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This has been explained many times, to no avail.
So instead of one of us explaining it yet again, I suggest that you do the following experiment. It requires only a transmitter, one or two dummy loads, an SWR meter, and no more than five minutes of your time. 1. Connect the transmitter to either a dummy load or an antenna through the SWR meter and measure the SWR. 2. Connect the transmitter in parallel with a dummy load by using a tee connector. Connect this parallel combination to the input of the SWR meter, and the output of the SWR meter to the same load as before (dummy load or antenna). Do you see any change in the SWR? If you don't, then something is wrong with your theory -- since the source impedance is clearly different for the two measurements --, and you should take the effort of resolving it with your recent observations. Roy Lewallen, W7EL Richard Fry wrote: "Ian White, G3SEK"wrote: Richard Fry wrote: "Ian White, G3SEK wrote The meter measures nothing that involves the source, except the level of RF that it supplies. It does not respond in any way whatever to the source impedance. Not that I said it did in my part of the thread, but nevertheless the above statement is not strictly true. In the case where the source Z of the tx PA does not match its load Z (which is typical), power reflected from the load mismatch will at least partly be re-reflected from the PA -- which then contributes to the power sensed by a "wattmeter" in the output path. Sorry, that statement cannot be correct. It would mean that the impedance you measure at the near end of a transmission line (terminated by some arbitrary load at the far end) would depend on the internal impedance of the device that's doing the measuring - and that is not true, either in transmission-line theory or in the real world. It is a function only of the line and the load. etc ____________ How, then, do you explain the "ghost image" that can occur* in analog(ue) TV transmission systems arising from reflections at/near the antenna end of the station's transmission line? *with sufficient round-trip propagation time in the transmission line RF |
Ian White, G3SEK wrote:
Reg Edwards wrote: For those who have forgotten how or have never measured SWR. Hang on, Reg - didn't you spend your career working on VLF cables that went under the ocean? How did you keep the water out of the slotted cable? And how far did you have to swim between Vmax and Vmin? Roy Lewallen, W7EL |
Let me suggest an additional exercise for Richard and anyone else that
believes that source impedance affects the SWR. Those of us who believe otherwise can easily calculate the SWR which will exist on a line, and the SWR that will be read by an SWR meter at any point in a system, by knowing simply the line length and impedance and the load impedance. We don't require knowledge of the source impedance. The equations we use can be found in numerous places, and these have been used for over a century to design working systems. You must use other equations to predict SWR -- equations which include source impedance. It would be very interesting to see those equations. Your equations and ours will predict different results from the simple test I proposed. So if you'll show us the equation you use to calculate SWR which includes source impedance, it'll be easy to see whether it's correct or not. Roy Lewallen, W7EL |
Richard Clark wrote:
I see you have yet to respond to this very matter attended to quite at length by Chipman. I have recently realized that those terms in Chipman's equations are interference terms. EM wave interference is not understood very well by RF people although it is understood very well by optics people. For instance, the superposing of two coherent voltages in a Z0 environment is well known. Vtot = V1 + V2 (assume V1 and V2 are in phase) Squaring both sides and dividing by Z0 yields the power. Vtot^2/Z0 = (V1+V2)^2/Z0 Vtot^2/Z0 = V1^2/Z0 + V2^2/Z0 + (2*V1*V2)/Z0 Note that the first term to the right of the equals sign is the power associated with the V1 wave and the second term is the power associated with the V2 wave. The third term is the interference term. If V1 and V2 are in phase, the third term will be constructive interference. If the phase angle between V1 and V2 is less than 90 degrees, the interference is constructive, i.e. cos(theta) is positive. If V1 and F2 were 180 degrees out of phase, the interference would be destructive. If the phase angle between V1 and V2 is between 90 degrees and 180 degrees, the interference is destructive, i.e. cos(theta) is negative. Interference is the reason for those extra terms in Chipman's equations. It always happens when the sum of two voltages are squared to get the power. Reference: _Optics_, by Hecht. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
"Roy Lewallen" wrote
Let me suggest an additional exercise for Richard and anyone else that believes that source impedance affects the SWR. (etc) ____________________ Just one sec, please. I didn't say that the true SWR connected to the tx output connector was affected. I said that the RF power measured at the sample point(s) in the transmitter can be affected by the source and load impedances of the tx, for the reasons stated. The true load SWR does not change under these conditions, but it cannot then be determined by such a meter. Attempting to do so will yield some value, but it will be wrong. RF |
"Walter Maxwell" wrote in message ... On Fri, 3 Sep 2004 17:16:48 -0300, "Another Voice" wrote: **** Post for FREE via your newsreader at post.usenet.com **** "Richard Harrison" Just how short can a transmission line be and still enforce its Zo? The whole thing is perfectly clear if one imagines applying a step function (rising edge) to any short, even VERY short, length of transmission line. The current in the short line will be equal to V/Zo - at least until the reflections (if any) start arriving back at the input. If the line happen to be terminated with Zo, then no reflections and I=V/Zo is the steady state. The only issue of shortness is that a very short line means very short time until the reflections arrive. The step function makes things a lot easier to understand than RF. It 'enforces' the distinction between the transient period and steady state. IMO, the length of the line is irrelevant when using a device such as the Bruene bridge or a Bird 43. Each of those instruments are designed or adjusted to indicate the forward or reflected power, based on three things: 1) ratio of the foward and reflected voltages, the voltage reflection coefficient 2) the scale numbered from 0 to 1, where 0 indicates the reflection is zero, and 1 equals total reflection, but the significant point is that a 3:1 mismatch gives a reflection coefficient of 0.5, which then means that the half-scale reading of 0.5 indicates the 3:1 mismatch, or a 3:1 SWR, and 3) the device is so designed or adjusted so that the voltage ratios indicate the correct value because it's inherent characteristic impedance, Zo, is 50 ohms. Thus, no transmission line is necessary. For example, the device can be connected directly to the antenna terminals, or any other device you desire to determine the mismatch, and power it directly from the signal source--no transmission line is needed on either port for the device to indicate the degree of mismatch. Walt, W2DU Walt, I hope people are listening to what you are saying. I built up a Bruene meter in SWCAD using 0% tolerance components and other ideal parts. Works exactly like Bird claims their meter does, except that the error only depends on the PC floating point arithmetic. Transmission line or not makes no difference. BTW, it is kind of neat to see the directional coupler properties, by driving the two sides with different signals, and then being able to separate them. Tam/WB2TT |
Sorry, I must have misinterpreted your earlier posting.
But we seem to now have a "true SWR" as opposed to some other kind of SWR. And "true SWR connected to the tx output" doesn't have any meaning at all to me. I also have no idea of what "sample points within the transmitter" might be. So let me explain what I (and virtually all published literature) mean by SWR. If we connect a transmitter to an SWR meter, and then to a long piece of lossless cable with the same Z0 as the SWR meter, and finally to a load, the SWR meter reading will be the same as the VSWR on the cable, i.e., the ratio of maximum to minimum voltages on the line. This ratio of voltages is, by definition, the VSWR -- which equals the ISWR, and is often referred to simply as SWR. If we measure or calculate the impedance seen looking into the line, then disconnect the line from the SWR meter and replace it and the load with lumped elements of the same impedance, the SWR meter reading won't change(*). Now, I can calculate the what the SWR meter reading will be under this condition also. In both cases, the source impedance won't affect the SWR meter reading, the positions or relative magnitudes of the maximum and minimum voltages on the line, or the voltage or current within the SWR meter line section. (This last condition assumes that the net power delivered by the source stays the same; otherwise, the ratio of voltage to current, and their phase angles, stay constant, regardless of the power delivered.) I have no idea how all this relates to your "true SWR". But do you agree with what I've said above? If not, I'll describe a couple of simple experiments which will test it against any alternative view you might propose. (*) We can also replace them with a load at the end of a line of different Z0. As long as we choose the load Z and the line length to make the impedance seen at the line input the same as before, the SWR meter will read the same as before -- even though it no longer equals the actual VSWR on the transmission line. The SWR meter is really indicating the impedance seen looking into the line, not in this case the actual line VSWR. (That's the essence of Reg's objection to the SWR meter designation. Of course, if I connect my ammeter across a resistor, it's not measuring the current through the resistor, either.) Roy Lewallen, W7EL Richard Fry wrote: "Roy Lewallen" wrote Let me suggest an additional exercise for Richard and anyone else that believes that source impedance affects the SWR. (etc) ____________________ Just one sec, please. I didn't say that the true SWR connected to the tx output connector was affected. I said that the RF power measured at the sample point(s) in the transmitter can be affected by the source and load impedances of the tx, for the reasons stated. The true load SWR does not change under these conditions, but it cannot then be determined by such a meter. Attempting to do so will yield some value, but it will be wrong. RF |
Ian White, G3SEK wrote:
Reg Edwards wrote: For those who have forgotten how or have never measured SWR. Hang on, Reg - didn't you spend your career working on VLF cables that went under the ocean? How did you keep the water out of the slotted cable? And how far did you have to swim between Vmax and Vmin? Roy Lewallen, W7EL =============================== Water can be kept out of slotted cables by the ship's radio operator who never has anything else to do. We used to toss him overboard with a ladle and pump. On occasions we used the ship's doctor when he wasn't propping up the bar boozing duty-free scotch. Didn't have to swim anywhere. The propagation velocity is so low at ELF in sea water it is necessary only to sit on a stool in a diving suit and wait for the max's and min's to pass by. --- Reg. |
"Roy Lewallen" wrote
But we seem to now have a "true SWR" as opposed to some other kind of SWR. And "true SWR connected to the tx output" doesn't have any meaning at all to me. My "true SWR" term is used is an attempt to differentiate between the SWR of the antenna system, and the inaccuracies associated with trying to measure it with devices that cannot isolate the incident power in the system from internal reflections of that power. For the conditions and reasoning outlined in my earlier posts in this thread, and even though the system SWR is a constant -- the normal SWR meter used in/with an operating transmitter working into a mismatched load won't have the ability to give strictly accurate measurement of that SWR. That is all I'm saying. I also have no idea of what "sample points within the transmitter" might be. In broadcast gear, these are the directional couplers whose pickup probes are inserted transversely into the coaxial line between the harmonic filter output and the tx output connector. I haven't been a licensed ham for over 40 years (when I went into the broadcast field), but I expect some ham txs might have the same setup. Otherwise it could be a Model 43 or the like inserted between the output connector of the ham tx and the transmission line to the antenna. I hope this is understandable now. RF |
Richard Fry wrote:
Ian White wrote: The so-called SWR meter is a steady-state instrument, so it always makes sense to use that quicker, easier way of thinking. Since you're the one who chooses to think of this particular situation in terms of multiple reflections, any difficulties you encounter are entirely yours. This reads to me as though you know they are there, but choose to ignore them...? Oh no, quite the opposite - but since these difficulties are entirely of your own making, you get to do the work :-) If you ever see a conflict between two different theories that explain the same observed facts, then there's an error somewhere. We agree on the subject of conflict resolution, but apparently not on the location of the error. Thank you for the more detailed explanation below... which, sure enough, revealed where the error is. If the multiple-reflection theory is extrapolated to infinite time, so that it calculates results for the steady state, it *must* give identical results to the steady-state theory. But whenever the steady-state theory can be used, it will always get you there much more quickly. This is true only to the extent that all the power ever generated by the transmitter eventually either is radiated by the antenna or is dissipated by losses somewhere. That is exactly true in the steady state. For simplicity, let's assume a tx with a source impedance of zero ohms feeds a lossless transmission line of uniform impedance throughout its length to a mismatch at the far end. The mismatch reflects a percentage of the incident power back down the line to the tx, and continues to do so as long as the transmitter generates power. The tx will re-reflect the reflected power back to the far end -- in this case all of the reflected power it ever sees, in fact. To this easily-seen, real-world reality you agreed above ("Yes, that is a true observation, ..."). The re-reflections combine with the power generated by the tx at that instant to create a vector sum at the sample point used by the meter. There's the error: you can't "combine... power" in that way. You can only create vector sums of voltage; and separately, vector sums of current. To make the multiple-reflection theory work correctly, you have to do two separate vector sums at output port of the transmitter. First you add all the voltage vectors: the 1st (original) forward, the 1st reflected, the 2nd forward (re-reflected), 2nd reflected... and so on, summed to infinity to give the correct result for the steady state. Then you do the exactly same for all the forward and reflected current vectors. In order to account for reflection from the transmitter, you have to assume some value of source impedance. Any value will do, for reasons we'll see in a moment. Now you can calculate two things: the vector ratio, which is the complex impedance that the transmitter sees as a load; and the scalar product, which is the power the transmitter can deliver into that load. If you vary the source impedance of the transmitter, it will change all the summed voltage vectors and all the summed current vectors - but each voltage term in the sum will be changed by exactly the same factor as its corresponding current term. Certainly the product (the output power) will change, but the ratio (the load impedance) will not. So, when correctly worked out, the load impedance is *not* a function of the transmitter output impedance or the output power. Likewise, the indication of the SWR meter is not a function of either the transmitter output impedance or the output power - this last one being a well-known fact. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
"Richard Clark" wrote
To date in this matter, I have yet to see any concrete value of source Z offered from those of the NOT 50 Ohms camp. Further, I have yet to see any of them offer any experimental confirmation of their assertion ________________ Please see the following. In the quote there, note the text starting "The transmitter's output source impedance must be low...", and the following sentences. + + + Below is a quote from a paper titled "A Study of RF Intermodulation Between FM Broadcast Transmitters Sharing Filterplexed or Co-located Antenna Systems," by Geoffrey Mendenhall. Mendenhall is a registered professional engineer, and now a VP for Harris Broadcast Division in Mason, OH. He is responsible for the engineering research and design of the entire broadcast transmitter product line for Harris: AM, FM & TV. Harris is the world's largest supplier of broadcast transmitters. This paper and quote has to be read here with some interpretation, because it is an analysis of what happens when an in-band signal from one transmitter is coupled into another transmitter when their antennas are close together and/or when adequate filtering of the external signal is not provided. But it is strictly applicable also for single tx and antenna systems, where an antenna mismatch produces reflections back toward the transmitter. In this case the "interfering signal" is not external, but a reflection of the incident power of that tx. QUOTE: Output return loss is a measure of the interfering signal that is coupled into the output circuit versus the amount that is reflected back from the output circuit without interacting with the non-linear device. To understand this concept more clearly, we must remember that although the output circuit of the transmitter is designed to work into a fifty ohm load, the output source impedance of the transmitter is not fifty ohms. If the source impedance were equal to the fifty ohm line impedance, half of the transmitter's output power would be dissipated in its internal output source impedance. The transmitter's output source impedance must be low compared to the load impedance in order to achieve good efficiency. The transmitter therefore looks like a voltage source driving a fifty ohm load. While the transmission line is correctly terminated looking toward the antenna (high return loss), the transmission line is greatly mismatched looking toward the output circuit of the transmitter (low return loss). This means that power coming out of the transmitter is completely absorbed by the load while interfering signals fed into the transmitter are almost completely reflected by the output circuit. END QUOTE The transmitter topology in this study was a single PA tube operating Class C. For these designs, an on-carrier return loss value of 2 dB or less is rather common. At 2 dB the reflection coefficient is over 79%. PAs comprised of multiple devices combined by balanced methods (e.g. 3dB hybrids, Wilkinsons) can provide a source impedance closer to 50 ohms (higher return loss). In these cases, power that is reflected off the load and NOT re-reflected by the tx mostly is dissipated in resistive networks in the PA combiner. However these networks do not provide a load for the forward power from the tx, only for power reflected by the output termination. RF Visit http://rfry.org for FM broadcast RF system papers. |
Sorry, it still isn't clear.
Richard Fry wrote: "Roy Lewallen" wrote But we seem to now have a "true SWR" as opposed to some other kind of SWR. And "true SWR connected to the tx output" doesn't have any meaning at all to me. My "true SWR" term is used is an attempt to differentiate between the SWR of the antenna system, and the inaccuracies associated with trying to measure it with devices that cannot isolate the incident power in the system from internal reflections of that power. For the conditions and reasoning outlined in my earlier posts in this thread, and even though the system SWR is a constant -- the normal SWR meter used in/with an operating transmitter working into a mismatched load won't have the ability to give strictly accurate measurement of that SWR. That is all I'm saying. What, then, is "system SWR"? How do you define it? I also have no idea of what "sample points within the transmitter" might be. In broadcast gear, these are the directional couplers whose pickup probes are inserted transversely into the coaxial line between the harmonic filter output and the tx output connector. I haven't been a licensed ham for over 40 years (when I went into the broadcast field), but I expect some ham txs might have the same setup. Otherwise it could be a Model 43 or the like inserted between the output connector of the ham tx and the transmission line to the antenna. In your last posting, you said, Just one sec, please. I didn't say that the true SWR connected to the tx output connector was affected. I said that the RF power measured at the sample point(s) in the transmitter can be affected by the source and load impedances of the tx, for the reasons stated. So replacing "sample point(s) in the transmitter" with "Model 43 or the like inserted between the output connector of the ham tx and the transmission line to the antenna", you've said that the RF power measured by the (model 43) SWR meter can be affected by the source impedance of the transmitter. Obviously, if we have a voltage or current source of fixed value and change the source impedance, the power delivered by the source changes, and any means of measuring the power at the source, load, or in between should show that change. That follows from elementary circuit theory, and doesn't require any consideration or knowledge of transmission lines, waves, or SWR. On the model 43, both the "forward" and "reverse" powers will change, but by the same fraction. Perhaps that's what you mean. But if you mean that the SWR reading or the ratio of "forward" to "reverse" power changes as a result of changing the source impedance, that's easily shown to be false by the simple experiment I proposed. I hope this is understandable now. Almost, but not quite. |
"Roy Lewallen" wrote in message ... This has been explained many times, to no avail. So instead of one of us explaining it yet again, I suggest that you do the following experiment. It requires only a transmitter, one or two dummy loads, an SWR meter, and no more than five minutes of your time. 1. Connect the transmitter to either a dummy load or an antenna through the SWR meter and measure the SWR. 2. Connect the transmitter in parallel with a dummy load by using a tee connector. Connect this parallel combination to the input of the SWR meter, and the output of the SWR meter to the same load as before (dummy load or antenna). If you don't have 2 dummy loads, there is a simple alternative. Connect the TX to the meter through a 1/4 wave section of 75 Ohm line. Unless the TX output was 75 Ohms, the equivalent TX output impedance seen by the meter has changed. Tam/WB2TT Do you see any change in the SWR? If you don't, then something is wrong with your theory -- since the source impedance is clearly different for the two measurements --, and you should take the effort of resolving it with your recent observations. Roy Lewallen, W7EL Richard Fry wrote: "Ian White, G3SEK"wrote: Richard Fry wrote: "Ian White, G3SEK wrote The meter measures nothing that involves the source, except the level of RF that it supplies. It does not respond in any way whatever to the source impedance. Not that I said it did in my part of the thread, but nevertheless the above statement is not strictly true. In the case where the source Z of the tx PA does not match its load Z (which is typical), power reflected from the load mismatch will at least partly be re-reflected from the PA -- which then contributes to the power sensed by a "wattmeter" in the output path. Sorry, that statement cannot be correct. It would mean that the impedance you measure at the near end of a transmission line (terminated by some arbitrary load at the far end) would depend on the internal impedance of the device that's doing the measuring - and that is not true, either in transmission-line theory or in the real world. It is a function only of the line and the load. etc ____________ How, then, do you explain the "ghost image" that can occur* in analog(ue) TV transmission systems arising from reflections at/near the antenna end of the station's transmission line? *with sufficient round-trip propagation time in the transmission line RF |
On Sat, 4 Sep 2004 17:57:26 -0500, "Richard Fry" wrote:
"Richard Clark" wrote To date in this matter, I have yet to see any concrete value of source Z offered from those of the NOT 50 Ohms camp. Further, I have yet to see any of them offer any experimental confirmation of their assertion ________________ Please see the following. In the quote there, note the text starting "The transmitter's output source impedance must be low...", and the following sentences. + + + Below is a quote from a paper titled "A Study of RF Intermodulation Between FM Broadcast Transmitters Sharing Filterplexed or Co-located Antenna Systems," by Geoffrey Mendenhall. Mendenhall is a registered professional engineer, and now a VP for Harris Broadcast Division in Mason, OH. He is responsible for the engineering research and design of the entire broadcast transmitter product line for Harris: AM, FM & TV. Harris is the world's largest supplier of broadcast transmitters. This paper and quote has to be read here with some interpretation, because it is an analysis of what happens when an in-band signal from one transmitter is coupled into another transmitter when their antennas are close together and/or when adequate filtering of the external signal is not provided. But it is strictly applicable also for single tx and antenna systems, where an antenna mismatch produces reflections back toward the transmitter. In this case the "interfering signal" is not external, but a reflection of the incident power of that tx. A critical point made in the quote below is evidence of a serious misunderstanding concerning the relationship between the source impedance of the tx and the load impedance. QUOTE: Output return loss is a measure of the interfering signal that is coupled into the output circuit versus the amount that is reflected back from the output circuit without interacting with the non-linear device. To understand this concept more clearly, we must remember that although the output circuit of the transmitter is designed to work into a fifty ohm load, the output source impedance of the transmitter is not fifty ohms. If the source impedance were equal to the fifty ohm line impedance, half of the transmitter's output power would be dissipated in its internal output source impedance. The last sentence in the paragraph above is incorrect. This shows that the writer of the quote is in the unbelievably large group that still believes incorrectly that half of the tx power would be lost if if it were conjugately matched. But we all know that efficiencies greater than 80% is achieved by Class C amps, and greater than 60% is achieved by Class B amps when the source impedance of the tx is 50 ohms resistive and the load impedance is also 50 ohms resistive. I have made appropriate measurements in a professional RF laboratory that prove this point. The data from these measurements and the procedure used is available for downloading from my web site at http://home.iag.net/~w2du under the title "On the Nature of the Source of Power in Class B and C Amplifiers." This piece is Chapter 19 in Reflections II, and also appears in QEX,, May/Jun 2001. Unfortunately, like the statement made in the 'quote' above, there are all too many RF engineers who fail to appreciate the true relationship between the two separate resistances in the amp, the resistance resulting in dissipation and the resistance responsible for delivering the power to the load. I guarantee the reader of the piece referenced above will come away with something to think about. The transmitter's output source impedance must be low compared to the load impedance in order to achieve good efficiency. The transmitter therefore looks like a voltage source driving a fifty ohm load. While the transmission line is correctly terminated looking toward the antenna (high return loss), the transmission line is greatly mismatched looking toward the output circuit of the transmitter (low return loss). This means that power coming out of the transmitter is completely absorbed by the load while interfering signals fed into the transmitter are almost completely reflected by the output circuit. END QUOTE The transmitter topology in this study was a single PA tube operating Class C. For these designs, an on-carrier return loss value of 2 dB or less is rather common. At 2 dB the reflection coefficient is over 79%. snip RF 73, Walt, W2DU |
On Sat, 4 Sep 2004 14:46:38 -0500, "Richard Fry"
wrote: |"Roy Lewallen" wrote | Let me suggest an additional exercise for Richard and anyone else that | believes that source impedance affects the SWR. (etc) |____________________ | |Just one sec, please. I didn't say that the true SWR connected to the tx |output connector was affected. I said that the RF power measured at the |sample point(s) in the transmitter can be affected by the source and load |impedances of the tx, for the reasons stated. Not so fast yourself. You said, "The generic function of this meter is to measure the degree of match between a source and a load." There is no power mentioned in your statement. I, and others, stated that your first statement was incorrect and since that time you have been introducing prodigious amounts of verbiage in an attempt to obfuscate and avoid the obvious error in your earlier statement. Just slap your forehead and say, "Shucks, I blew it with that one" and we can all forget about it. I do it all of the time. | |The true load SWR does not change under these conditions, but it cannot then |be determined by such a meter. Attempting to do so will yield some value, |but it will be wrong. Oh please. If an SWR meter, direction bridge, TLI or whatever you want to call it has decent directivity, i.e. the ability to discern forward and reflected power, forward and reflected waves, reflection coefficient, scattering parameters, or whatever you want to call them, then the applied power is immaterial. We are trying to measure a RATIO, not some absolute value of power. |
"Roy Lewallen" wrote If we connect a transmitter to an SWR meter, and then to a long piece of lossless cable with the same Z0 as the SWR meter, and finally to a load, the SWR meter reading will be the same as the VSWR on the cable, i.e., the ratio of maximum to minimum voltages on the line. ========================================= It is at this point where impressionable novices are led astray by old wives, never again to return to logical thought on the subject. They imagine that because the meter happens to indicate the swr on the line, the meter is actually responding to the swr on it. Whereas the meter is actually responding to the modulus of the reflection coefficient caused by the line's input impedance regardless of what its Zo may be. The act of making the line's Zo and the meter's resistance both equal to the transmitter's designed-for load resistance, has put additional infomation into the system. Cooking the books! If there's an SWR to be indicated it is on a long line between meter and the transmitter. In the absence of such a line the meter wastefully discards half of the information it is presented with and indicates the modulus of the reflection cofficient. A more appropriate name is TLI. ---- Reg, G4FGQ |
On Sat, 4 Sep 2004 14:08:53 +0000 (UTC), "Reg Edwards"
wrote: NOTE: In the above description and calculation there is no mention of Zo, terminating impedance, source impedance, reflection coefficient, forward power, reflected power, reflected volts, reflected current, Smith charts, or conjugate matches. All these things are superflous to the determination. No information other than the two voltage measurements is needed. Hi All, No mention merely means there is no offer of accuracy (not very important, eh what?). These two measurements (repeated at intervals) can reveal a SWR that varies along the length of the line like a snake - UNLESS of course, you DO observe unmentionables like Load reflection co-efficients and Source reflection co-efficients. As such, a description of how not to measure SWR, but rather how to exhibit error if you perchance have the misfortune of having a transmitter that is unmatched to a 50 Ohm transmission system whose load is in fact mismatched also. Need I point out that if both ends are matched - what's the point in measuring SWR? ;-) 73's Richard Clark, KB7QHC |
In message , Reg Edwards
writes "Roy Lewallen" wrote If we connect a transmitter to an SWR meter, and then to a long piece of lossless cable with the same Z0 as the SWR meter, and finally to a load, the SWR meter reading will be the same as the VSWR on the cable, i.e., the ratio of maximum to minimum voltages on the line. ========================================= It is at this point where impressionable novices are led astray by old wives, never again to return to logical thought on the subject. They imagine that because the meter happens to indicate the swr on the line, the meter is actually responding to the swr on it. Whereas the meter is actually responding to the modulus of the reflection coefficient caused by the line's input impedance regardless of what its Zo may be. The act of making the line's Zo and the meter's resistance both equal to the transmitter's designed-for load resistance, has put additional infomation into the system. Cooking the books! If there's an SWR to be indicated it is on a long line between meter and the transmitter. In the absence of such a line the meter wastefully discards half of the information it is presented with and indicates the modulus of the reflection cofficient. A more appropriate name is TLI. ---- Reg, G4FGQ Would this help? On the subject of whether the TX impedance affected the SWR reading, I propose the following practical test: Using standard CATV bits and pieces, connect up the following- Signal source directional coupler #1 directional coupler #2 load. DC#1 picks off the forward signal, DC#2 picks off the reverse. Use a spectrum analyser to measure signal levels. Beforehand check the DCs for go directivity, and chose a frequency where it is best (at least 25dB). This will probably be around 20MHz. With good load, measure forward and reverse signals. Repeat with known load mismatch. Screw up source impedance (eg add T-piece at source o/p, and double-terminate). Repeat the above. Think about what the results mean. Ian. -- |
Richard Fry quoted Geoffrey Mendenhall who is responsible for the entire
Harris broadcast transmitter line design as saying: "If the source impedance were equal to the 50 ohm line impedance, half the transmitter`s output would be dissipated in its internal output source impedance." Mendenhall is wrong. A Class C amplifier`s "internal output source impedance" is largely "dissipationless resistance" produced by the non-conduction time during its RF cycle. A matched Class C amplifier typically produces efficiencies exceeding 50% by a comfortable margin. That`s why they are used despite their harmonic generating nonlinearity. Best regards, Richard Harrison, KB5WZI |
"Wes Stewart" wrote:
Oh please. If an SWR meter, direction bridge, TLI or whatever you want to call it has decent directivity, i.e. the ability to discern forward and reflected power, forward and reflected waves, reflection coefficient, scattering parameters, or whatever you want to call them, then the applied power is immaterial. We are trying to measure a RATIO, not some absolute value of power. _______________ However an SWR meter, direction(al) bridge, TLI or whatever you want to call it has NO ability to discern between two waves traveling in the same direction, unless their detectors are operating in the time domain -- which normally they do not. That is the "feature" causing the anomalies I have been writing about. RF |
Richard Fry wrote:
"It doesn`t matter whether we state the result of measurement in units of SWR, return loss, or as a reflection coefficient -- they all give the same information -- ." Correct. The units above are fungible. All are an expression of the mismatch of a load to the Zo of the transmission line. With a Bird wattmeter, the reflection coefficient (rho) is the sq. rt. of the reflected power divided by the forward power. SWR = 1+rho / 1-rho Return loss in dB = 20 log (rho) Return loss in dB = 10 log (Ref.Pwr./Fwd.Pwr.) Rho = (ZL/Zo)-1 / (ZL/Zo)+1 None of the expressions above include the source Z, therefore it does not apply. Best regards, Richard Harrison, KB5WZI |
"Roy Lewallen" wrote:
Sorry, it still isn't clear. What, then, is "system SWR"? How do you define it? System SWR is the net SWR of a component assembly present at its input terminals. "Antenna system SWR" then is comprised of the net SWR of everything in the RF path from the output of the SWR meter to and including the antenna. In a transmitter, the antenna system begins electrically at the output of the SWR meter -- physically close to the output connector of the tx. Obviously, if we have a voltage or current source of fixed value and change the source impedance, the power delivered by the source changes, But the mechanism I've described considers the re-reflection by a mismatched source of power not initially absorbed by a mismatched load -- not that a change of source impedance changed the total power flowing out of the source. ...both the "forward" and "reverse" powers will change, but by the same fraction... Agree. I'm not so sure that the Model 43 or equivalent methods used in/with transmitters accurately preserves the power ratios under these conditions, though. RF |
What's a directional coupler?
What do they look like? Don't bother answering those questions. Why do the arguers, when caught in a tight corner, always escape to UHF for help from directional couplers? There are NO directional couplers at HF. They are as scarce as real swr meters. So they cannot be used in futile attempts to explain what really happens at HF. You're next move will be to drag in scattering-matrices. --- Reg ;o) |
"Reg Edwards" wrote in message ... What's a directional coupler? What do they look like? Don't bother answering those questions. Why do the arguers, when caught in a tight corner, always escape to UHF for help from directional couplers? There are NO directional couplers at HF. So what is the element in my Bird Thruline then, and how does it work? I'd dying to hear at what frequencies directional couplers suddenly begin to "exist". A sudden change in the laws of Physics at some arbitary frequency named by man. (Most SWR meters I've seen use a directional coupler, by the way. Even the cheap ones.) I think we've got to the root of Reg's problem. Just like the last time he raised this nonsense. -- Brian Reay www.g8osn.org.uk www.amateurradiotraining.org.uk FP#898 |
"Richard Clark" wrote
To date in this matter, I have yet to see any concrete value of source Z offered from those of the NOT 50 Ohms camp. Further, I have yet to see any of them offer any experimental confirmation of their assertion Richard Fry wrote: Below is a quote from a paper titled "A Study of RF Intermodulation Between FM Broadcast Transmitters Sharing Filterplexed or Co-located Antenna Systems," by Geoffrey Mendenhall. (clip). Quoting Mendenhall, "...If the source impedance were equal to the fifty ohm line impedance, half of the transmitter's output power would be dissipated in its internal output source impedance..." Walter Maxwell wrote The last sentence in the paragraph above is incorrect. This shows that the writer of the quote is in the unbelievably large group that still believes incorrectly that half of the tx power would be lost if if it were conjugately matched. But we all know that efficiencies greater than 80% is achieved by Class C amps, and greater than 60% is achieved by Class B amps when the source impedance of the tx is 50 ohms resistive and the load impedance is also 50 ohms resistive. _______________ To Walter Maxwell: 1. You may be interested in reading Mendenhall's complete paper, which I will email to you. The lab measurements reported in it used two, operating, high-power FM broadcast transmitters -- and support his statements about amplifier source impedance and its consequences. 2. I will ask again, if transmitters have a 50 ohm source impedance, what accounts for the fact that TV ghosts are produced by an antenna system reflection having a sufficient delay time? Calculations and measured data show that the energy that produced the ghost originated by re-reflection off the TV transmitter output stage of far-end reflections in the antenna system. If the tx source impedance was 50 ohms, it would absorb the far-end reflection, which would be incapable of producing a ghost image. Further, if the tx source impedance was 50 ohms, then the RF intermodulation measured and reported in Mendenhall's paper -- and verified in real-world installations by the radiated interference those IM products produced -- would not occur. RF |
Reg, G4FGQ wrote:
"There are NO directional couplers at HF." Behold the Bird wattmeter! Why not call the SWR indicator a "mismatch meter"? Best regards, Richard Harrison, KB5WZI |
The meaning of this paragraph in my last post in this thread is more clearly
understood when two commas are added... But the mechanism I've described considers the re-reflection, by a mismatched source, of power not initially absorbed by a mismatched load -- not that a change of source impedance changed the total power flowing out of the source. |
In message , Reg Edwards
writes What's a directional coupler? What do they look like? Don't bother answering those questions. Why do the arguers, when caught in a tight corner, always escape to UHF for help from directional couplers? There are NO directional couplers at HF. They are as scarce as real swr meters. So they cannot be used in futile attempts to explain what really happens at HF. You're next move will be to drag in scattering-matrices. --- Reg ;o) Reg, I thought that a DC was essentially any device which sampled some of the RF signal travelling in one direction, and (if perfect) none of the signal travelling in the other. Over the past 40 years, I have had lots of dealings with them. I've even designed some which were bought in their thousands (if not millions). Some of them seemed to work from below 5MHz to over 870MHz (well, I THOUGHT they did, and so did the people who bought them). Where did I go wrong? On the positive side, I never went as far as to claim that they would measure SWR. Ian. -- |
On Sun, 5 Sep 2004 12:18:09 +0000 (UTC), "Reg Edwards"
wrote: |What's a directional coupler? |What do they look like? |Don't bother answering those questions. | |Why do the arguers, when caught in a tight corner, always escape to UHF for |help from directional couplers? | |There are NO directional couplers at HF. They are as scarce as real swr |meters. So they cannot be used in futile attempts to explain what really |happens at HF. | |You're next move will be to drag in scattering-matrices. Why not. I have used an HP3577 network analyzer with an S-parameter test set that was specified to work over the frequency range of 100 Hz to 200 Mhz. I guess the guys at HP didn't realize that you can't do this. |
On Sun, 5 Sep 2004 07:49:06 -0500, "Richard Fry"
wrote: if transmitters have a 50 ohm source impedance, Hi OM, There you go again "IF." IF indeed! It seems you find controversy where there is none. :-) I would again suggest you read what I wrote, and point out what exactly your contention is with IT. 73's Richard Clark, KB7QHC |
And another example in point:
From the input to a TV transmit antenna system, tx disconnected, I have personally measured the far-end antenna system reflections of a 2T sin² video pulse (0.25 µs H.A.D.) modulated onto a TV channel carrier, and detected by a vestigial sideband demodulator tuned to that TV channel. A high-directivity directional coupler at the input to the main line, a display device, calibrated attenuators, and the time difference between the incident and reflected pulse enable accurate measurement of the reflection coefficient of the antenna system. This was a common practice after a new antenna system installation to measure and optimize the far-end match for the best quality radiated signal, and was pioneered by RCA Broadcast Eqpt Div, my employer at the time. More elegant means are used these days. When this test shows a 5% pulse return 2 µs after the incident pulse time (for example), then the same pulse passed through the tx also shows nearly exactly the same reflection % and time separation -- assuming there is enough RF delay in the system for the reflection to be resolved in the demodulated waveform. As the directional coupler driving the normal demodulator at the TV station is looking at forward power only, it is clear that the reflection from the far end of the antenna system has been re-reflected from the TV tx output stage, and NOT absorbed by it in its "conjugate impedance." RF |
|
Richard Clark wrote
"Richard Fry" wrote: if transmitters have a 50 ohm source impedance, It seems you find controversy where there is none. :-) I would again suggest you read what I wrote, and point out what exactly your contention is with IT. _________________ OK. Earlier you wrote, "To date in this matter, I have yet to see any concrete value of source Z offered from those of the NOT 50 Ohms camp. Further, I have yet to see any of them offer any experimental confirmation of their assertion (made rather simple by the exhibition of uncertainty)." Our controversy is illustrated by my posts with an opposite conclusion, beginning last night and continuing this morning. As for experimental evidence, I report some in my post here of a few minutes ago about making refl coeff measurements of TV transmit antenna systems. Mendenhall's paper also has experimental evidence of this. I will email it to you. I trust my contention is now clear to you. RF |
"Richard Clark" wrote
But this gets curiouser and curiouser (as Alice through the Looking Glass would offer). Cited as an example of the "NOT 50 Ohm" society (and one of its leading proponents) we find that Geoff Mendenhall's notable achievement in 1968 was building a 400W FM amplifier. Truly a hands-on achievement. Now if we simply review the historical archive and ask Geoff himself what the Z of his design was, we find by his own hand: RF Output Impedance: 50 Ohms Let's see, no technical argument, and sources that are self-contradicting. Whatchagonnado? Punt? :-) 73's Richard Clark, KB7QHC |
On Sun, 5 Sep 2004 11:05:24 -0500, "Richard Fry"
wrote: I trust my contention is now clear to you. Hi OM, Actually no. Your reference, Mendenhall, specifically writes about his design: "It was necessary to determine the plate load impedance (formula) = 1000 Ohms where Emin min drop across the tube in saturation I1 ac plate current. "Since this Zp was to be coupled into a Z output of 50 Ohm, a impedance transformation of 20:1 was needed." Fairly straightforward stuff there. Geoff's own notes are the model of economy and directly to the nut of the issue. His notes are also straight from old school first principles. As often happens, the more elaborate the discussion of rather simple matters belies the inference of hucksterism. Seems like ALL my arguments, references, citations, data offered and so on are congruent with Geoff's own description of amplifiers. I certainly need no further testimony from him as I am perfectly capable of finding his own work and offering it here. Thanx anyway, but no thanx. It would also serve you better to read more and write less, after all I SPECIFICALLY mandated a discussion of transistor amateur amplifiers. When I allowed this divergence to tubes, Mendenhall himself proved that NOTHING changes in the physics of sources. 73's Richard Clark, KB7QHC |
"Richard Clark" wrote
Now if we simply review the historical archive and ask Geoff himself what the Z of his design was, we find by his own hand: RF Output Impedance: 50 Ohms Let's see, no technical argument, and sources that are self-contradicting. Whatchagonnado? Punt? :-) ________________ You assume he refers to the source impedance of/at output of the amplifier. More likely he is following convention and stating the load impedance that the amplifier was designed to work into. The source impedance of most transmitters is not published even today. If it was, probably we wouldn't be having all of this confusion about it, and its effects. RF |
Brian Reay wrote:
"---dying to hear at what frequencies directional couplers suddenly begin to exist." It isn`t sudden. They sure work at audio frequencies. In telephones, they are used to prevent the user`s voice from overpowering the distant party`s voice in the user`s ear. They are called hybrids. Hybrids are also used to couple a 2-wire circuit which simultaneously carries both directions of transmission with a 4-wire circuit consisting of a transmit pair and a receive pair. Best regards, Richard Harrison, KB5WZI |
"Richard Clark" wrote
I SPECIFICALLY mandated a discussion of transistor amateur amplifiers. ____________________ At least there appears to be an acknowledgement that some RF amplifiers do not have a source impedance that is the conjugate of their load impedance. So progress has been made. RF |
On Sun, 05 Sep 2004 16:21:27 GMT, Richard Clark
wrote: On Sun, 5 Sep 2004 11:05:24 -0500, "Richard Fry" wrote: I trust my contention is now clear to you. Hi OM, Actually no. Your reference, Mendenhall, specifically writes about his design: Hi All, I would add further, Mendenhall's notes of his design, as the model of clarity, include references, one of which is particularly notable and estimable within this group: "Treman [sic], F.E.; "Electronic and Radio Engineering"; Mc Graw - Hill Book Co.; 1955" the same publication I've had since the same date that Geoff built his transmitter. Geoff's attachments also include the data sheets from Eimac which show quite plainly that ALL of his formulas and computations are congruent with ALL sources of information in his references. Another reference: "Goodman, Byron (Ed.); 'The Radio Amateur's Handbook'; American Radio Relay League; Newton, Conn.; 1966" (I used to have that publication, back then, too) I also vaguely note some inference of peculiar intermodulation products that would be produced by a transmitter with 50 Ohm output characteristic - in that I may be mistaken because when the verbiage gets particularly dense to explain simple matters, I must admit my own filters kick in. However, Mendenhall's work was not simply that of an amateur's project, nor was it a school term paper, nor was it the speculation of an engineering sales pitch. The report I am drawing upon was Geoff's own Type Acceptance application to the FCC which included all the technical specifications of spurs, intermodulation products, stability, efficiency (80%), class of operation, modulation, out-of-band responses.... I don't think I need go much further. :-) For those who wish to read the COMPLETE story of how to build a rig, how to specify it, how to measure it, and to note how it exactly conforms to conventional wisdom; then visit: http://www.techatl.com/wrek/docs/gnm_0011.htm where you will find all of one page of theory, and 40 odd pages of reality: The WREK 425 Watt RF power amplifier, also known as the "Goat-Mitter" was designed by Geoffrey N. Mendenhall (dubbed the Goatman by WREK announcer, Ed Esserman) and constructed entirely with hand tools by Geoff and the WREK staff in August of 1968. 73's Richard Clark, KB7QHC |
On Sun, 5 Sep 2004 11:25:02 -0500, "Richard Fry"
wrote: You assume Hi OM, That is called a punt. 73's Richard Clark, KB7QHC |
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