"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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