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#201
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#202
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It occurs to me I might be able to do a whole lot better than actual
measurements. I should be able to simulate a power meter in the SWCadIII Spice simulator, and do a transient analysis. This will let me go from RF to DC out. They also have models for lossless and lossy transmission lines, which should make it possible to see how steady state is reached from turning the source on at T=0. Tam/WB2TT |
#203
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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 |
#204
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Tom,
I am making progress with the SWCad model of the power meter. The current to voltage converter is working, which should be the hardest part. What I like about doing it this way is that all components have 0 tolerance, and there is nothing in the circuit that I don't put on the schematic. Unfortunately, I won't be able to do anything the next couple of days. Tam/WB2TT |
#205
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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 |
#206
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Tarmo Tammaru wrote:
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. Chipman alludes to such a "phenomenon of resonance" in Chapter 10, "Resonant Transmission Line Circuits". For instance, at a conjugate match point where 100+j100 is seen looking in one direction and 100-j100 is seen looking in the opposite direction, there seems to be a *localized* exchange of extra energy between +j100 and -j100 that can adversely affect the value indicated by an SWR meter placed between those two values. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#207
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Goodness, are we talking about energy moving back and forth on a
cycle-to-cycle basis? Instantaneous power? Are you saying boar hog tits do have a use after all -- in Texas, anyway? Roy Lewallen, W7EL Cecil Moore wrote: Chipman alludes to such a "phenomenon of resonance" in Chapter 10, "Resonant Transmission Line Circuits". For instance, at a conjugate match point where 100+j100 is seen looking in one direction and 100-j100 is seen looking in the opposite direction, there seems to be a *localized* exchange of extra energy between +j100 and -j100 that can adversely affect the value indicated by an SWR meter placed between those two values. |
#208
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Cecil, W5DXP wrote:
"For instance, at a conjugate match point where 100+j100 is seen looking in one direction and 100-j100 is seen looking in the opposite direction, there seems to be a "localized" exchange between +j100 and -j100 that can adversely affect the value indicated by an SWR meter placed between these values." If we have a resonant LC circuit, there is only resistance to limit current. If the resonant circuit is a series combination, we can place a certain voltage of the resonant frequency across the series combination. Voltage across either L or C can be much larger than the applied voltage as the reactive Z`s are equal and opposite. This leaves the applied voltage equal to (I)(R). Some day I hope to see Chipman`s analysis. Transmission lines have distributed inductance and capacitance. A "conjugate match point" seems an oxymoron to me. A conjugately matched circuit demonstrates this condition no matter where it is sliced to look in both directions. A resonant length of transmission line with reflections will have more loss than a similar matched line simply because the msatched line has no opportunity to lose some of the reflected energy. Seems to me, we correct power factor at a load to eliminate reactive current in the power line. We are resonating the load and eliminating a reflection from the load. If loss from reflected power is trivial, we don`t need to worry with matching at the load and can match at the sending end of the line to get the power we need. Best regards, Richard Harrison, KB5WZI |
#209
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Roy Lewallen wrote:
Goodness, are we talking about energy moving back and forth on a cycle-to-cycle basis? We are talking about a localized energy exchange between an inductive reactance and a capacitive reactance during a cycle - that third term in your energy equation - and the possible effects on an averaging RMS wattmeter. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#210
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Richard Harrison wrote:
Some day I hope to see Chipman`s analysis. I just bought the book on half.com. Transmission lines have distributed inductance and capacitance. A "conjugate match point" seems an oxymoron to me. A conjugately matched circuit demonstrates this condition no matter where it is sliced to look in both directions. Nope, it doesn't, Richard. A flat system is conjugately matched, i.e. you see 50 + j0 in one direction and 50 - j0 in the other direction. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
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