Tom, Cecil, etc

Well, I got the SwCAD model of the SWR/power meter operating. Very
interesting. Learned a lot that I would never have thought of by just
contemplating.

Here is the circuit:

A) An opamp with a gain bandwidth of 10000 MHz that senses the current in
the line. The current to voltage gain conversion constant is 50 I.

B) Another opamp that does the RF sum of K(V + 50 I). I am calling this
output VF.

C) A third opamp that does the RF subtraction of K(V - 50 I). Gee, lets call
this VR.

D) It can be shown that SWR=(VF + VR) / (VF - VR). I love statements like
this, but it is easy enough to prove. Let I=V/RL, and plug the first two
equations into the third.

I did a calibration run at 5W with the source set to 15.811V, ZS=0. With my
K, I get VF=3.13, VR=0, PF=5W

Now for Cecil 1. ZL = 50 - j400. VF=1.62, VR=1.56, SWR=53, PF=1.33W,
PR=1.24W.

Now for Cecil 2. ZL=50-j400, BUT ZS= 0 + J400. VF=11.2, VR=10.9, SWR=73.7,
PF=64W, PR=60.6W. I am at such a high impedance here, that I suspect the 10K
sampling resistors are loading down the circuit somewhat. (I might try 100K
instead).

Note that there is absolutely nothing explicit in the circuit that has
anything to do with transmission lines. All components are perfect; there
are no stray inductances or stray capacitors.

Tam/WB2TT


"Tom Bruhns" wrote in message
m...
Hi Tam,

Yes, the lower reactance -- lower Q, and the lower frequency, will
both help keep the disturbance from the meter at a reasonable level.
If you simply re-tune the 50pF cap in Cecil's 7.2MHz 'speriment, you
still end up with about 1.2:1 SWR, because it's effectively a "T"
impedance matching network. But the same line at 1.8MHz with C and L
at 50 ohms reactance, re-tuned to resonance after insertion of the
meter, gives about 50dB return loss, and you'd be lucky to resolve
that with a typical SWR meter. Of course, you're stuck with 1.8nF of
capacitance too.

Let us know how it works out when you have time. I like your idea of
peeking inside the bridge; I had the same thought.

Cheers,
Tom

"Tarmo Tammaru" wrote in message
...
"Tom Bruhns" wrote in message
m...

(Tam: my recommendation is to do the test yourself. It will be a lot
easier to play with "what-ifs" and to check out things that don't at
first make sense if you have direct control of the experiment.)

Cheers,
Tom
Tom,

I read you, but first I have to paint the kitchen. I was going to use 50
+/& -j50. I also want to get inside the meter and look at the voltage
and
current separately. It's a Kenwood, no sealed slugs. Good point about
the
meter changing the reactance; 160 m might be a good place to do this, or
I
might use a variable capacitor.


Tam/WB2TT