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#242
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An antenna question--43 ft vertical
Roger Hayter wrote:
wrote: John S wrote: On 7/7/2015 1:58 PM, wrote: John S wrote: On 7/5/2015 7:21 PM, wrote: John S wrote: On 7/5/2015 5:24 PM, wrote: Roger Hayter wrote: wrote: The output impedance of an amateur transmitter IS approximately 50 Ohms as is trivially shown by reading the specifications for the transmitter which was designed and manufactured to match a 50 Ohm load. Do you think all those manuals are lies? You are starting with a false premise which makes everything after that false. A quick google demonstrates dozens of specification sheets that say the transmitter is designed for a 50 ohm load, and none that mention its output impedance. If the source impedance were other than 50 Ohms, the SWR with 50 Ohm coax and a 50 Ohm antenna would be high. It is not. Where is the source impedance found on a Smith chart? Also, if you have EZNEC, you will not find a place to specify source impedance but it will show the SWR. A Smith chart is normalized to 1. EZNEC allows you to set the impedance to anything you want and assumes the transmission line matches the transmitter. The EZNEC help file is very comprehensive. Please find any reference to your assertion that there is an assumption of source impedance there and provide information for us to verify your assertion. Why don't you email the author and get his take on your assumptions? Why don't YOU? You are the one in need of knowledge. If I do it and report back here you will just doubt it or find something else to argue about. Better you should do it first-hand. EZNEC calculates the SWR presented to the SOURCE which is usually placed at the antenna terminals. EZNEC also calculates the SWR presented to the SOURCE which can be modeled as a SOURCE at one end of a transmission line and the antenna at the other end. SWR is defined in terms of SOURCE impedance and LOAD impedance. I am tired of typing in the same equations over and over again. Zo is the characteristiic impedance of the transmission line and nothing to do with the source impedance of whatever generator is supplying power to the system. No, Zo is the source impedance and it is irrelevant what the source physically is and has nothing to do with power. https://en.wikipedia.org/wiki/Reflection_coefficient http://www.antenna-theory.com/tutori...nsmission3.php -- Jim Pennino |
#243
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An antenna question--43 ft vertical
wrote in message news Ralph Mowery wrote: Can you show any place where the SWR definition mentions the Source impedance ? I have several times now, but once again: SWR = (1 + |r|)/(1 - |r|) Where r = reflection coefficient. r = (Zl - Zo)/(Zl + Zo) Where Zl = complex load impedance and Zo = complex source impedance. https://en.wikipedia.org/wiki/Reflection_coefficient http://www.antenna-theory.com/tutori...nsmission3.php YOu have just proven my point. Read carefully from your refernce to Wikipedia : "The reflection coefficient of a load is determined by its impedance and the impedance toward the source." Notice it says TOWARD and not THE SOURCE. From the second referaence notice that it says load impedance and impedance of the transmission line. Nothing mentions the source at all: "The reflection coefficient is usually denoted by the symbol gamma. Note that the magnitude of the reflection coefficient does not depend on the length of the line, only the load impedance and the impedance of the transmission line. Also, note that if ZL=Z0, then the line is "matched". In this case, there is no mismatch loss and all power is transferred to the load." |
#244
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An antenna question--43 ft vertical
Ralph Mowery wrote:
wrote in message news Ralph Mowery wrote: Can you show any place where the SWR definition mentions the Source impedance ? I have several times now, but once again: SWR = (1 + |r|)/(1 - |r|) Where r = reflection coefficient. r = (Zl - Zo)/(Zl + Zo) Where Zl = complex load impedance and Zo = complex source impedance. https://en.wikipedia.org/wiki/Reflection_coefficient http://www.antenna-theory.com/tutori...nsmission3.php YOu have just proven my point. Read carefully from your refernce to Wikipedia : "The reflection coefficient of a load is determined by its impedance and the impedance toward the source." Notice it says TOWARD and not THE SOURCE. Notice it actually says "the impedance toward the source". From the second referaence notice that it says load impedance and impedance of the transmission line. Nothing mentions the source at all: What the hell do you think the transmission line is in this case if not the source? "The reflection coefficient is usually denoted by the symbol gamma. Note that the magnitude of the reflection coefficient does not depend on the length of the line, only the load impedance and the impedance of the transmission line. Also, note that if ZL=Z0, then the line is "matched". In this case, there is no mismatch loss and all power is transferred to the load." Perhaps you would like the second link better as it has pictures. Of maybe this one that explains it all starting with lumped equivelant circuits. http://www.maximintegrated.com/en/ap...dex.mvp/id/742 Notice that ALL the links talk about the source impedance. -- Jim Pennino |
#245
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An antenna question--43 ft vertical
On 7/8/2015 3:38 PM, John S wrote:
On 7/8/2015 10:48 AM, rickman wrote: On 7/8/2015 10:09 AM, John S wrote: On 7/2/2015 1:38 PM, rickman wrote: On 7/2/2015 1:56 PM, Ralph Mowery wrote: "Jerry Stuckle" wrote in message ... Try this - connect the output of an HF transmitter to an SWR bridge. Now connect a piece of 75 ohm coax such as RG-59 to the output of the SWR meter, and connect that to a 75 ohm resistive load. Do you think the SWR bridge will show a 1:1 SWR? Not a chance. It will be 1.5:1. What you have described is a case of using the wrong swr bridge. You are trying to use a 50 ohm bridge on a 75 ohm system. If a 75 ohm bridge is used it will show a 1:1 SWR. The real SWR is 1:1. With a 75 ohm line and 75 ohm load there is no reflected power. My knowledge of antenna systems is limited, but I do know that this is correct, there will be no reflection from the antenna. If there is no reflections from the antenna, how can there be a loss in the source end? There is NO power returned according to your own statement. I don't see any contradiction. The power comes from the source through the source impedance. The source impedance will create a loss, no? If the transmitter output is 50 ohms there will be a loss in this matching that will result in less power being delivered to the feed line, but that will not result in reflections in the feed line. Why? What causes the loss? The transmitter output resistance? So that would mean that one can never achieve more that 50% efficiency at the transmitter's OUTPUT! And that would mean that a 1000W transmitter is dissipating 500 watts under the BEST circumstances. Good luck on getting that to work to your satisfaction. Maybe "loss" isn't the right term then. The output of a 50 ohm source driving a 75 ohm load will deliver 4% less power into the load than when driving a 50 ohm load. That comes to -0.177 dB. Is there any part of that you disagree with? All of it. Let's say you have a 1A source and it has a 50 ohm impedance in series with its output. With a 50 ohm load it will provide 50W to the load. With a 75 ohm load it will provide 75W to the load. The only difference is that the 50 ohm load will cause the source voltage (before the series impedance) to be 100V while the 75 ohm load will require 112V (before the series impedance). If the series impedance is 0 +/- j75 ohms, it will have no power loss. If the series impedance is 50 + j0 it will have a 50W loss. I was referring to a voltage source. -- Rick |
#246
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An antenna question--43 ft vertical
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#247
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An antenna question--43 ft vertical
rickman wrote:
On 7/8/2015 3:38 PM, John S wrote: On 7/8/2015 10:48 AM, rickman wrote: On 7/8/2015 10:09 AM, John S wrote: On 7/2/2015 1:38 PM, rickman wrote: On 7/2/2015 1:56 PM, Ralph Mowery wrote: "Jerry Stuckle" wrote in message ... Try this - connect the output of an HF transmitter to an SWR bridge. Now connect a piece of 75 ohm coax such as RG-59 to the output of the SWR meter, and connect that to a 75 ohm resistive load. Do you think the SWR bridge will show a 1:1 SWR? Not a chance. It will be 1.5:1. What you have described is a case of using the wrong swr bridge. You are trying to use a 50 ohm bridge on a 75 ohm system. If a 75 ohm bridge is used it will show a 1:1 SWR. The real SWR is 1:1. With a 75 ohm line and 75 ohm load there is no reflected power. My knowledge of antenna systems is limited, but I do know that this is correct, there will be no reflection from the antenna. If there is no reflections from the antenna, how can there be a loss in the source end? There is NO power returned according to your own statement. I don't see any contradiction. The power comes from the source through the source impedance. The source impedance will create a loss, no? If the transmitter output is 50 ohms there will be a loss in this matching that will result in less power being delivered to the feed line, but that will not result in reflections in the feed line. Why? What causes the loss? The transmitter output resistance? So that would mean that one can never achieve more that 50% efficiency at the transmitter's OUTPUT! And that would mean that a 1000W transmitter is dissipating 500 watts under the BEST circumstances. Good luck on getting that to work to your satisfaction. Maybe "loss" isn't the right term then. The output of a 50 ohm source driving a 75 ohm load will deliver 4% less power into the load than when driving a 50 ohm load. That comes to -0.177 dB. Is there any part of that you disagree with? All of it. Let's say you have a 1A source and it has a 50 ohm impedance in series with its output. With a 50 ohm load it will provide 50W to the load. With a 75 ohm load it will provide 75W to the load. The only difference is that the 50 ohm load will cause the source voltage (before the series impedance) to be 100V while the 75 ohm load will require 112V (before the series impedance). If the series impedance is 0 +/- j75 ohms, it will have no power loss. If the series impedance is 50 + j0 it will have a 50W loss. I was referring to a voltage source. Instead of arguing about it, one can download QUCS for free which will simulate the whole thing and one can see what really happens. Download QUCS for your operating system: http://qucs.sourceforge.net/ Generate a model consisting of a voltage source with a series resistance of a few Ohms to simulate a solid state source or a much higher resistance to simulate a vacuum tube source. Chose a convienient frequency for the source. Go to: http://home.sandiego.edu/~ekim/e194r.../matcher2.html to calculate an impedance matching network to match the resistance you've chosen to 50 Ohms. Put the matching circuit in the model. Add a transmission line to the model. Terminate the transmission line with a 50 Ohms resistor. Add a fixed frequency AC simulation at the desired frequency. Change various parameters to your heart's content to see what happens. Change the matching network such that the output of your transmitter is no longer 50 Ohms and see what happens. When the QUCS output disagrees with your beliefs, you can argue with the program. -- Jim Pennino |
#248
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An antenna question--43 ft vertical
Wayne wrote:
"John S" wrote in message ... On 7/7/2015 1:44 PM, wrote: Ian Jackson wrote: In message , Jerry Stuckle writes Sure, there is ALWAYS VSWR. It may be 1:1, but it's always there. If there's no reflection, there can be no standing wave. So, being pedantic, there's no such thing as an SWR of 1:1! Despite the name, VSWR is defined in terms of complex impedances and wavelengths, not "waves" of any kind. Actually, VSWR is defined as the ratio of Vmax/Vmin. That's also my understanding of the definition. That is one definition. In fact since SWR is defined as the maximum to minimum voltage ratio, the "V" in VSWR is redundant. There are other kinds of "SWR", but you will never see them refered to in Amateur literature. But, using the voltage ratio definition, you can work yourself back around to various source and load impedances, reflection coefficients, etc. Or you can start from source and load impedances. The whole Vmax/Vmin definition is a leftover from the early days when RF measurments were done on Lecher lines or slotted lines and you actually looked for the min and max points on the line. I doubt few under about 50 have heard of slotted lines or Lecher lines much less ever used one. -- Jim Pennino |
#249
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An antenna question--43 ft vertical
On 7/8/2015 4:48 PM, wrote:
John S wrote: On 7/8/2015 12:47 PM, wrote: John S wrote: So, at 1Hz the law has changed, eh? What new law do I need to use? To be pendatic, there is only one set of physical laws that govern electromagnetics. However for DC all the complex parts of those laws have no effect and all the equations can be simplified to remove the complex parts. In the real, practical world people look upon this as two sets of laws, one for AC and one for DC. A good example of this is the transmission line which does not exist at DC; at DC a transmission line is nothing more than two wires with some resistance that is totally and only due to the ohmic resistance of the material that makes up the wires. So, is .01Hz AC or DC, Jim? How about 1Hz? 10Hz? Where does AC begin and DC end? It is called a limit. If there is NO time varying component, it is DC, otherwise it is AC. Are you playing devil's advocate or are you really that ignorant? Then there is no such thing as DC because even a battery looses voltage over a period of time. DC voltage sources have noise. Are just being argumentative or are you really that ignorant? |
#250
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An antenna question--43 ft vertical
On 7/8/2015 4:51 PM, wrote:
John S wrote: On 7/8/2015 1:14 PM, wrote: John S wrote: On 7/7/2015 1:44 PM, wrote: Ian Jackson wrote: In message , Jerry Stuckle writes Sure, there is ALWAYS VSWR. It may be 1:1, but it's always there. If there's no reflection, there can be no standing wave. So, being pedantic, there's no such thing as an SWR of 1:1! Despite the name, VSWR is defined in terms of complex impedances and wavelengths, not "waves" of any kind. Actually, VSWR is defined as the ratio of Vmax/Vmin. Actually, VSWR can be defined several ways, one of which is: (1 + |r|)/(1 - |r|) Where r is the reflection coefficient which can be defined a: (Zl - Zo)/(Zl + Zo) Where Zl is the complex load impedance and Zo is the complex source impedance. Note that a complex impedance has a frequency dependant part. So, since Vmax/Vmin (the base definition) has no frequency dependent part, does that invalidate it? The "base definition" can be whatever set of equations you pick that are true. BTW, the Vmax/Vmin DOES have a frequency dependant component that determines WHERE Vmax and Vmin occur. You are just being argumentative. The WHERE doesn't matter in measuring VSWR. You still measure correct VSWR wherever the locations. |
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