited, half wavelength transmission line. Put your magic lossless
Bird wattmeter in the line and measure the forward and reverse power:

Pf = 100 watts
Pr = 100 watts

The "transmitter" is perfectly matched to the line.
The "reflected power" is 100 watts.
The dissipation of the "transmitter" source impedance is zero. Not 100
watts. Not even one watt. Zero.

No, how can anyone possibly say that when the transmitter is matched,
the reflected power is absorbed by the transmitter?

Any number of other examples can easily be found. You'll find a few
others in the "Food For Thought" series available from
ftp://eznec.com/pub/food_for_thought/.

I'm firmly in agreement with Bill and Ian on this one.

Roy Lewallen, W7EL


  #110   Report Post  
Old July 18th 03, 11:19 PM
Roy Lewallen
 
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Dr. Slick wrote:
Roy Lewallen wrote in message ...

Y'see, if you really, really want an antenna to be a kind of automobile,
you can cook up a bunch of reasons to convince yourself that it is. The
same method works for astrology and fortune telling, too.



Shall i call this a Straw man argument? Or putting words in
someone's mouth?


Feel free to call it what you want. I believe I've made as valid an
argument for an antenna being an automobile as you did for it being a
transformer, and based on the same criteria.




Well, if you agree that two antennas/transducers in close
proximity will make a transformer (albeit a somewhat inefficient
one!), then i don't think i was that far off base.


I agree.




The optimization of an antenna depends on many factors, only one of
which is the nature of the medium in which it's immersed. And among the
medium's important properties are its permeability, permittivity, and
the velocity of a wave propagating in it. The phase velocity and
characteristic impedance can both be calculated from the permeability
and permittivity, so you can't really say any one of these is more
important than the other.

It doesn't make any sense to throw out the concept of free space
impedance just because it confuses people who don't know what it means.
It's an extremely useful and well-understood concept. For example,
reflection of a wave from a plane conductor or the ground can easily be
found by calculating a reflection coefficient based on the impedance of
the reflecting surface and the impedance of the impinging wave. (The
impedance of a wave can be quite different close to an antenna than it
is after it's traveled some distance.) If you look in some of those
texts I recommended, you'll find the impedance of free space cropping up
all over the place.

What needs to be thrown away is the belief that all impedances are the
ratio of a voltage to a current, along with the notion that only
resistors can have resistance.

Roy Lewallen, W7EL




You have convinced me that you are correct about both of these
points.


Good. Then the effort was worthwhile.


But i don't think that an antennas impedance will not be affected
by the permeability of the medium that surrounds it. An antennas
input impedance will be different in free space as opposed to being
immersed in water, for example.


Indeed it will.


This indicates to me that the antenna is indeed "matching" 50
Ohms to the impedance of free space, even if it is a different type of
impedance.


That's a leap I'm unable to make or to follow.

Do you think that the characteristics of a transformer of a
specific turns ratio, gauge wire, and core geometry, will NOT depend
on the core material? I would say definitely it WILL depend on the
material.


Actually, an adequate core shouldn't appear as a significant factor in
transformer performance. Naturally, an inadequate core will adversely
affect it. But I just don't accept that as evidence, let alone "proof"
that an antenna is fundamentally an impedance matching device.

I see that you won't be swayed from your visualization. But hopefully
some of the other readers can see the fallacy of the concept. I think
I've done all I can, so I'll leave this topic now.

*Chuckle* I was just reminded of something that happened years ago, when
my son was a small boy. He learned that I was an engineer, so he
couldn't wait to see the train I drove. After a great deal of repeated,
patient, explanation, I finally got across (I thought) a description of
what I did, and that it had nothing to do with trains. Well, he had
occasion to visit me at work quite a long time later. He kept wandering
off. When I asked why, he explained that he was trying to find where the
train was kept. Yeah, I might not drive trains, but I must have
*something* to do with trains.

Slick, you've got the right concepts now, but you're still looking for
that train.

Roy Lewallen, W7EL



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