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Old July 18th 03, 12:50 PM
Tom Bruhns
 
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"Reg Edwards" wrote in message ...
....
====================================
Tom, To add a bit more -

50-ohm generators as used in laboratories (so that measured reflexion loss,
mismatch loss etc, mean something) are effectively constant voltage
generators in series with a 50-ohms resistor, or constant current generators
in shunt with a 50-ohm resistor. They may be followed by an ampifier whose
output impedance is held constant at 50-ohms by some automatic means. None
of these circuits bear much resemblance to a pair of 807's and a tuned tank.

The best that can be said about Rg of the usual HF radio transmitter is that
Rg is indeterminate. IT EVEN VARIES AS THE LOAD IMPEDANCE IS CHANGED which
most of the Guru's contributing to this newsgroup appear to be unaware of or
at least choose to disregard. So what does "adjusting RL to equal Rg" mean?
To use it in a description of feeder + antenna behaviour further propagates
myths, including those surrounding SWR, forward power, reflected power, SWR
meters, etc.

Does Terman ever bother to mention Rg of a Tx PA? If he doesn't it can't
matter very much to him. The ARRL handbook, when numerically designing a
transistor linear HF PA, makes no mention of Rg.


Amen, brother. I was thinking after making my last posting to this
thread that the one thing I DON'T bother thinking about when designing
a PA is what source impedance it will present. I worry about
currents, voltages, efficiency, distortion, a network to present the
proper load to the active device(s)... but not Rg.

In precision instrumentation systems, the output is commonly levelled
or monitored through a levelling splitter (not to be confused with a
power divider), so that a virtual zero-impedance point can be
established, with a 50-ohm (or other Zo) resistor from that point to
each output. And network analyzers are commonly calibrated with
precision loads so that the imperfections in their outputs and
reflectometers and cabling can be backed out by the calibration
software.

Cheers,
Tom

  #82   Report Post  
Old July 18th 03, 01:54 PM
Tarmo Tammaru
 
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"Roy Lewallen" wrote in message
...
Most simple derivations for the input impedance of a dipole assume it's
infinitely thin.


That is why I used #30 wire



The general problem of a dipole made from wire of
finite diameter is a lot tougher, and is the topic of the papers by the
authors I listed in another recent posting. With EZNEC, you'll find that
the dipole impedance will continue to change as you make the wire
diameter smaller and smaller, until it gets too small for the program to
handle at all.



When I switched to #14, the impedance did not change more than a few tenths
of an Ohm. However, resonance went down about 10 KHz.

BTW, when I took my first Junior level EE Fields & Waves course I asked my
prof about the same question that Slick brought up. Was convinced that 300
Ohm TV antennas were really 377 Ohms!

Tam/WB2TT


  #83   Report Post  
Old July 18th 03, 02:03 PM
Tarmo Tammaru
 
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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


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Old July 18th 03, 02:28 PM
Tarmo Tammaru
 
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"Dilon Earl" wrote in message
...
On Thu, 17 Jul 2003 09:06:32 -0500, "William E. Sabin"
[email protected] wrote:
If I have a 100 watt transmitter and my wattmeter shows 3 watts
reflected. Is 3 watts actually being dissipated in the tank and final
PA?

That would depend on the output impedance of the transmitter. If it is 50
Ohms, all the reflected power would be absorbed by the transmitter. If it is
0 Ohms or its Norton equivalent, it is all reflected, and none is
dissipated.

Tam/WB2TT


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Old July 18th 03, 02:47 PM
Tarmo Tammaru
 
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This whole discussion points out the fallacy of trying to understand
reflections by sending a CW signal. Consider a 100W pulse radar transmitter.
Also, assume the transmitter rurns off between pulses ( a good assumption),
and that the antenna mismatch causes 3% of the power to be reflected.

When the pulse reaches the antenna, 97W is transmitted, and 3W is returned.
All of this 3 W is reflected by the transmitter, and a second pulse is
produced where 2.91W is radiated and .09 W is reflected back. This is
re-reflected, and a third pulse is produced where .0873 W is radiated and
..0027 W is reflected, etc, etc.

Tam/WB2TT
"W5DXP" wrote in message
...
William E. Sabin wrote:
Keeping everything simple and not getting into peripheral issues is
desirable at this point.


The assumption that the source is outputting (forward power
minus reflected power) is essentially saying that all amplifiers
are perfectly matched and re-reflect all reflected power.
That seems like a stretch to me.
--
73, Cecil http://www.qsl.net/w5dxp



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Old July 18th 03, 03:08 PM
William E. Sabin
 
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Tarmo Tammaru wrote:
This whole discussion points out the fallacy of trying to understand
reflections by sending a CW signal.


There is nothing wrong with considering the
steady-state solution. We do that all the time.
The steady-state is achieved mathematically by
considering an infinite sum of forward and
reflected waves (for a CW signal) until it
converges to a steady state. In the final solution
the source impedance is found to have no effect on
the standing wave pattern (the SWR).

Reference: W.C. Johnson "Transmission Lines and
Networks", McGraw-Hill 1950, chapter 4, Equation 4.23.

In the steady state the final result is according
to my discussion.

Bill W0IYH

  #87   Report Post  
Old July 18th 03, 04:36 PM
Richard Clark
 
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On Fri, 18 Jul 2003 09:03:38 -0400, "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.


That seems to be ignored to complement its use as analogy.


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


Hi Tam,

That material has been around for a couple of decades, but is quite
heavy. You might find yourself with a fleet of stealth bumper cars as
a result.

73's
Richard Clark, KB7QHC
  #88   Report Post  
Old July 18th 03, 04:46 PM
Richard Clark
 
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On Fri, 18 Jul 2003 09:51:35 -0700, W5DXP
wrote:

Dick Carroll wrote:
You can be just as sure that the speaker presents its rated impedance only
at one frequency, too.


Is it still 1000 Hz?


Hi All,

Such easy things to confirm.....

I have a pair of Pioneer speakers, looked at their spec.s pasted to
the back, rated flat from 10Hz to 25KHz (flat being order of two from
nominal 10 Ohms) with a small (BW) peak at 80Hz.

Years ago in my metrology lab, we abandoned laboratory grade
amplifiers and speakers for commercial ones as the market offered far
better product at much cheaper rates.

Sound, like light, makes virtual experts of everyone through their
misperception of its common features. Eyes and ears are fine
instruments clouded by the brain.

73's
Richard Clark, KB7QHC
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Old July 18th 03, 05:03 PM
Richard Clark
 
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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


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