Tam/WB2TT wrote:
I am not quite sure what you are saying. But, I ran a SPICE simulation of
the following:
1V 1MHz source with resistor R0 feeding a 50 Ohm 250 ns transmission line
shorted at the far end. Independent of R0, in steady state the voltage at
the input end of the transmission line will be 1V. The effect of R0 is to
limit how long it takes to reach steady state. For R0 = 50 Ohms, it is one
cycle; for R0 = 500 Ohms, it is about 8 cycles, as eyeballed off the
waveform display.

Does SPICE report the steady-state forward and reflected waves
or just the superposition of those two waves? We all know what
they look like when superposed. The question is whether the
identity of the forward and reflected waves disappear after
they are superposed. To the best of my knowledge, the very
existence of standing waves requires the existence of a forward-
traveling wave and a rearward-traveling wave.

I have asked for examples of standing waves void of rearward-
traveling waves and none has been forthcoming.
--
73, Cecil http://www.qsl.net/w5dxp



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"Cecil Moore" wrote in message
...
Tam/WB2TT wrote:
I am not quite sure what you are saying. But, I ran a SPICE simulation
of
the following:
1V 1MHz source with resistor R0 feeding a 50 Ohm 250 ns transmission
line
shorted at the far end. Independent of R0, in steady state the voltage
at
the input end of the transmission line will be 1V. The effect of R0 is
to
limit how long it takes to reach steady state. For R0 = 50 Ohms, it is
one
cycle; for R0 = 500 Ohms, it is about 8 cycles, as eyeballed off the
waveform display.

Does SPICE report the steady-state forward and reflected waves
or just the superposition of those two waves? We all know what
they look like when superposed. The question is whether the
identity of the forward and reflected waves disappear after
they are superposed. To the best of my knowledge, the very
existence of standing waves requires the existence of a forward-
traveling wave and a rearward-traveling wave.

I have asked for examples of standing waves void of rearward-
traveling waves and none has been forthcoming.
--
73, Cecil http://www.qsl.net/w5dxp
It shows the composite voltage waveform, and the net current. Exactly what a
Bird wattmeter would do. Of course the Bird only shows you steady state,
Spice (SWCAD) swhows how you got there.

Tam/WB2TT

Tam/WB2TT wrote:
It shows the composite voltage waveform, and the net current. Exactly what a
Bird wattmeter would do.

That's not what a Bird wattmeter does. A Bird wattmeter possesses
a directional coupler. SPICE apparently does not. Is it possible
to add a directional coupler to SPICE? If you know the Z0, the net
voltage/current magnitudes/phases, it should be possible to use
phasor addition/subtraction to obtain the forward and reflected
components, just like the Bird wattmeter does.
--
73, Cecil http://www.qsl.net/w5dxp



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"Cecil Moore" wrote in message
...
Tam/WB2TT wrote:
It shows the composite voltage waveform, and the net current. Exactly
what a
Bird wattmeter would do.

That's not what a Bird wattmeter does. A Bird wattmeter possesses
a directional coupler.

Sort of. I have built circuits that subtract out the transmitted signal,
leaving the received signal.signal. The Bird is cruder than that.

SPICE apparently does not.

I was driving it with a sine wave, but did a transient analysis. The whole
point is it does not have to know about reflections. It calculates the
waveform by using the same rules that are used to derive standing waves and
reflections.

Is it possible
to add a directional coupler to SPICE?

I have built a SPICE model of a Kenwood power/SWR meter (Have better
schematic than for a Bird). Actually, an idealized version that is not
physically realizable; I did this on purpose. Clearly shows what the
limitations are. Interesting thing is that there is information present that
no SWR meter that I know of displays. For an SWR other than 1:1, you can
deduce whether RL is bigger or smaller than Z0 by comparing two voltages.


If you know the Z0, the net
voltage/current magnitudes/phases, it should be possible to use
phasor addition/subtraction to obtain the forward and reflected
components, just like the Bird wattmeter does.

But there is only one voltage sample, which is the sum of Vf and Vr. There
*are* two current samples, but they are exactly the same, only one is 180
degrees out of phase due to looking at the opposite end of the current
transformer.

Here is what happens. Say you want a meter that shows 100W full scale when
feeding a 50 Ohm load. That is 70.7 V and 1.414 A. In the "Forward"
direction this leads to

100 = 70.7K1 + 1.414K2

In the "Reverse"direction, we know that Pr=0, so

0 = 70.7K1 - 1.414K2 ( The minus sign comes from reversing the current
reading).

You have 2 equations, so you can solve for K1 and K2. You know it can't
*really* measure power, because there is no multiplier. Just like the Bird,
it *adds* (vector wise) voltage and current.
--
73, Cecil http://www.qsl.net/w5dxp



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Tam/WB2TT wrote:
You have 2 equations, so you can solve for K1 and K2. You know it can't
*really* measure power, because there is no multiplier. Just like the Bird,
it *adds* (vector wise) voltage and current.

Exactly, and if you work out the math, you will find it yields
a meter deflection that can be calibrated in watts of forward
or reflected power.

In your example, assume that 70.7v yields a 5v sample and 1.414a
yields a 5v sample. If they are in phase, 10v will indicate 100
watts forward and zero volts will indicate zero watts reflected.
If they are not equal and not in phase, their sum still indicates
forward watts and their difference still indicates reflected watts.
--
73, Cecil http://www.qsl.net/w5dxp



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