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#201
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An antenna question--43 ft vertical
On 7/7/2015 2:32 PM, Ian Jackson wrote:
In message , Jerry Stuckle writes On 7/7/2015 6:25 AM, 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! Wrong. An SWR of 1:1 indicates a perfect match, with no reflected power. It is recognized by all electronics texts and experts. My suggestion would be for you to learn some transmission line theory. Your statement here just showed you have no knowledge of it at all. Even when I took my novice test many years ago I had to understand SWR better than that. The point I'm trying to make is not technical. It's simply one of verbal logic. Without the presence of a standing wave, you can't possibly have something called a "standing wave ratio". But, like all RF engineers, an SWR of 1-to-1 is something I too strive to achieve! "Yesterday, upon the stair, I met a man who wasn't there. He wasn't there again today, I wish, I wish he'd go away..." It doesn't matter what your verbal logic is, Ian. The correct term is "Standing Wave Ratio", and an SWR of 1:1 means there is no reflected power and you have a perfect match. That is the technical definition of a technical term. -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
#203
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An antenna question--43 ft vertical
On 7/7/2015 11:09 AM, Ian Jackson wrote:
In message , rickman writes On 7/7/2015 6:25 AM, 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! Why do you say that? If there is no reflection the voltage on the line is purely due to the forward signal and so the VSWR is 1:1. What's wrong with that? A standing wave is caused by a reflection. If there IS no reflection, there is NO standing wave. So while you can have an SWR of 1.00000000000001-to-1 (because a standing wave DOES exist), you can't really have one of 1-to-1 (because there IS no standing wave). ;o)) [Just a bit of pedantic, lateral thinking on my part. Don't worry too much about it. It has absolutely no bearing whatsoever on the current discussions.] Sounds great, but that is not how the VSWR is defined. -- Rick |
#204
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An antenna question--43 ft vertical
On 7/7/2015 11:02 AM, John S wrote:
On 7/7/2015 8:33 AM, rickman wrote: On 7/7/2015 8:52 AM, John S wrote: On 7/7/2015 7:19 AM, rickman wrote: On 7/7/2015 3:19 AM, John S wrote: On 7/6/2015 12:02 PM, rickman wrote: On 7/6/2015 12:50 PM, John S wrote: On 7/5/2015 11:39 PM, rickman wrote: On 7/5/2015 4:45 PM, Jerry Stuckle wrote: On 7/5/2015 3:58 PM, Roger Hayter wrote: Jerry Stuckle wrote: On 7/5/2015 9:23 AM, Ian Jackson wrote: In message , writes Wayne wrote: "Jeff Liebermann" wrote in message ... On Sat, 04 Jul 2015 19:04:01 -0400, Jerry Stuckle wrote: Think of it this way, without the math. On the transmitter side of the network, the match is 1:1, with nothing reflected back to the transmitter. So you have a signal coming back from the antenna. You have a perfect matching network, which means nothing is lost in the network. The feedline is perfect, so there is no loss in it. The only place for the signal to go is back to the antenna. Wikipedia says that if the source is matched to the line, any reflections that come back are absorbed, not reflected back to the antenna: https://en.wikipedia.org/wiki/Impedance_matching "If the source impedance matches the line, reflections from the load end will be absorbed at the source end. If the transmission line is not matched at both ends reflections from the load will be re-reflected at the source and re-re-reflected at the load end ad infinitum, losing energy on each transit of the transmission line." Well, I looked at that section of the writeup. And, I have no idea what the hell they are talking about. Looks like a good section for a knowledgeable person to edit. If the termination matches the line impedance, there is no reflection. Both the antenna and the source are terminations. This is a bit difficult to visualze with an RF transmitter, but is more easily seen with pulses. Being essentially a simple soul, that's how I sometimes try to work out what happening. You'll be better off if you killfile the troll. You'll get a lot less bad information and your life will become much easier. The wikipedia entry is correct as written. In the real world, the output of an amateur transmitter will seldom be exactly 50 Ohms unless there is an adjustable network of some sort. I've always understood that the resistive part of a TX output impedance was usually less than 50 ohms. If a transmitter output impedance WAS 50 ohms, I would have thought that the efficiency of the output stage could never exceed 50% (and aren't class-C PAs supposed to be around 66.%?). Also, as much power would be dissipated in the PA stage as in the load. Fixed output amateur transmitters are a nominal 50 ohms. It can vary, but that is due to normal variances in components, and the difference can be ignored in real life. But output impedance has little to do with efficiency. A Class C amplifier can run 90%+ efficiency. It's output may be anything, i.e. high with tubes but low with solid state. But the output impedance can be converted to 50 ohms or any other reasonable impedance through a matching network. A perfect matching network will have no loss, so everything the transmitter puts out will go through the matching network. Of course, nothing is perfect, so there will be some loss. But the amount of loss in a 1:1 match will not be significant. Also, the amplifier generates the power; in a perfect world, 100% of that power is transferred to the load. The transmitter doesn't dissipate 1/2 of the power and the load the other 1/2. It's not like having two resistors in a circuit where each will dissipate 1/2 of the power. BTW - the resistive part of the impedance is not the same as resistance. For a simple case - take a series circuit of a capacitor, an inductor and a 50 ohm resistor. At the resonant frequency, the impedance will be 50 +j0 (50 ohms from the resistor, capacitive and inductive reactances cancel). But the DC resistance is infinity. Again, a simple example, but it shows a point. The resistive part of the impedance is exactly the same as a resistance as far as the frequency you are using is concerned. And if the amplifier output impedance *and* the feeder input resistance were *both* matched to 50 ohms resistive then 50% of the power generated (after circuit losses due of inefficiency of generation) would be dissipated in the transmitter. It would, at the working frequency, be *exactly* like having two equal resistors in the circuit each taking half the generated power. So the amplifier has a much lower output impedance than 50ohms and no attempt is made to match it to 50 ohms. OK, so then please explain how I can have a Class C amplifier with 1KW DC input and a 50 ohm output, 50 ohm coax and a matching network at the antenna can show 900 watt (actually about 870 watts due to feedline loss)? According to your statement, that is impossible. I should not be able get more than 450W or so at the antenna. It is true that if the transmitter is thought of as a fixed voltage generator in series with, say, a 5 ohm resistor then the maximum power transfer in theory would occur with a 5 ohm load. But to achieve this the output power would have to be 100 times higher (ten times the voltage) and half of it would be dissipated in the PA. Not the best way to run things, it is better to have a voltage generator chosen to give the right power with the load being much bigger than its generator resistance. So why do all fixed-tuning amateur transmitters have a nominal 50 ohm output instead of 1 or two ohms? And why do commercial radio stations spend tens of thousands of dollars ensuring impedance is matched throughout the system? The maximum power transfer at equal impedance theorem only applies if you started with a *fixed* output voltage generator. We don't; we start with a load impedance (50 ohm resistive), then we decide what power output we want, and we choose the voltage to be generated accordingly. (Thank you for giving me the opportunity to think about this!) Actually, it doesn't matter if it's a fixed or a variable output voltage - maximum power transfer always occurs when there is an impedance match. How about we quit with the speculation and come up with some numbers? Here is a simulation of a 50 ohm load with a 50 ohm matched series output impedance and a voltage source of 200 VAC peak. Power into the load is 100 W. http://arius.com/sims/Matched%20Load%20Power.png Same exact circuit with the series impedance of just 1 ohm, power into the load is 385 W. http://arius.com/sims/UnMatched%20Load%20Power.png I'd say that is pretty clear evidence that matched loads are not the way to maximize power transfer when the load impedance is fixed and the output impedance is controllable. There can be a lossless resistive part of source impedance according to the IEEE (and most every other well educated EEs). After all, a transmission line has a resistance but it's loss resistance is much lower. Can you provide a reference to any of this? IEEE Standard Dictionary of Electrical and Electron Terms, IEEE Std 100-1972. For a complete treatment on the topic, see Reflections III by Walter Maxwell, W2DU, Appendix 10. Anything more accessible? I will not lead you by the hand. I have supplied the requested references. If you can't find the referenced material, then you need to learn how to research, buy books, go to the library, etc. It is your loss. Education is more than online chatter. Lol. You are a trip. I'm not going to spend $100 on a book just to see if you are right. I was intrigued by the idea that a wire could carry a signal without the resistance dissipating power according to P = I^2 R. I guess there is some communication failure. Yes, there is. I did NOT say that a wire was dissipationless. More communication failure. I never said *that*. Please reread the above. Ditto. I said "There can be a lossless resistive part of source impedance". What you said is clearly quoted above. Source impedance may or may not contain resistance which dissipates. A 50 ohm piece of coax has an impedance of approximately 50+j0 ohms. So, does the 50 ohms dissipate? Look at the specifications of the coax. How much loss? I've never seen a coax specified with a complex impedance. But I'll take your word that this is accurate. However, if you drive a 50 ohm cable, how much current flows? If it is unterminated, I believe the result is none because it all flows back from the reflection and is returned to the source. If you terminate it with a 50 ohm load it is all absorbed and dissipates the appropriate wattage. So I don't see how the 50 ohm impedance of the cable is in any way a useful analogy to say the real part of the impedance does not dissipate. A transmission line is not a load. If you are unwilling to go to the library or do research or buy a book on you own, then you are beyond help. What is your ignorance worth to you? If you don't want to discuss this, why do you type so many words? Maybe it would be useful to explain how an impedance matching network would provide a conjugate match. That is, show an example with real values for components. If this is too much work and you don't wish to do that, fine. I'm ok with it. But no need to insult me. Better to just not respond. -- Rick |
#205
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An antenna question--43 ft vertical
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#206
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An antenna question--43 ft vertical
On 7/7/2015 12:47 PM, Dave Platt wrote:
In article , rickman wrote: Lol. You are a trip. I'm not going to spend $100 on a book just to see if you are right. I was intrigued by the idea that a wire could carry a signal without the resistance dissipating power according to P = I^2 R. I guess there is some communication failure. Yah. It's a question of terminology. Unfortunately, one term has come to be used for two (related but different) concepts. There is "resistance", as in the E=I^2*R sort. If I recall correctly, Maxwell refers to this as "dissipative" impedance. If you put current through a dissipative resistance, a voltage drop develops across the resistance, and power is dissipated. There are plenty of examples of this, with which I'm sure you're familiar. There is also "resistance", as in "the 'real', non-reactive component of a complex impedance, in which current is in phase with voltage." This type of "resistance" is fundamentally non-dissipative - that is, you can run power through it without dissipating the power as heat. There are also good examples of this. One "textbook" example would be a perfectly-lossless transmission line... say, one made out of a wire and tube of a superconductor, cooled to below the superconducting temperature. You can (in principle) build such a superconducting coax to have almost any convenient impedance... 50 or 75 ohms, for example. Since we're theorizing, let's assume we can built one a few trillion miles long... so long that the far end is light-years away. If you hook a transmitter to one end of this and start transmitting, it will "look" to the transmitter like a 50-ohm dummy load. The transmitter itself won't be able to tell the difference. The transmitter puts out an RF voltage, and the line "takes current" exactly in phase with the voltage, in a ratio of one RF ampere per 50 RF volts. But, there's a fundamental difference between this "resistance" and that of a dummy load. A 50-ohm dummy load's resistance is dissipative... all of the power going into it turns into heat, and is dissipated in accordance with the fundamental laws of thermodynamics. *None* of the power being fed into the superconducting coax, is dissipated as heat in the coax. All of the power still exists, in its original RF form. It's being stored/propagated down the coax without loss. When it hits a load at the other end, it may be dissipated as heat there. Or, perhaps not. What if what's at the other end of the superconducting coax is a superconducting antenna, tweaked to present an impedance of exactly 50 ohms? The RF will be radiated into space. And, "free space" is another great example of a medium that has a well-defined "resistance" (in the non-dissipated sense). https://en.wikipedia.org/wiki/Impedance_of_free_space One of the fundamental jobs of an antenna, is to match the impedance of its feedline to the impedance of free space. Now, any coax you can buy at the store has *both* types of "resistance", of course. It has a dissipative component, and a non-dissipative component. Typically, the more you spend and the more you have to strain your back carrying it around, the lower the amount of dissipative resistance (which is only good for keeping the pigeons' feet warm) and the more predictable and precisely-defined the non-dissipative part. Excellent explanation, Dave. I don't have the ability to educate like you do. Thanks. John |
#207
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An antenna question--43 ft vertical
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. |
#208
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An antenna question--43 ft vertical
On 7/7/2015 1:37 PM, wrote:
John S wrote: On 7/6/2015 1:03 PM, wrote: John S wrote: On 7/6/2015 11:01 AM, Jerry Stuckle wrote: On 7/6/2015 4:20 AM, Ian Jackson wrote: In message , rickman writes How about we quit with the speculation and come up with some numbers? Here is a simulation of a 50 ohm load with a 50 ohm matched series output impedance and a voltage source of 200 VAC peak. Power into the load is 100 W. http://arius.com/sims/Matched%20Load%20Power.png Same exact circuit with the series impedance of just 1 ohm, power into the load is 385 W. http://arius.com/sims/UnMatched%20Load%20Power.png I'd say that is pretty clear evidence that matched loads are not the way to maximize power transfer when the load impedance is fixed and the output impedance is controllable. Quite simply, if your prime objective is to get maximum power out of a power (energy?) source, the source having an internal resistance is a BAD THING. You don't design the source to have an internal resistance equal to its intended load resistance. No one designs lead-acid batteries that way (do they?), so why RF transmitters? While theoretically you can extract the maximum power available from the source when the load resistance equals the source resistance, you can only do so provided that the heat you generate in the source does not cause the source to malfunction (in the worst case, blow up). Because DC power transfer is not the same as AC power transfer. Why not? Does something happen to the laws of physics with AC? Yes, quite a lot, you get a whole new set of laws. If you apply 1vDC to a 1 ohm resistor, you get 1A of current. If you apply 1vAC RMS (at any frequency) to a 1 ohm resistor, you get 1A of current. How does the AC change the law? What part of "you get a whole new set of laws" was it you failed to understand? Here's a clue for you; at DC the reactive components of a length of wire are irrelevant but at AC they are not. So, at 1Hz the law has changed, eh? What new law do I need to use? |
#209
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An antenna question--43 ft vertical
On 7/7/2015 10:05 AM, Jerry Stuckle wrote:
On 7/7/2015 3:05 AM, John S wrote: On 7/6/2015 1:03 PM, wrote: John S wrote: On 7/6/2015 11:01 AM, Jerry Stuckle wrote: On 7/6/2015 4:20 AM, Ian Jackson wrote: In message , rickman writes How about we quit with the speculation and come up with some numbers? Here is a simulation of a 50 ohm load with a 50 ohm matched series output impedance and a voltage source of 200 VAC peak. Power into the load is 100 W. http://arius.com/sims/Matched%20Load%20Power.png Same exact circuit with the series impedance of just 1 ohm, power into the load is 385 W. http://arius.com/sims/UnMatched%20Load%20Power.png I'd say that is pretty clear evidence that matched loads are not the way to maximize power transfer when the load impedance is fixed and the output impedance is controllable. Quite simply, if your prime objective is to get maximum power out of a power (energy?) source, the source having an internal resistance is a BAD THING. You don't design the source to have an internal resistance equal to its intended load resistance. No one designs lead-acid batteries that way (do they?), so why RF transmitters? While theoretically you can extract the maximum power available from the source when the load resistance equals the source resistance, you can only do so provided that the heat you generate in the source does not cause the source to malfunction (in the worst case, blow up). Because DC power transfer is not the same as AC power transfer. Why not? Does something happen to the laws of physics with AC? Yes, quite a lot, you get a whole new set of laws. If you apply 1vDC to a 1 ohm resistor, you get 1A of current. If you apply 1vAC RMS (at any frequency) to a 1 ohm resistor, you get 1A of current. How does the AC change the law? You apply 1vdc to a 0.159 microfarad capacitor and you get 0 amps flowing (open circuit). You apply 1vac at 1MHz to that same capacitor and you get 1 amp flowing, with the current leading the voltage by 90 degrees. You apply 1vdc to a 0.159 microhenry inductor and you get infinite amps flowing (short circuit). You apply 1vdc at 1MHz to that same inductor, and you get 1 amp flowing with the voltage leading the current by 90 degrees. You place the capacitor and inductor in series. Fed with DC, you get 0 amps flowing (open circuit). Fed with 1MHz AC, you get infinite current flowing (short circuit). You place the capacitor and inductor in parallel. Fed with DC, you get infinite current flowing (short circuit). Fed with 1MHz AC you get 0 amps flowing (open circuit). There is a huge difference between ac and dc! Yes, but the LAWS have not changed. The components have changed. So, changing the components changes the laws of physics? Suppose you apply .01Hz AC RMS to the components you specified. What then? |
#210
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An antenna question--43 ft vertical
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