Richard Clark wrote:

On Tue, 25 May 2004 07:27:03 -0500, "Richard Fry"
wrote:

REALITY CHECK: If tx source Z really was 50 ohms, a tx connected to a 50
ohm load would lose 1/2 of the RF power it generates to that internal Z.


Dear Richard,

This is the time-honored misreading and misapplication of Thevenin's
theorem posed by Edison to confound investors in Westinghouse's AC
generation plants. Look at the ratio of DC to AC power plants
constructed in the past century to find the poor accuracy of your
reading.

73's
Richard Clark, KB7QHC

Boy, I'm gonna have to try and find my old EE5xx Non-Linear Transistor
Design notebook. I doubt I could translate it into words for this
section of the thread, but it is incredibly applicable.

I took it not knowing what it really was - RF amplifiers, tank circuits
and matching circuits of every flavor, and how much DC power was used
and RF delivered at the primary freq and all harmonics. He also covered
doublers, triplers, phase locked loops (from scratch), the whole 9
yards. I learned more in that one class than in any other 5 I took.
The professor was fantastic and very very tough. It was also the most
demanding course I ever took.

tom
K0TAR

On Tue, 25 May 2004 19:01:31 -0500, Tom Ring
wrote:

Boy, I'm gonna have to try and find my old EE5xx Non-Linear Transistor
Design notebook. I doubt I could translate it into words for this
section of the thread, but it is incredibly applicable.

Hi Tom,

It's not that difficult. Edison corrupted the Thevenin model
demanding that there be a RESISTANCE equal to the load to accomplish
this equivalency (a case of being overly literal for a market
advantage). The Thevenin model only requires an IMPEDANCE, hence
these allusions to 50% maximum efficiency (an imposed condition much
like debate here) were an invention by Edison to detract investment
away from AC power transmission and attract it back to his DC dynamos.
The real pro's new how to dance with numbers and concepts too.

This resolves how the common Ham rig can present 50 Ohms Impedance and
still offer more than 50% efficiency - although, a simple examination
of the power in vs. the power out reveals it isn't nearly as good as
50% for ANY mode. Such is the gulf between reality and theory - some
folk's mileage may vary. ;-)

73's
Richard Clark, KB7QHC

Richard Clark wrote:

Hi Tom,

It's not that difficult. Edison corrupted the Thevenin model
demanding that there be a RESISTANCE equal to the load to accomplish
this equivalency (a case of being overly literal for a market
advantage). The Thevenin model only requires an IMPEDANCE, hence
these allusions to 50% maximum efficiency (an imposed condition much
like debate here) were an invention by Edison to detract investment
away from AC power transmission and attract it back to his DC dynamos.
The real pro's new how to dance with numbers and concepts too.

This resolves how the common Ham rig can present 50 Ohms Impedance and
still offer more than 50% efficiency - although, a simple examination
of the power in vs. the power out reveals it isn't nearly as good as
50% for ANY mode. Such is the gulf between reality and theory - some
folk's mileage may vary. ;-)

73's
Richard Clark, KB7QHC

Understood. I still want to find it. Been about 30 years since I wrote
it all down in 4 colors. I took serious notes.

tom
K0TAR

Richard Clark wrote:
"The Thevenin model only requires an IMPEDANCE."

Yes, but "To secure maximum power output from a generator whose emf and
whose internal impedance are constant, the load must have an impedance
equal to the conjugate of the generator`s internal impedance." (page 43,
"Transmission Lines, Antennas, and Wave Guides", King, Mimno, and Wing)

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:

Richard Clark wrote:
"The Thevenin model only requires an IMPEDANCE."

Yes, but "To secure maximum power output from a generator whose emf and
whose internal impedance are constant, the load must have an impedance
equal to the conjugate of the generator`s internal impedance." (page 43,
"Transmission Lines, Antennas, and Wave Guides", King, Mimno, and Wing)

Anything about dissipationless resistances or negative resistances?
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil, W5DXP wrote:
"Anything about dissipationless resistances or negative resistances (in
"Transmission Lines, Antennas, and Wave Guides")?"

On page 73, the characteristic resistance of free-space is defined as
the sq rt of the permeability of space devided by the dielectric
constant of space. The units are henries/m and farads/m. The solution is
a voltage to current ratio of 376.7 ohms, or 120 pi ohms.

As free-space is a perfect (lossless) medium for radio waves, it is a
dissipationless resistance.

On page 13, the characteristic resistance (Ro) of a transmission line is
given in formula (14.3) as the sq rt of L/C, but this is an
approximation for low-loss lines as there are no perfect lines.

Negative resistance is a gain instead of a loss. The authors of
"Transmission Lines, Antennas, and Wave Guides" were writing for WW-2
officers being trained at Harvard University in radio and radar. The
phone system was then using some "negative resistance repeaters" but
neither these nor "active antennas" were big at that time. You can only
make up the loss in a two-wire phone loop with amplification. Any more
gain and the loop "sings" (breaks into oscillation).

Best regards, Richard Harrison, KB5WZI

On page 73, the characteristic resistance of free-space is defined as
the sq rt of the permeability of space devided by the dielectric
======================

Why must engineere resort to quoting from their worshipped Guru's who are
hardly more likely to be correct than they are themslves.

In all probability the recipient of the advice does't have a copy of the
sacred text. If he did he wouldn't be asking the question anyway. And it
must be extremely rare for anybody to spend weeks attempting to find a copy.
He knows he will have lost interest long before he finds one.

Do engineers these days, in their daily work, depend entirely on the 'gospel
truths' to be found in text books on their extensive book shelves without
much understanding of what its all about? It's a recipe for time-wasting
errors!

Or are they content to obtain money under false pretences?

Sorry about the diversion. I'm in that sort of mood tonight. I'm on Pinot
Noir 2001, red - a product of Romania, bottled before Romania became a EU
member. But it's no worse than the Italian, Spanish, French, or even the
Californian plonk!

Never sampled a 6-shooter, John Wayne, Texan vintage.
----
Reg.

Richard Harrison wrote:
Negative resistance is a gain instead of a loss.

Therefore, doesn't an amplifier have negative resistance?
--
73, Cecil http://www.qsl.net/w5dxp



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On Wed, 26 May 2004 09:05:30 -0500, (Richard
Harrison) wrote:

Richard Clark wrote:
"The Thevenin model only requires an IMPEDANCE."

Yes, but "To secure maximum power output from a generator whose emf and
whose internal impedance are constant, the load must have an impedance
equal to the conjugate of the generator`s internal impedance." (page 43,
"Transmission Lines, Antennas, and Wave Guides", King, Mimno, and Wing)


Hi Richard,

Not if it is strictly resistive. This is the rhetorical dance of
conditions. Too many arguments turn on a resistive load, and then
demand efficiency in terms of a resistive Thevenin model. That just
doesn't pass.

73's
Richard Clark, KB7QHC

Richard Clark wrote:

(Richard Harrison) wrote:
Yes, but "To secure maximum power output from a generator whose emf and
whose internal impedance are constant, the load must have an impedance
equal to the conjugate of the generator`s internal impedance." (page 43,
"Transmission Lines, Antennas, and Wave Guides", King, Mimno, and Wing)

Not if it is strictly resistive.

Of course, it works for the purely resistive.
R+j0 is the conjugate of R-j0
--
73, Cecil http://www.qsl.net/w5dxp



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