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#141
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An antenna question--43 ft vertical
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). -- Ian |
#143
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An antenna question--43 ft vertical
"Jeff Liebermann" wrote in message ... On Sun, 5 Jul 2015 17:08:56 -0700, "Wayne" wrote: I'm off on a different approach. I have an RF ammeter mounted in a box. The box is in the shack between the ATU and the antenna. I simply adjust the ATU for max current on the ammeter. Rewind. I just noticed that you're planning to put the RF ammeter between the ATU and the antenna. That will work, but with approximately 1200 ohms antenna impedance, you are going to see 50/1200 = 0.04 times the antenna current that you would see on the 50 ohm line between the xmitter and the ATU. If you're running lots of power, that might work, but offhand, methinks not. Also, you can't adjust the ATU for maximum current. It adjusts itself based on it's own internal VSWR sensor. All you can do is watch the light show and listen to the relays clatter. You might be able to have some control if it were a motorized antenna tuner. Certainly a manual antenna tuner would work. This isn't something planned. I have been using the ammeter between the tuner and antenna for many years. With an automatic tuner, there is no feedback from the ammeter to the tuner. In that case it is simply an indicator that current is present. Some of my antennas have SWR outside the capabilities of my automatic tuner. A manual tuner is used in that case. With a manual tuner, I don't look for a specific reading, just a peak in the current from the tuner to the antenna. |
#144
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An antenna question--43 ft vertical
On 7/6/2015 12:39 AM, 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. Your simplified response shows you never took an EE course in your life. I suggest you take some EE courses and learn how things work. It's very obvious you don't have any more knowledge than you get with ohm's law. But if what you say is correct, then I should be able to get a lot of power out of my 100 watt transmitter feeding a 1 ohm antenna. Never mind the 50:1 SWR. -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
#145
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An antenna question--43 ft vertical
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. If what you say is correct, then it wouldn't matter what antenna impedance I had, as long as it matches the transmission line. VSWR would be immaterial. That is demonstrably false. -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
#146
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An antenna question--43 ft vertical
On 7/6/2015 11:59 AM, Jerry Stuckle wrote:
On 7/6/2015 12:39 AM, 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: 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. Your simplified response shows you never took an EE course in your life. I suggest you take some EE courses and learn how things work. It's very obvious you don't have any more knowledge than you get with ohm's law. But if what you say is correct, then I should be able to get a lot of power out of my 100 watt transmitter feeding a 1 ohm antenna. Never mind the 50:1 SWR. Would anyone else like to explain to Jerry the fallacy of his argument? I get tired of explaining the obvious sometimes. -- Rick |
#147
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An antenna question--43 ft vertical
On 7/5/2015 7:08 PM, Wayne wrote:
"Jeff Liebermann" wrote in message ... On Sun, 05 Jul 2015 20:22:19 +0100, Brian Reay wrote: As for a simpler way, I'd recommend a remote auto-matcher like an SGC at the antenna base. It will minimise coax losses and should give you a good match, at least for most bands. I've used a similar set up (with radials) and achieved a good match even on 80m. If your radio has a built in tuner, then it can be used to 'tweak' the match in the event the radio isn't 'seeing' 1.5:1. Turn it off initially. Let the SGC find a match. If it isn't ideal, use the local ATU for a final tweak. I never found this was required but YMMV. Not everyone is a true believer in antenna tuners: http://www.qsl.net/g3tso/Hombrew-Mobile%20Antennas.html Interesting. I'm off on a different approach. I have an RF ammeter mounted in a box. The box is in the shack between the ATU and the antenna. I simply adjust the ATU for max current on the ammeter. Hey, Wayne - As a matter of curiosity on my part, can you find a way to measure the ammeter's resistance and let me know the full-scale value? Many thanks, John |
#148
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An antenna question--43 ft vertical
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? If what you say is correct, then it wouldn't matter what antenna impedance I had, as long as it matches the transmission line. VSWR would be immaterial. There is no VSWR nor ISWR if the load matches the line. That is demonstrably false. Please demonstrate this for us as we wish to learn. |
#149
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An antenna question--43 ft vertical
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? Because RF transmitters deliver high frequency AC to a transmission line. -- Jim Pennino |
#150
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An antenna question--43 ft vertical
John S wrote:
On 7/6/2015 12:19 AM, 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. So, it can't be used in a 50 ohm environment? What does that have to do with anything? The chart has a SWR graph and nowhere does it need source impedance. If you disagree, please link to one. EZNEC allows you to set the impedance to anything you want and assumes the transmission line matches the transmitter. Please show the EZNEC statement that "assumes the transmission line matches the transmitter". Look in the help section if you have EZNEC and can cut and paste or just refer me to the chapter and verse. Also, if you have EZNEC, you can insert a transmission line with arbitrary characteristic impedance, put a load on the far end matching the line, and look at the SWR. It will still be 1:1 because the LOAD matches the LINE. Not because EZNEC assumes a source impedance. Try it with and report back here. There is no way that a source initiates reflections. That is a property of the line and load only. It may re-reflect a wave reflected from the load, but that is all. You can also verify this in LTSPICE if you wish. What happens if you take any off the shelf commercial amateur radio transmitter that does not have a built in tuner and: Attach a 10 Ohm load. Attach a 200 Ohm load. Attach a 1,000 Ohm load. Attach a 1 Ohm load. Attach a 50 Ohm load. Please address my questions first before setting up another strawman. Start with Electromagnetics by Kraus and Carver, Chapter 13. -- Jim Pennino |
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