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Mythbusters: V/I ratio is forced to Z0
The myth: Measurements with a Bird 43 of the conditions on the
Thruline section are invalid unless it has some minimum length of 50 ohm line on both sides of itself. I have performed a test using components at hand where the Bird 43 has 75 ohm line on both sides of itself, and the test configuration is designed to present a 50+j0 ohm load at the point where the Bird 43 sampling element is located. The question is, how does the Bird respond? The test in detail. Each component is in a list from the source to the load: IC706IIG 1m RG58 with UHF connectors MFJ949E ATU 3m RG6 (Zo=75 ohm) with BNC connectors Bird 43 5.27m Belden 9275 (Zo=75 ohm, vf=0.83) with BNC connectors 50 ohm dummy load Short adapters were used to connect to the Bird's Type N connectors, the dummy load's Type N connectors and the MFJ949 UHF connector. The half wave resonance of the 5.27m length of RG6 was determined by s/c one end and connecting the other end via an adapter to a MFJ259B and finding the impedance dip at 23.05MHz. The calculated vf from this test is 0.81, which reconciles reasonably with the specs. Free space wavelenght at the test frequency is 13m. The transmitter was set to 23.05MHz, and the ATU tuned to develop rated power output. The ATU is only used to present the rated load to the transmitter so as to obtain 100W for the test, to suit the Bird 43 element. It is inconsequential to the DUT (the Bird 43). With this configuration, it is expected that the impedance at the Bird 43 is approximately 50+j0, and that there would be almost zero reflected power. The Bird 43 indicated 100W forward power and a quarter of a needle width detection on reflected power. The Bird 43 would appear to provide valid readings for the conditions on the Bird 43 Thruline section in this case, notwithstanding that there is not any 50 ohm transmission line attached to the Bird 43 + N-BNC adapters. The myth that measurements with a Bird 43 of the conditions on the Thruline section are invalid unless it has some minimum length of 50 ohm line on both sides of itself is BUSTED. Has anyone experimental evidence to the contrary? Owen PS: I hasten to add / apologise, I do not watch much television, but I was forced to endure Mythbusters when visiting a friend recently. It's about as scientific as what goes on here, so I thought the style appropriate! -- |
Owen Duffy wrote: The myth: Measurements with a Bird 43 of the conditions on the Thruline section are invalid unless it has some minimum length of 50 ohm line on both sides of itself. I have performed a test using components at hand where the Bird 43 has 75 ohm line on both sides of itself, and the test configuration is designed to present a 50+j0 ohm load at the point where the Bird 43 sampling element is located. The question is, how does the Bird respond? The test in detail. Each component is in a list from the source to the load: IC706IIG 1m RG58 with UHF connectors MFJ949E ATU 3m RG6 (Zo=75 ohm) with BNC connectors Bird 43 5.27m Belden 9275 (Zo=75 ohm, vf=0.83) with BNC connectors 50 ohm dummy load Short adapters were used to connect to the Bird's Type N connectors, the dummy load's Type N connectors and the MFJ949 UHF connector. The half wave resonance of the 5.27m length of RG6 was determined by s/c one end and connecting the other end via an adapter to a MFJ259B and finding the impedance dip at 23.05MHz. The calculated vf from this test is 0.81, which reconciles reasonably with the specs. Free space wavelenght at the test frequency is 13m. The transmitter was set to 23.05MHz, and the ATU tuned to develop rated power output. The ATU is only used to present the rated load to the transmitter so as to obtain 100W for the test, to suit the Bird 43 element. It is inconsequential to the DUT (the Bird 43). With this configuration, it is expected that the impedance at the Bird 43 is approximately 50+j0, and that there would be almost zero reflected power. The Bird 43 indicated 100W forward power and a quarter of a needle width detection on reflected power. The Bird 43 would appear to provide valid readings for the conditions on the Bird 43 Thruline section in this case, notwithstanding that there is not any 50 ohm transmission line attached to the Bird 43 + N-BNC adapters. The myth that measurements with a Bird 43 of the conditions on the Thruline section are invalid unless it has some minimum length of 50 ohm line on both sides of itself is BUSTED. Has anyone experimental evidence to the contrary? Owen What made you decide to use 23.05 MHz and 5.27 meters of .81 VF feedline? ac6xg |
On Wed, 12 Oct 2005 16:13:24 -0700, Jim Kelley
wrote: Owen Duffy wrote: The myth: Measurements with a Bird 43 of the conditions on the Thruline section are invalid unless it has some minimum length of 50 ohm line on both sides of itself. I have performed a test using components at hand where the Bird 43 has 75 ohm line on both sides of itself, and the test configuration is designed to present a 50+j0 ohm load at the point where the Bird 43 sampling element is located. The question is, how does the Bird respond? The test in detail. Each component is in a list from the source to the load: IC706IIG 1m RG58 with UHF connectors MFJ949E ATU 3m RG6 (Zo=75 ohm) with BNC connectors Bird 43 5.27m Belden 9275 (Zo=75 ohm, vf=0.83) with BNC connectors 50 ohm dummy load Short adapters were used to connect to the Bird's Type N connectors, the dummy load's Type N connectors and the MFJ949 UHF connector. The half wave resonance of the 5.27m length of RG6 was determined by s/c one end and connecting the other end via an adapter to a MFJ259B and finding the impedance dip at 23.05MHz. The calculated vf from this test is 0.81, which reconciles reasonably with the specs. Free space wavelenght at the test frequency is 13m. The transmitter was set to 23.05MHz, and the ATU tuned to develop rated power output. The ATU is only used to present the rated load to the transmitter so as to obtain 100W for the test, to suit the Bird 43 element. It is inconsequential to the DUT (the Bird 43). With this configuration, it is expected that the impedance at the Bird 43 is approximately 50+j0, and that there would be almost zero reflected power. The Bird 43 indicated 100W forward power and a quarter of a needle width detection on reflected power. The Bird 43 would appear to provide valid readings for the conditions on the Bird 43 Thruline section in this case, notwithstanding that there is not any 50 ohm transmission line attached to the Bird 43 + N-BNC adapters. The myth that measurements with a Bird 43 of the conditions on the Thruline section are invalid unless it has some minimum length of 50 ohm line on both sides of itself is BUSTED. Has anyone experimental evidence to the contrary? Owen What made you decide to use 23.05 MHz and 5.27 meters of .81 VF feedline? I have performed a test using components at hand where the Bird 43 has ac6xg -- |
Owen Duffy wrote:
The myth: Measurements with a Bird 43 of the conditions on the Thruline section are invalid unless it has some minimum length of 50 ohm line on both sides of itself. I have performed a test using components at hand where the Bird 43 has 75 ohm line on both sides of itself, and the test configuration is designed to present a 50+j0 ohm load at the point where the Bird 43 sampling element is located. The question is, how does the Bird respond? The test in detail. Each component is in a list from the source to the load: IC706IIG 1m RG58 with UHF connectors MFJ949E ATU 3m RG6 (Zo=75 ohm) with BNC connectors Bird 43 5.27m Belden 9275 (Zo=75 ohm, vf=0.83) with BNC connectors 50 ohm dummy load Short adapters were used to connect to the Bird's Type N connectors, the dummy load's Type N connectors and the MFJ949 UHF connector. The half wave resonance of the 5.27m length of RG6 was determined by s/c one end and connecting the other end via an adapter to a MFJ259B and finding the impedance dip at 23.05MHz. The calculated vf from this test is 0.81, which reconciles reasonably with the specs. Free space wavelenght at the test frequency is 13m. The transmitter was set to 23.05MHz, and the ATU tuned to develop rated power output. The ATU is only used to present the rated load to the transmitter so as to obtain 100W for the test, to suit the Bird 43 element. It is inconsequential to the DUT (the Bird 43). With this configuration, it is expected that the impedance at the Bird 43 is approximately 50+j0, and that there would be almost zero reflected power. The Bird 43 indicated 100W forward power and a quarter of a needle width detection on reflected power. The Bird 43 would appear to provide valid readings for the conditions on the Bird 43 Thruline section in this case, notwithstanding that there is not any 50 ohm transmission line attached to the Bird 43 + N-BNC adapters. The myth that measurements with a Bird 43 of the conditions on the Thruline section are invalid unless it has some minimum length of 50 ohm line on both sides of itself is BUSTED. Actually, the results of your experiment proves the myth to be true and not to be a myth at all. There's 104.17 watts of forward power through the Bird and 4.17 watts of reflected power back through the Bird. Why does the Bird ignore those actual power values? Has anyone experimental evidence to the contrary? Yes, your experiment. Assuming 100 watts delivered to the load, the forward power on the 75 ohm coax is actually about 104.17 watts so the Bird's forward power reading is in error by 4.17 watts. The reflected power on the 75 ohm coax is about 4.17 watts so the Bird's reflected power reading is in error by close to an infinite percentage. The Bird 43 is reading neither of the actual power values correctly. All you have just proven is that the Bird 43 gives invalid readings when it is in a 75 ohm environment. THERE ARE ABOUT 4.17 WATTS OF REFLECTED ENERGY FLOWING BACK THROUGH THE BIRD AND THE BIRD COMPLETELY IGNORES IT. So the Bird is not even yielding valid readings for forward and reflected power through itself. That's exactly what I have been saying all along. If it were calibrated for 75 ohms, it would indicate the correct values. -- 73, Cecil http://www.qsl.net/w5dxp |
"Cecil Moore" wrote in message . .. Owen Duffy wrote: The myth: Measurements with a Bird 43 of the conditions on the Thruline section are invalid unless it has some minimum length of 50 ohm line on both sides of itself. I have performed a test using components at hand where the Bird 43 has 75 ohm line on both sides of itself, and the test configuration is designed to present a 50+j0 ohm load at the point where the Bird 43 sampling element is located. The question is, how does the Bird respond? The test in detail. Each component is in a list from the source to the load: IC706IIG 1m RG58 with UHF connectors MFJ949E ATU 3m RG6 (Zo=75 ohm) with BNC connectors Bird 43 5.27m Belden 9275 (Zo=75 ohm, vf=0.83) with BNC connectors 50 ohm dummy load Short adapters were used to connect to the Bird's Type N connectors, the dummy load's Type N connectors and the MFJ949 UHF connector. The half wave resonance of the 5.27m length of RG6 was determined by s/c one end and connecting the other end via an adapter to a MFJ259B and finding the impedance dip at 23.05MHz. The calculated vf from this test is 0.81, which reconciles reasonably with the specs. Free space wavelenght at the test frequency is 13m. The transmitter was set to 23.05MHz, and the ATU tuned to develop rated power output. The ATU is only used to present the rated load to the transmitter so as to obtain 100W for the test, to suit the Bird 43 element. It is inconsequential to the DUT (the Bird 43). With this configuration, it is expected that the impedance at the Bird 43 is approximately 50+j0, and that there would be almost zero reflected power. The Bird 43 indicated 100W forward power and a quarter of a needle width detection on reflected power. The Bird 43 would appear to provide valid readings for the conditions on the Bird 43 Thruline section in this case, notwithstanding that there is not any 50 ohm transmission line attached to the Bird 43 + N-BNC adapters. The myth that measurements with a Bird 43 of the conditions on the Thruline section are invalid unless it has some minimum length of 50 ohm line on both sides of itself is BUSTED. Actually, the results of your experiment proves the myth to be true and not to be a myth at all. There's 104.17 watts of forward power through the Bird and 4.17 watts of reflected power back through the Bird. Why does the Bird ignore those actual power values? Has anyone experimental evidence to the contrary? Yes, your experiment. Assuming 100 watts delivered to the load, the forward power on the 75 ohm coax is actually about 104.17 watts so the Bird's forward power reading is in error by 4.17 watts. The reflected power on the 75 ohm coax is about 4.17 watts so the Bird's reflected power reading is in error by close to an infinite percentage. The Bird 43 is reading neither of the actual power values correctly. All you have just proven is that the Bird 43 gives invalid readings when it is in a 75 ohm environment. THERE ARE ABOUT 4.17 WATTS OF REFLECTED ENERGY FLOWING BACK THROUGH THE BIRD AND THE BIRD COMPLETELY IGNORES IT. So the Bird is not even yielding valid readings for forward and reflected power through itself. That's exactly what I have been saying all along. If it were calibrated for 75 ohms, it would indicate the correct values. -- 73, Cecil http://www.qsl.net/w5dxp math please??? where do you get 4.17watts?? to me it looks like a 50 ohm load on a 50 ohm meter so zero reflected power. |
Dave wrote:
math please??? where do you get 4.17watts?? to me it looks like a 50 ohm load on a 50 ohm meter so zero reflected power. Here's the setup. 100W--tuner--75 ohm coax--Bird--1/2WL 75 ohm coax--50 ohm load Assumptions: Losses are negligible. The tuner provides a Z0-match so 100 watts is delivered to the load, i.e. all reflected power is re-reflected. I think this was actually the case for the experiment. Also assume that the impedance bump caused by the insertion of the Bird is negligible, i.e. the same net voltage and net current exists whether the Bird is in or out of the circuit. The voltage reflection coefficient at the load is (50-75)/(50+75) equals -0.2. That makes the power reflection coefficient (-0.2)^2 equals 0.04, i.e. 4% of the power incident upon the load is reflected. If 100 watts is delivered to the load while 4% is being reflected then the power incident upon the load is 100w/(1-0.04)=100w/0.96= 104.1667 watts. Incident power minus delivered power = reflected power so the reflected power on the 75 ohm coax is 4.1667 watts. The SWR on the 75 ohm coax is (1+|rho|)/(1-|rho|)= 1.2/0.8 = 1.5 THE SWR ON THE 75 OHM COAX IS 1.5:1. The Bird is in error when it reports the SWR to be 1:1. The SWR is *NOT* 1:1 anywhere on the load side of the system. The Bird reports a forward power of 100w. The actual forward power is 104.1667w. The Bird reports a reflected power of near zero. The actual reflected power is 4.1667w. The Bird reports an SWR of 1:1. The actual SWR is 1.5:1. The Bird is reporting false values because it is embedded in a 75 ohm environment and is calibrated for 50 ohms. If the ratio of voltage to current equals 50 within a 75 ohm system, there exists an SWR of 1.5:1 and an SWR meter calibrated for 75 ohms will verify that fact. -- 73, Cecil http://www.qsl.net/w5dxp |
Cecil Moore wrote:
The SWR is *NOT* 1:1 anywhere on the load side of the system. Sorry, this should have read: The SWR is *NOT* 1:1 anywhere on the load side of the tuner. -- 73, Cecil http://www.qsl.net/w5dxp |
Dave wrote:
math please??? where do you get 4.17watts?? to me it looks like a 50 ohm load on a 50 ohm meter so zero reflected power. Let's compare the following two configurations. 100W--tuner--75 ohm coax--Bird--1/2WL 75 ohm coax--50 ohm load 100W--tuner--75 ohm coax--Bird--1/4WL 75 ohm coax--50 ohm load The actual SWR on the coax is the same in both cases. The actual forward power and reflected power are the same in both cases. The SWR on the coax is 75/50 = 1.5:1. The actual forward power is 104.1667w and the actual reflected power is 4.1667w. In the first case the Bird will report an SWR of 1:1, a forward power of 100w, and a reflected power of zero simply because the ratio of net voltage to net current is 50 ohms at the Bird measurement point. In the second case the Bird will report an SWR of 2.25:1, a forward power of 117.4w and a reflected power of 17.4w simply because the ratio of net voltage to net current is 112.5 ohms at the Bird measurement point. The ratio of net voltage to net current alone is useless for calculating SWR unless the Z0 environment (characteristic impedance) exists and is known. Minus the humor, I believe this is Reg's main point. -- 73, Cecil http://www.qsl.net/w5dxp |
"Cecil Moore" wrote in message .. . Dave wrote: math please??? where do you get 4.17watts?? to me it looks like a 50 ohm load on a 50 ohm meter so zero reflected power. Here's the setup. 100W--tuner--75 ohm coax--Bird--1/2WL 75 ohm coax--50 ohm load Assumptions: Losses are negligible. The tuner provides a Z0-match so 100 watts is delivered to the load, i.e. all reflected power is re-reflected. I think this was actually the case for the experiment. Also assume that the impedance bump caused by the insertion of the Bird is negligible, i.e. the same net voltage and net current exists whether the Bird is in or out of the circuit. STOP! you can not assume this, especially if you consider the actual results of the experiment. the bird is a 50 ohm transmission line segment and it is seeing a 50 ohm load. the 50 ohm load plus 1/2 wave of anything is 50 ohms. so as far as the bird knows there is no reflected power and the real world reading is correct. |
"Cecil Moore" wrote in message
.. . Dave wrote: math please??? where do ---- ----alone is useless for calculating SWR unless the Z0 environment (characteristic impedance) exists and is known. Minus the humor, I believe this is Reg's main point. -- 73, Cecil http://www.qsl.net/w5dxp Did I miss something? As far as the Bird is concerned, all it sees is 50 ohms, indistinguishable from a 50 ohm termination, on its connector. 73, Frank |
Dave wrote:
"Cecil Moore" wrote in message Also assume that the impedance bump caused by the insertion of the Bird is negligible, i.e. the same net voltage and net current exists whether the Bird is in or out of the circuit. you can not assume this, especially if you consider the actual results of the experiment. the bird is a 50 ohm transmission line segment and it is seeing a 50 ohm load. the 50 ohm load plus 1/2 wave of anything is 50 ohms. so as far as the bird knows there is no reflected power and the real world reading is correct. Remove the Bird, reconnect the two pieces of 75 ohm coax, and I'll bet you will measure 70.7 volts and 1.414 amps at that point, 1/2 WL back from the 50 ohm load. If so, the Bird is NOT changing the conditions when it is inserted. And we know there has to be reflected power all up and down the 75 ohm coax. There is no place in the system on the load side of the tuner where reflected power is zero. The reflected power exists and the Bird reports a bogus reading for it. With or without the Bird in the circuit the ratio of net voltage to net current at the measurement point, 1/2WL away from the 50 ohm load, will be 50 ohms according to transmission line theory. That's all the Bird is seeing - a V/I ratio of 50 which is irrelevant to SWR unless Z0 is known and Z0 is known to be 75 ohms, NOT 50 ohms. The ratio of net voltage to net current is All THE BIRD EVER SEES. "As far as the bird knows ..." it is embedded in a 50 ohm environment. But the Bird is not all-knowing. In this case, it is embedded in a 75 ohm environment and is giving bogus readings because all it sees (samples) is a net voltage to net current whose ratio is 50 ohms. It is what the Bird doesn't know that is important. The Bird doesn't know that it is not embedded in a 50 ohm environment, and in its ignorance of that fact, reports bogus results. The actual SWR on a lossless line doesn't change. Yet, in another posting, I showed that moving the Bird 1/4WL closer to the load caused a reported SWR change by the Bird from 1:1 to 2.25:1. How could both results possibly be right? -- 73, Cecil http://www.qsl.net/w5dxp |
Frank wrote:
Did I miss something? As far as the Bird is concerned, all it sees is 50 ohms, indistinguishable from a 50 ohm termination, on its connector. Exactly the point, Frank. The Bird doesn't know it is embedded in a 75 ohm environment and reports bogus results because it assumes it is embedded in a 50 ohm environment, which it is not. This is Reg's main point in wanting to call the SWR meter a TLI meter. The Bird is not to blame for reporting bogus results. It is the operator who is to blame for using a 50 ohm meter in a 75 ohm environment and reporting the bogus results as valid. 100w--tuner--75 ohm coax--x--1/2WL 75 ohm coax--50 ohm load Without a Bird installed at point 'x', most everyone would agree that the SWR all up and down the 75 ohm coax is 1.5:1 and the voltage and current at point 'x' are the same as the in-phase voltage and current at the 50 ohm load. The voltage at point 'x' is 70.7 volts, and the current at point 'x' is 1.414 amps. The SWR on the line is 1.5:1. Now, installing the Bird at point 'x' doesn't change anything appreciably. The voltage at point 'x' is still very close to 70.7 volts and the current at point 'x' is still very clost to 1.414 amps. Those two parameters are what the Bird samples and all the Bird sampoles. And since the Bird is calibrated for 50 ohms, it reports a bogus SWR of 1:1. If the Bird is recalibrated for 75 ohms, it will report the correct SWR of 1.5:1. -- 73, Cecil http://www.qsl.net/w5dxp |
"Cecil Moore" wrote in message .. . Dave wrote: "Cecil Moore" wrote in message Also assume that the impedance bump caused by the insertion of the Bird is negligible, i.e. the same net voltage and net current exists whether the Bird is in or out of the circuit. you can not assume this, especially if you consider the actual results of the experiment. the bird is a 50 ohm transmission line segment and it is seeing a 50 ohm load. the 50 ohm load plus 1/2 wave of anything is 50 ohms. so as far as the bird knows there is no reflected power and the real world reading is correct. Remove the Bird, reconnect the two pieces of 75 ohm coax, and I'll bet you will measure 70.7 volts and 1.414 amps at that point, 1/2 WL back from the 50 ohm load. If so, the Bird is NOT changing the conditions when it is inserted. And we know there has to be reflected power all up and down the 75 ohm coax. There is no place in the system on the load side of the tuner where reflected power is zero. The reflected power exists and the Bird reports a bogus reading for it. With or without the Bird in the circuit the ratio of net voltage to net current at the measurement point, 1/2WL away from the 50 ohm load, will be 50 ohms according to transmission line theory. That's all the Bird is seeing - a V/I ratio of 50 which is irrelevant to SWR unless Z0 is known and Z0 is known to be 75 ohms, NOT 50 ohms. The ratio of net voltage to net current is All THE BIRD EVER SEES. "As far as the bird knows ..." it is embedded in a 50 ohm environment. But the Bird is not all-knowing. In this case, it is embedded in a 75 ohm environment and is giving bogus readings because all it sees (samples) is a net voltage to net current whose ratio is 50 ohms. It is what the Bird doesn't know that is important. The Bird doesn't know that it is not embedded in a 50 ohm environment, and in its ignorance of that fact, reports bogus results. The actual SWR on a lossless line doesn't change. Yet, in another posting, I showed that moving the Bird 1/4WL closer to the load caused a reported SWR change by the Bird from 1:1 to 2.25:1. How could both results possibly be right? -- 73, Cecil http://www.qsl.net/w5dxp obviously at 1/4 wave from the load the impedance seen by the bird is not 50 ohms. you just can't ignore that internally the bird is a 50 ohm line, even though it is only a couple inches long it is 50 ohms, put a 50 ohm load on it and it will read no reflected power. this is just how it works, you can't wish it to be anything else, especially when the actual measurements prove me correct. you may try to assume away the bird and its characteristic impedance, but it just won't go away as evidenced by the real world measurements. you have to realize that the bird is not measuring what is going on outside it's case, the only thing it can measure is the voltage and current in its little 50 ohm internal world. so look at it from the meter's point of view... it looks out the load side and sees 50 ohms, it can't know that it is really a 75 ohm line, in fact it can't know there is any line there at all, it just sees a 50 ohm load, therefore no reflection into the meter, and no reflected power reading. qed. |
Dave wrote:
obviously at 1/4 wave from the load the impedance seen by the bird is not 50 ohms. That's the whole point, Dave. The poor ignorant Bird doesn't know what is going on external to itself. You and I have to be smarter than the Bird and not report its bogus results as "correct". The Bird reports a 9:1 SWR on a matched-line 450 ohm ladder-line. Is the SWR really 9:1? this is just how it works, you can't wish it to be anything else, especially when the actual measurements prove me correct. The actual measurements prove you wrong! The forward power reported by the Bird is NOT the actual forward power. The reflected power reported by the Bird is NOT the actual reflected power. The SWR reported by the Bird does not exist anywhere in the system on the load side of the tuner. The Bird is totally confused because it is being abused by the operator. The operator is totally ignorant when he reports the readings are "correct". you have to realize that the bird is not measuring what is going on outside it's case, the only thing it can measure is the voltage and current in its little 50 ohm internal world. That's exactly the point and that's why some readings reported by the Bird are obviously bogus when compared to the broader knowledge of actual external conditions. The operator needs to be smarter than the Bird. so look at it from the meter's point of view... it looks out the load side and sees 50 ohms, it can't know that it is really a 75 ohm line, in fact it can't know there is any line there at all, it just sees a 50 ohm load, therefore no reflection into the meter, and no reflected power reading. qed. But you and I know that 75 ohm reflected energy is flowing through the Bird and the Bird is ignoring it. Of course, we cannot blame the Bird for the bogus readings. We can only blame the ignorant operator who reports that the bogus readings are "correct" when they are obviously false. -- 73, Cecil http://www.qsl.net/w5dxp |
Cecil Moore wrote: The actual SWR on a lossless line doesn't change. Yet, in another posting, I showed that moving the Bird 1/4WL closer to the load caused a reported SWR change by the Bird from 1:1 to 2.25:1. How could both results possibly be right? You need to keep thinking about that. What if they are both right? Is it really one continuous, uniform transmission line? Is SWR the same everywhere in a tee stub circuit? ac6xg |
"Cecil Moore" wrote in message ... Dave wrote: obviously at 1/4 wave from the load the impedance seen by the bird is not 50 ohms. That's the whole point, Dave. The poor ignorant Bird doesn't know what is going on external to itself. You and I have to be smarter than the Bird and not report its bogus results as "correct". The Bird reports a 9:1 SWR on a matched-line 450 ohm ladder-line. Is the SWR really 9:1? well DUH! obviously if you want to measure swr on a 450 ohm ladder line you don't use a 50 ohm bird! anyone using a 50 ohm bird has to expect results to be referenced to its internal 50 ohm line. i.e. if you actually do measure no reflected power so you know the load on the bird is 50 ohms, and if you really are in a 75 ohm environment then you can compute what the swr is on the 75 ohm line... just don't expect the bird to show it to you directly. |
"Cecil Moore" wrote - Minus the humor, I believe this is Reg's main point. ==================================== Dear Cec, I assume you are referring to me, Reg Edwards, G4FGQ. Before responding I have found it necessary to have several glasses of Australian, Banrock Station, 2004, Shiraz Mataro, Red. It can be recommended with confidence. I have only ever had ONE point. It is that arguments and discussions about SWR, invariably involving "ifs" and "buts" and vivid imaginations of the contributors, are of entertainment value only. In any event, CB-ers and novices are unlikely to learn anything from it. There is only one transmission line on which SWR matters. It is that which runs between the so-called SWR meter or tuner and the antenna. It is generally unknown. But whatever it is the so-called SWR meter, even a Bird, is incapable of measuring it. Having got that off my chest. now perhaps I can finish the bottle with a cigarette. ---- Reg, G4FGQ. |
On Thu, 13 Oct 2005 14:31:22 GMT, Cecil Moore wrote:
The Bird is in error when it reports the SWR to be 1:1. This is the poor carpenter blaming his tools. The instrument can only be valid within its presumed operating conditions. Deliberate misuse is not a reason to crow about inaccuracy. The SWR is *NOT* 1:1 anywhere on the load side of the tuner. This is the poor carpenter asking for his wage for his "craft." The Bird is accurately responding to the operating conditions it is found within. The manufacturer of the Bird wattmeter makes no claim as to the state of match BEFORE the meter; and especially when it is so obviously and deliberately misused - which in this sliver of specificity is transparent to the reading. What is being busted is the claim that a necessary condition of operation for the Bird was the requirement for a length of 50 Ohm line to "force" a purely mythical presumption. That myth has been exposed and discarded. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Owen, To respond to your last question: Has anyone experimental evidence to the contrary? is consistently NO. Your own time at the bench has already drained the pool of ability in that regard. Your only expectation ever after having bellied up to the bench is to watch your work being gummed to death. However, for completeness' sake, and as no one here really understands what accuracy is about anyway, there is one factor to be considered. The numbers offered verge on the limit of the Bird's ability to resolve a power anyway. There is a built in probability of ±5W of error from the get-go, and any snake oil salesman can craft an argument leveraging that error to prove anything. We have seen that ±5W error in the form of an argument that uses both + and - (not simply one or the other) to please a theory. Owen, the same experiment with a deliberate mismatch of 3:1 would be just as effective at busting the myth AND providing data that overwhelmed the inherent meter inaccuracy. 73's Richard Clark, KB7QHC |
Jim Kelley wrote:
Cecil Moore wrote: The actual SWR on a lossless line doesn't change. Yet, in another posting, I showed that moving the Bird 1/4WL closer to the load caused a reported SWR change by the Bird from 1:1 to 2.25:1. How could both results possibly be right? You need to keep thinking about that. What if they are both right? Here's the example sans the Bird. Between the tuner output and the load, where exactly is the actual SWR = 1:1 and where exactly is the actual SWR = 2.25:1? Answer: nowhere! XMTR--tuner---1 WL 75 ohm coax---50 ohm load -- 73, Cecil http://www.qsl.net/w5dxp |
Dave wrote:
well DUH! obviously if you want to measure swr on a 450 ohm ladder line you don't use a 50 ohm bird! Yes, now you are getting it. If you want to measure SWR on a 75 ohm coax line, you don't use a 50 ohm Bird. I couldn't have said it better myself. -- 73, Cecil http://www.qsl.net/w5dxp |
Richard Clark wrote:
Cecil Moore wrote: The Bird is in error when it reports the SWR to be 1:1. This is the poor carpenter blaming his tools. Exactly! You got my point. It is operators who refuse to recognize the errors in the Bird's readings and report a bogus SWR as "correct" who are at fault. -- 73, Cecil http://www.qsl.net/w5dxp |
Cecil Moore wrote: Jim Kelley wrote: Cecil Moore wrote: The actual SWR on a lossless line doesn't change. Yet, in another posting, I showed that moving the Bird 1/4WL closer to the load caused a reported SWR change by the Bird from 1:1 to 2.25:1. How could both results possibly be right? You need to keep thinking about that. What if they are both right? Here's the example sans the Bird. Between the tuner output and the load, where exactly is the actual SWR = 1:1 and where exactly is the actual SWR = 2.25:1? Answer: nowhere! XMTR--tuner---1 WL 75 ohm coax---50 ohm load And the answer would be different still in this circuit. XMTR---1/4 WL 75 ohm coax---50 ohm load Amazin', what happens when you change the circuit! ac6xg |
"Reg Edwards" wrote CB-ers and novices are unlikely to learn anything from it. ================================ And it appears some professional IEEE engineers and university professors also remain in difficulty. Just because, from it's sales blurb and scale markings, an instrument is purported to measure SWR with forward and reflected power, should not be taken as being the gospel truth. It can be highly misleading. And from this newsgroup it seems it is! There's still some left in the bottle. Hic! Hic! ---- Reg. |
On Thu, 13 Oct 2005 17:27:01 GMT, Cecil Moore wrote:
Richard Clark wrote: Cecil Moore wrote: The Bird is in error when it reports the SWR to be 1:1. This is the poor carpenter blaming his tools. Exactly! You got my point. It is operators who refuse to recognize the errors in the Bird's readings There was no error in the reading beyond the inherent ±5% specified as the meter movement's. Owen no where at any time makes any appeal to measuring or presenting SWR so the following claim is entirely fabricated to present something not under his, my, or other poster's consideration: and report a bogus SWR as "correct" who are at fault. Persistence in wedging a new picture into a valuable frame does not make it a classic portrait by a master of the craft. This finger painting being offered is no more notable than yet another tacky Elvis on velvet. |
Jim Kelley wrote:
Cecil Moore wrote: Here's the example sans the Bird. Between the tuner output and the load, where exactly is the actual SWR = 1:1 and where exactly is the actual SWR = 2.25:1? Answer: nowhere! XMTR--tuner---1 WL 75 ohm coax---50 ohm load And the answer would be different still in this circuit. XMTR---1/4 WL 75 ohm coax---50 ohm load No, it wouldn't. The answer is exactly the same. *NOWHERE* is the SWR 1:1 or 2.25:1. In both examples, the SWR on the coax is 1.5:1. The SWR is *always* set by the relationship of Z0 to the load. In both examples above, that relationship is *identical*. Amazin', what happens when you change the circuit! Z0 didn't change. The load didn't change. Therefore, the SWR didn't change. What exactly do you think changed? -- 73, Cecil http://www.qsl.net/w5dxp |
Richard Clark wrote:
Cecil Moore wrote: Exactly! You got my point. It is operators who refuse to recognize the errors in the Bird's readings There was no error in the reading beyond the inherent ±5% specified as the meter movement's. The Bird is supposed to measure power. The Bird's forward power readings are in error unless used in a 50 ohm environment. I previously talked about using a hammer on a screw. I was hoping even you could understand that metaphor without me having to explain it to you. -- 73, Cecil http://www.qsl.net/w5dxp |
Cecil Moore wrote: Z0 didn't change. The load didn't change. Therefore, the SWR didn't change. What exactly do you think changed? I think the circuit changed. Don't you? I also think that if you change the circuit, it's possible to measure the effect of that change. The meter measures what takes place at its insertion point in the circuit. What you seem to be upset about is that it might not in every case accurately display the conditions at some arbitrary position away from its insertion point. Like within a shorted quarterwave stub for example. Are you still unwilling to accept that the meter itself can present a perturbation? ac6xg |
On Thu, 13 Oct 2005 18:48:05 GMT, Cecil Moore wrote:
The Bird is supposed to measure power. The Bird's forward power readings are in error That has not been demonstrated by Owen's example. Elvis is getting moldy, and the velvet is becoming tattered at the frame where it was nailed in. |
"Cecil Moore" wrote in message om... Dave wrote: well DUH! obviously if you want to measure swr on a 450 ohm ladder line you don't use a 50 ohm bird! Yes, now you are getting it. If you want to measure SWR on a 75 ohm coax line, you don't use a 50 ohm Bird. I couldn't have said it better myself. -- then why are you complaining about it not showing the swr on the 75 ohm coax?? you should know that no one in their right mind would expect it to do that. |
"Cecil Moore" wrote in message . .. Jim Kelley wrote: Cecil Moore wrote: Here's the example sans the Bird. Between the tuner output and the load, where exactly is the actual SWR = 1:1 and where exactly is the actual SWR = 2.25:1? Answer: nowhere! XMTR--tuner---1 WL 75 ohm coax---50 ohm load And the answer would be different still in this circuit. XMTR---1/4 WL 75 ohm coax---50 ohm load No, it wouldn't. The answer is exactly the same. *NOWHERE* is the SWR 1:1 or 2.25:1. In both examples, the SWR on the coax is 1.5:1. The SWR is *always* set by the relationship of Z0 to the load. In both examples above, that relationship is *identical*. Amazin', what happens when you change the circuit! Z0 didn't change. The load didn't change. Therefore, the SWR didn't change. What exactly do you think changed? -- 73, Cecil http://www.qsl.net/w5dxp obviously the problem here is that cecil thinks he is the only one that knows better than to try to measure reflected power with a 50 ohm meter in a 75 ohm coax. |
Richard Clark wrote:
Cecil Moore wrote: The Bird is supposed to measure power. The Bird's forward power readings are in error That has not been demonstrated by Owen's example. Richard, if you don't understand why a 50 ohm power meter yields erroneous readings when installed in a 75 ohm environment, I don't know what else to tell you. What is it about using a hammer on a screw that you don't understand? -- 73, Cecil http://www.qsl.net/w5dxp |
Dave wrote:
then why are you complaining about it not showing the swr on the 75 ohm coax?? you should know that no one in their right mind would expect it to do that. You at first essentially implied that a Bird wattmeter calibrated for 50 ohms yields valid readings in a 75 ohm environment. I was questioning whether you were in your right mind or not. Others have been strangely silent on the subject. Still others seem not to have gotten it yet. -- 73, Cecil http://www.qsl.net/w5dxp |
Jim Kelley wrote:
Cecil Moore wrote: Z0 didn't change. The load didn't change. Therefore, the SWR didn't change. What exactly do you think changed? I think the circuit changed. Don't you? The circuit changed without changing the forward power, reflected power, and SWR so nothing of interest to the present topic (V/I ratio) changed. Do you know what dictates the SWR in a distributed network? Certainly not the length of the feedline or the removal of a tuner (assuming lossless conditions). -- 73, Cecil http://www.qsl.net/w5dxp |
Dave wrote:
obviously the problem here is that cecil thinks he is the only one that knows better than to try to measure reflected power with a 50 ohm meter in a 75 ohm coax. From your posturing, it wasn't readily apparent to me that you were agreeing with me. -- 73, Cecil http://www.qsl.net/w5dxp |
"Cecil Moore" wrote in message . .. Dave wrote: obviously the problem here is that cecil thinks he is the only one that knows better than to try to measure reflected power with a 50 ohm meter in a 75 ohm coax. From your posturing, it wasn't readily apparent to me that you were agreeing with me. -- i'm not agreeing with how you assume the 50 ohm impedance of the meter out of the circuit. and i am not agreeing that there should be reflected power measured by the meter in the case of the 50 ohm load on the end of a 1/2 wave 75 ohm line. obviously it will not measure any reflected power which is perfectly correct for the whole circuit as defined and as actually measured. |
On Thu, 13 Oct 2005 19:17:23 GMT, Cecil Moore wrote:
you don't understand why a 50 ohm power meter yields erroneous readings Any error of misunderstanding Owen's post is entirely your own. My career in calibrating RF Wattmeters while you were flipping bits is a good indicator of the tectonic trench you stand in. The fact remains, there has been no error displayed (beyond the casual 5% reading error inherent in the meter) nor his results refuted as to how much power has impinged upon the cabled load as he explicitly described being attached to the Bird's measurement port. As there is no other use for such a wattmeter, any appeals to the contrary are idle chatter. Owen's claim stands: the myth of requiring a 50 Ohm transmission line at the measurement port of the Bird wattmeter has been debunked. This comes as no surprise but for one poster to this board. |
On Thu, 13 Oct 2005 16:45:51 +0000 (UTC), "Reg Edwards"
wrote: Before responding I have found it necessary to have several glasses of Australian, Banrock Station, 2004, Shiraz Mataro, Red. It can be recommended with confidence. Entirely OT: Reg, you are way to impatient. There are very few Australian Shiraz that should be taken in less than three years. I can't recall the "instructions" on the back of the bottle that tell one what to drink it with and when to drink it, but Banrock Station produce "everday drinking" quality reds that should stand a few years cellaring, but most wines in this part of the market will state "enjoy now" for marketing purposes. IIRC, the labels carry a bit of a story on their wildlife refuge, do they get to tell you what to eat with it? Anyway, now that it is open, enjoy it. Owen -- |
On Thu, 13 Oct 2005 09:53:08 -0700, Richard Clark
wrote: Owen, To respond to your last question: Has anyone experimental evidence to the contrary? is consistently NO. Your own time at the bench has already drained the pool of ability in that regard. Your only expectation ever after having bellied up to the bench is to watch your work being gummed to death. I fully expected someone to object, not only to object, but to do so without any original experimental evidence and to devalue the experiment that I did so that some readers who do not have even a meager understanding of transmission line theory fall to being convinced by whoever is most tenacious is defending their position. However, for completeness' sake, and as no one here really understands what accuracy is about anyway, there is one factor to be considered. The numbers offered verge on the limit of the Bird's ability to resolve a power anyway. There is a built in probability of ±5W of error from the get-go, and any snake oil salesman can craft an argument leveraging that error to prove anything. We have seen that ±5W error in the form of an argument that uses both + and - (not simply one or the other) to please a theory. Owen, the same experiment with a deliberate mismatch of 3:1 would be just as effective at busting the myth AND providing data that overwhelmed the inherent meter inaccuracy. Indeed, and I considered a number of other experiments that did so, but this one was based on components at hand, and should have been easily understood by a person with the most basic understanding of transmission line theory. It was important to surround the Bird with line different to 50 ohms. I expect the argument to twist an turn, to focus on everything but the assertions that: - there should be approximately a 50+j0 Z presented to the load side terminals of the Bird Thruline (ie the ratio of V/I is 50+j0 where V is the net or forward and reflected voltages, and I is the net of forward and reflected currents); - the Bird Thruline is a 120mm section of 50 ohm transmission line; - in the region of the Bird Thruline sampler element, the ratio of V/I is approximately 50+j0; and that the observed Bird 43 readings were reasonably consistent with those assertions. The arguments that knowing that the Bird measurements are valid at the point of measurement is of little value are unrelated to the issue and a diversionary tactic, but wrong nevertheless. I won't add to the diversion to identify them. I will extract the essence of the analysis and write a separate web page on it that may in the longer term assist others in their development, go being "gummed to death" doesn't totally devalue the information behind the case, and it might just be the cost of exposing the proposition to review. Thank you for your support. Owen PS: I am planning my next mythbusters (oh no! I hear...) Myth: SWR meters measure SWR. Now this is not to bag SWR meters, I think that they are very useful instruments, but they have limitations, and the greatest problem is not the meters, but probably the knowledge base of those (ab)using them. -- |
Owen Duffy wrote:
The arguments that knowing that the Bird measurements are valid at the point of measurement is of little value are unrelated to the issue and a diversionary tactic, but wrong nevertheless. Hi Owen, If it's wrong to argue that Bird wattmeter measurements are valid at the point of measurement, I don't wanna be right. ;-) PS: I am planning my next mythbusters (oh no! I hear...) Undoubtedly it's the collective "oh goody!" that you hear. Myth: SWR meters measure SWR. Now this is not to bag SWR meters, I think that they are very useful instruments, but they have limitations, and the greatest problem is not the meters, but probably the knowledge base of those (ab)using them. You must be new around here. :-) 73, ac6xg |
On Thu, 13 Oct 2005 15:02:37 -0700, Jim Kelley
wrote: You must be new around here. :-) Hi Jim, Give Owen more credit than that, after all he knows at least one prolific poster: I expect the argument to twist an turn, to focus on everything but the assertions As long as you keep on the message, then that profligacy twists and turns the postings like yellowing leaves starved of nourishment. A faint rustle of reality causes them to fall unnoticed to the way-side. 73's Richard Clark, KB7QHC |
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