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Reg Edwards wrote:
"Cecil Moore" wrote On my system, there's a 50 ohm cable from the transceiver to the input of the SWR meter and another 50 ohm cable from the output of the SWR meter to the balun. Each of these cables forces the ratio of the voltage to current in each of the traveling waves to a value of 50 ohms. I have an in-line Autek WM-1 and no tuner. -- ==================================== Cec, You can't measure SWR on a line which is less than 1/4-wave long. Preferably it should be as long as 1/2-wavelength to ensure the max and min voltage points both occur on it. Total and absolute nonsense. Where do you come up with this stuff? snip arm waving speech -- Jim Pennino Remove .spam.sux to reply. |
Owen Duffy wrote:
On Tue, 27 Sep 2005 01:36:15 +0000 (UTC), wrote: Owen Duffy wrote: On Mon, 26 Sep 2005 21:39:27 +0000 (UTC), wrote: Jim, that seems inconsistent with your earlier statemetn "No, the SWR being measured is on the load side of the meter." The load side is the side with the load, i.e. the antenna, on it. In the example you quoted with a 100 ohm load on a 100 ohm line, were the line loss low, and the line long enough to be sure to sample a fully developed voltage maximum and voltage minimum it would be found that the VSWR was 1:1. Not for a 50 Ohm system, i.e. a transmitter expecting 50 Ohms and a meter calibrated for a 50 Ohm system. I am sorry Jim, the VSWR is a property of the transmission line and its termination, and the VSWR on that 100 ohm line with a 100 ohm termination is 1:1. The VSWR could be *MEASURED* on that line by sampling the magnitude of the voltage at different points on the line and it would be found that the magnitude of the voltage was constant, which means VSWR=1:1. No, the measured SWR is relative to the design impedance of the SWR meter which is normally 50 Ohms. The SWR on the line depends on the characteristic impedance of the line and the impedance of the termination of the line. 50 ohms does not come into it. We are not talking about SWR on the line, we are talking about SWR at the input END of the line; big difference. The SWR on your proposed 100 ohm line with a 100 ohm termination is 1:1. If your measurement indicates anything else, then you need to consider your measurement as invalid. Once again, we are not talking about SWR *ON* the line, we are talking about SWR as seen at the input *END* of the line; big difference. Furthermore, the SWR *ANYWHERE* on the line is *NOT* 1:1 for a 50 Ohm reference. Try hooking a length of 93 Ohm line (which is easier to get than 100 Ohm line) terminated with a 93 Ohm resistor to any 50 Ohm SWR meter of any type. Then hook just the 93 Ohme resistor to the meter and tell me what the difference is in the readings. Owen -- If anything is misnamed it is the term SWR. SWR is nothing more than a dimensionless impedance ratio. You do NOT need a transmission line to have SWR in spite of the W in SWR standing for 'wave'. -- Jim Pennino Remove .spam.sux to reply. |
On Tue, 27 Sep 2005 02:25:11 +0000 (UTC),
wrote: SWR is nothing more than a dimensionless impedance ratio. Is that so... Owen -- |
Owen Duffy wrote:
On Tue, 27 Sep 2005 02:25:11 +0000 (UTC), wrote: SWR is nothing more than a dimensionless impedance ratio. Is that so... Owen -- Yes, it is so and it is equal to: SWR = (A + B)/(A - B) Whe A = sqrt ( (R + Z)^2 + X^2 ) B = sqrt ( (R - Z)^2 + X^2 ) R = resistive component of load impedance in Ohms. X = reactive component of load impedance in Ohms. Z = reference impedance (purely resistive) in Ohms If you don't believe it, get some resistors, capacitors and a half way decent SWR meter and do some experiments; no transmission line required. -- Jim Pennino Remove .spam.sux to reply. |
Owen,
SWR meters with a sampling line. The only experience I've had has been I once made one for HF. It was of the type where a second wire is drawn alongside the inner conductor of a short length of coaxial line of impedance in the same street as the system it is to work with. Operating frequencies covered the whole of the HF band. That is a very wide band. Which indicates that line length plays no part in measuring accuracy once calibrated. To explain how the thing works it is necessary to return to what it really is. It is a resistance bridge. All so-called SWR meters, whatever the circuit or form of construction, are resistance bridges. The bridge has 3 internal ratio arms. The 4th arm is the variable transmitter load. If all 4 arms are of same resistance we have a very sensitive arrangement suitable for QRP transmitters. However, 3/4 of the TX power is dissipated in the 3 internal bridge arms. For higher power transmitters it is necessary to use high ratios for the ratio arms. In the case of meters which use a little ferrite ring as a current transformer, a resistor of the order of 30 to 100 ohms can be shunted across the current transformer secondary winding while the primary winding has an input resistance of the order of 0.1 ohms which forms the value of the ratio arm in series with the external load. This 0.1-ohm arm is capable of carrying the load current of several amps with only a small power loss. The other two ratio arms can be a pair of high value resistors in the same ratio as occurs via the current transformer. If the input resistance of the current transformer is 0.1 ohms then the bridge ratio is 50 / 0.1 = 500:1 where 50 ohms is the usual value of the load resistance when the bridge is balanced and SWR = 1:1 The two high impedance arms can be capacitors in the same ratio of 500:1 which have zero power dissipation but have a minor effect on accuracy. They introduce a small phase angle into the load as seen by the transmitter through the meter. The error increases with increasing frequency. It will be seen that the take-off point is effectively the same for both current and voltage. Returning to the so-called sampling line. There is a bridge configuration which is not quite so obvious. But instead of a current transformer the current is picked off by means of a short length of wire in parallel with the coaxial inner conductor by virtue of their mutual inductance. The line is too short for propagation effects to play a significant part. Voltage is picked off at the same point by virtue of the capacitance between the wire and coaxial inner conductor. The phase relationship between volts and amps can be reversed just by reversing the direction of propagation through the meter. The bridge ratio is set partially by the ratio of the impedances Zo of the additional wire and inner coax conductor. The length of coaxial line affects only the bridge sensitivity and power dissipated in the meter. As you must be aware, sensitivity falls of fast with decreasing frequency and 160 meters was my favourite band. So the home-brewed meter was soon discarded and I returned to ferrite rings. I was left with the impression it was very easy to make and that almost anything would work. Hope you can understand the foregoing. ---- Reg. |
Jim,
Perhaps there's some misunderstanding about location of the meter and what it is measuring. Let's try to clear it up. Would you please do us both a favour by answering the following simple question? There is a 50 ohm line feeding a 100 ohm antenna. There is an SWR meter located at the line-antenna junction. The meter has a reading. Does the reading apply to SWR of the antenna, or does it apply to the SWR along the feedline? Antenna or Feedline? ---- Reg. |
"Reg Edwards" wrote:
There is a 50 ohm line feeding a 100 ohm antenna. There is an SWR meter located at the line-antenna junction. The meter has a reading. Does the reading apply to SWR of the antenna, or does it apply to the SWR along the feedline? ______________ It applies to the match of the RF network that follows the SWR meter to the impedance for which the SWR meter was calibrated. And if in your example the SWR meter has been calibrated for 50 ohms, and is moved to the input end of that line+antenna RF network, it will also have a reading -- which will be the same as when it was inserted at the antenna-line junction, less the round-trip RF attenuation of the transmission line (assuming that the transmission line is 50 +/- j0 ohms throughout its length). In fact it is a common practice to optimise the transmission line/antenna match of commercial FM and TV broadcast antenna systems by use of a variable transformer inserted at the antenna input, whose adjustment is made by reference to the far-end reflection seen at the sending end of the transmission line, using a high-directivity reflectometer, or SWR meter. The same physics applies to ham antenna systems and methods/means of measurement. RF Visit http://rfry.org for FM transmission system papers. |
Reg Edwards wrote:
There is a bridge configuration which is not quite so obvious. But instead of a current transformer the current is picked off by means of a short length of wire in parallel with the coaxial inner conductor by virtue of their mutual inductance. The line is too short for propagation effects to play a significant part. The pickup lines in my Heathkit HM-15 are terminated on one end with a 50 ohm resistor. One pickup line thus attenuates the reflected traveling wave and allows the forward traveling wave to be rectified. The other pickup line attenuates the forward traveling wave and allows the reflected traveling wave to be rectified. Knowing the peak values of both of these two traveling waves allows a calibrated meter to indicate SWR. -- 73, Cecil http://www.qsl.net/w5dxp |
Owen Duffy wrote:
On Tue, 27 Sep 2005 02:54:31 +0000 (UTC), wrote: Owen Duffy wrote: On Tue, 27 Sep 2005 02:25:11 +0000 (UTC), wrote: SWR is nothing more than a dimensionless impedance ratio. The fundamental definition of SWR flows from the behaviour and properties of RF transmission lines. And power=EI. And it also equals I^2*R and E^2/R. SWR can be expressed in terms of power ratios, current ratios, and impedance ratios. When a transmission line is terminated in an impedance other than its characteristic impedance, there will be both a forward wave and a reflected wave of such magnitude to resolve the conditions that must apply at the termination. Irrelevant. The forward wave and the reflected wave sum at all points along the line having regard for their magnitudes and relative phase to produce a "standing wave". The Standing Wave Ratio (SWR or VSWR) is defined to mean the ratio of the maximum to the minimum of the magnitude of the standing wave voltage pattern along the line. Is is also defined as a current ratio and an impedance ration. The SWR on a lossless line can be calculated knowing the complex characteristic impedance of the line and the complex load impedance. What no waves, just impedences!! Now you are contidicting yourself. The SWR on the line does not depend in any way on some unrelated independent reference resistance as you suggest in your formula. Read it again. The R is the R of the thing at the end of the line. The X is the X of the thing at the end of the line. The X is the impedance of the line. You seem to be suggesting that your redefined SWR is a really good (obscure) way to talk about an impedance (independently of a transmission line) in terms of some standardised reference value, and you can throw away the fundamental meaning of SWR to support your SWR(50) concept. In your terms (independently of a transmission line), for instance, a Z of 60+j10 would be SWR(50)=1.299, and so would an infinite number of other Zs have SWR(50)=1.299... how is that of value. To know Z is 60+j10 is to know more than to know SWR(50)=1.299. The equations given are general and can be derived from first priciples. The Z in the equations is the Z of your reference, i.e. 50 for a 50 Ohm system. SWR is *ALWAYS* relative to some reference impedance. Owen -- -- Jim Pennino Remove .spam.sux to reply. |
Reg Edwards wrote:
Jim, Perhaps there's some misunderstanding about location of the meter and what it is measuring. Let's try to clear it up. Would you please do us both a favour by answering the following simple question? There is a 50 ohm line feeding a 100 ohm antenna. There is an SWR meter located at the line-antenna junction. What does this mean? The meter has a reading. Does the reading apply to SWR of the antenna, or does it apply to the SWR along the feedline? The reading is the SWR at that point. Antenna or Feedline? ---- Reg. An SWR meter reads the SWR of the thing connected to its output port with respect to the reference impedance the meter was designed for. The SWR meter reads the SWR *AT THE POINT OF CONNECTION* of the connected system. Not the middle of the system, not the other end (if it has one) of the system, but the input point. If you measure a SWR (50 Ohms reference) of 2:1 for a black box, what is in the box? A. A 25 Ohm resistor. B. A 100 Ohm resistor. C. A cable spool of coax with some impedance at the end of it. D. Could be any of the above. In general there is no guarantee that the SWR at any point of a transmission line will be equal to the SWR at any other point on a transmission line other than for special cases. What seems to have you terribly confused is that all the transmission lines, tuners, antennas, connectors and whatevers become a *SYSTEM* and the SWR at the input connector to the *SYSTEM* is not guaranteed to be the same as the SWR at some arbitrary point inside the system. When you measure the SWR of a line with a load on the end, you are measuring the SWR of the entire system relative to your reference, not the load. -- Jim Pennino Remove .spam.sux to reply. |
Cec, you have YOUR explanation and I have MY explanation.
Which is the most simple? There is a bridge. When the variable arm, the load, is 50 ohms the bridge is balanced and the meter indicates SWR = 1:1 When the variable arm is either 0 ohms or infinite ohms, the meter indicates SWR = infinity :1 What can be more simple than that? How it works can be visualised. But the meter is ambiguous. It cannot distinguish between loads of 0 ohms and infinite ohms. Additional information is required. This serious ambiguity also applies to your weird contraption. ;o) ---- Regards, Reg. |
Cec, I notice that you and others have begun to use my description of
"indicate" rather than "measure". ---- Reg. |
Jim, I'm sorry you are unable to answer the simple question "Feedline
or Antenna?". ---- Reg. |
Richard,
If's and But's are not required. The antenna is just an arbitrary load. Does the meter reading indicate SWR on the feedline (which is what is usually required), or does it not? This is not a "catch question". It is not a troll. "Antenna or Feedline?" please. KISS ---- Reg. ===================================== "Richard Fry" wrote in message ... "Reg Edwards" wrote: There is a 50 ohm line feeding a 100 ohm antenna. There is an SWR meter located at the line-antenna junction. The meter has a reading. Does the reading apply to SWR of the antenna, or does it apply to the SWR along the feedline? ______________ It applies to the match of the RF network that follows the SWR meter to the impedance for which the SWR meter was calibrated. And if in your example the SWR meter has been calibrated for 50 ohms, and is moved to the input end of that line+antenna RF network, it will also have a reading -- which will be the same as when it was inserted at the antenna-line junction, less the round-trip RF attenuation of the transmission line (assuming that the transmission line is 50 +/- j0 ohms throughout its length). In fact it is a common practice to optimise the transmission line/antenna match of commercial FM and TV broadcast antenna systems by use of a variable transformer inserted at the antenna input, whose adjustment is made by reference to the far-end reflection seen at the sending end of the transmission line, using a high-directivity reflectometer, or SWR meter. The same physics applies to ham antenna systems and methods/means of measurement. RF Visit http://rfry.org for FM transmission system papers. |
"Reg Edwards"
"Antenna or Feedline?" please. KISS ___________ Antenna. And a big smooch to you, too. RF |
Reg Edwards wrote:
Cec, you have YOUR explanation and I have MY explanation. Mine is a lot simpler. The Heath HM-15 has two pickup elements. If you install a Z0 resistor load at one end it "picks up" the forward wave. If you install a Z0 resistor load at the other end it "picks up" the reflected wave. The two pickup voltages are rectified and compared through a calibration procedure. The parts that came with the HM-15 kit in the 50s-60s included two 72 ohm resistors. RG-ll was very popular at the time. If one wanted a 72 ohm SWR meter, one installed the 72 ohm resistors. If one wanted a 50 ohm SWR meter, one installed the 50 ohm resistors. A switch could be installed that switched between 50 ohms and 72 ohms calibration. This serious ambiguity also applies to your weird contraption. ;o) Actually, the Heathkit design concept is easier to understand than is the bridge explanation or the toroid-pickup/phasor-addition explanation. The first SWR meter I built in the 50s, used two lengths of insulated wire shoved under the braid of the coax. It worked but, at the time, I had no idea why it worked. Heath's little slotted line pickup device was pretty slick. I sometimes see them for sale at hamfests. -- 73, Cecil http://www.qsl.net/w5dxp |
Reg Edwards wrote:
Cec, I notice that you and others have begun to use my description of "indicate" rather than "measure". What a meter movement "measures" is current. What a meter "indicates" can be anything in the world depending upon the calibration scale. -- 73, Cecil http://www.qsl.net/w5dxp |
Reg Edwards wrote:
"Antenna or Feedline?" please. If the meter is calibrated for the feedline Z0, it will read the SWR on the feedline. If the meter is calibrated for the antenna Z0, it will read the SWR on the antenna. -- 73, Cecil http://www.qsl.net/w5dxp |
On Tue, 27 Sep 2005 15:48:35 +0000 (UTC), "Reg Edwards"
wrote: But the meter is ambiguous. It cannot distinguish between loads of 0 ohms and infinite ohms. Additional information is required. Hi Reggie, Without recourse to that "additional information," explain how you achieve the unambiguous by your method of probing lines (be they parallel, coaxial, or waveguide). In other words, your objection is a non sequitur, it is meaningless because you need the same additional information and you cannot demonstrate any measurable difference between the manifold methods of coming to the same determination. Of course, if you throw a spanner in the other guy's gear-box, you might win the race. 73's Richard Clark, KB7QHC |
On Tue, 27 Sep 2005 17:29:10 +0000 (UTC), "Reg Edwards"
wrote: If's and But's are not required. The antenna is just an arbitrary load. Does the meter reading indicate SWR on the feedline (which is what is usually required), or does it not? This is not a "catch question". It is not a troll. No, of course it isn't (must be all those other posts then) "Antenna or Feedline?" please. KISS Hi Reggie, Ah yes! That "additional information" finally surfaces as a requirement doesn't it? SWR "on the feedline" is like dust thrown into the eyes of the rubes before the elephant appears in front of them. Presumably, "on the feedline" is akin to the ark holding a sacred artifact like the finger of St. Heavybottom. The traditional slotted line used for probe determination of SWR comes with two connectors like that commonplace SWR meter (and the slotted line probe connects to a - SWR METER! albeit, not the commonplace variety, but odd how the tide of time has not yet altered the name to TLI). And if we were to substitute the slotted line for commonplace SWR (or t'other way 'round), both/either/each would face the same issues and offer the same results. Imagine that, not a whit of difference, except that the probes can add error through in-expert use. Whoops! Same issues of how things can/do go wrong. So, the ultimate question of the universe (yadda-yadda-yadda) is what difference does having a transmission line between these two connectors make on the outcome of what SWR exists AT the load connector? A rhetorical difference! After-all, you would need "extraordinarily more information" to express where the SWR resided, wouldn't you? ;-) Any artificial constraint you toss in as an objection exists for yourself as well. I can see Kelvinator swinging his cane now. Did I say lower 6th? They would probably hoot you down to upper 5th. 73's Richard Clark, KB7QHC |
Cec, you can make the meter read anything you like just by twiddling
the calibration pot. Of what bloody use is that? !**?!! Just answer the obvious question. No If's or But's ---- Reg. "Cecil Moore" wrote in message ... Reg Edwards wrote: "Antenna or Feedline?" please. |
"Reg Edwards" "Antenna or Feedline?" please. KISS ___________ Antenna. And a big smooch to you, too. -------------------------------------- Rich, thanks for the smooch. But I'm afraid you are wrong. The meter correctly indicates SWR on the feedline when it is located at the antenna end of the line. But don't worry too much about it. It seems you are in good company. So much for the technical education of radio engineers. It all comes about because of so-called SWR meters being called SWR meters - which they are not. At least not when located at the transmitter end of transmission lines as they nearly always are. When located adjacent to the transmitter they are TLI's. Transmitter Loading Indicators. And SWR has very little or nothing to do with it. ---- Reg. |
Rich, your abuse of the English language renders it impossible for me
or anybody else to make any sense of what you are waffling about. ---- Punchinello, G4FGQ |
On Tue, 27 Sep 2005 19:13:16 +0000 (UTC), "Reg Edwards"
wrote: Rich, your abuse of the English language renders it impossible for me or anybody else to make any sense of what you are waffling about. Aw Reggie, Are you using a prescription grade wine glass when you were trying to read it? Or can we blame it on the grape? No ifs ands or buts now because with each new post the question becomes more remote and harder for you to answer. 73's Richard Clark, KB7QHC |
Reg Edawds, G4FGQ wrote:
"R&B`s cavity magnetron was developed at Birmingham University in the midst of the air raids on that industrial city." Prior to Randall & Boot, magnetrons were low-power devices, outclassed ny klystrons. R & B`s resonant-cavity magnetron, operating at 10 cm exuded 100x more power than previous magnetrons and allowed smaller hardware and imptoved image definition.. The 6 KW GEC manufactured version given to the U.S. in August 1940 was shipped by ordinary parcel post to arouse no suspicion of its importance. It has been called the most important shipment ever to arrive on U.S. shores. The resonant-cavity magnetron was an awesome contribution to victory in WW-2 against both Germany and Japan. Their radar development lagged far behind. My ship in WW-2 had only one spare part kept in the captain`s safe, the Raytheon resonant-cavity magnetron for our Raytheon navigational radar system. Fortunately, we never needed to replace it. After WW-2, Rayrheon shifted production to their bew "Radar Range". The Japanese soon caught up with their ubiquitous microwave ovens. They were more motivated. Our houses were already equipped with oil, gas, and electric ranges aplenty. Everyone, it turned out, was ready for microwave too. Klystrons were not washed up either. The most powerful generators ever built were klystrons. The speed detector used to ticket your car probably uses a laser. All radar isn`t pulsed radar. Radar altometers use separate transmitter and receiver antennas. They transmit an FM signal whose modulation frequency is changing at a certain rate. This is continuously compared with the modulation frequency of the received echo to tell how far away the reflection point is. The television signal you watch off the air was probably generated by a large klystron. Best regards, Richard Harrison, KB5WZI |
"Reg Edwards"
But I'm afraid you are wrong. The meter correctly indicates SWR on the feedline when it is located at the antenna end of the line. ______________ Remove the feedline, and connect the 100 ohm antenna through the SWR meter calibrated for 50 ohms, directly to the transmitter. The meter has a reading. The measurement will have the same value as before, neglecting any adjustment for having no line loss now. But there is no feedline, so how can there be any SWR on it, you write. The fact that there is not enough transmission line length in the system for literal standing wave maxima and minima to develop on it does not mean that reflections do not exist in the output load system. It is the value of those reflections that determines the corresponding value of SWR. Reflections can be measured by appropriate instruments regardless of the length of line in the measured system, or even the existence of any transmission line at all. The convention of the professional engineering community for many decades has been to convert incident and reflected waveform samples into the corresponding value of SWR, no matter if there is insufficient line length in the system for the corresponding maxima and minima to develop fully on it. It doesn't matter, electrically. Your constant diatribes stating that it does is a futile exercise. Disable the SWR protection in your ham tx and key it to full power into an open or short. There is no transmission line where standing wave maxima and minima could exist, but your tx will burn up anyway. Maybe save your response until tomorrow morning, when your Merlot buzz has worn off :) RF |
Teg, G4FGQ wrote:
"Err, no, the meter is telling what it sees at the point of measurement." Yes, and that is a ratio depending on "rho", the reflection coefficient. We know that rho equals: the square root of reflected pwr / fwd pwr And: VSWR = 1+rho / 1-rho VSWR is a function of the reflection coefficient You drive an automobile and glance at the speedometer. It is an electrical meter giving an indication proportional to vehicle speed. You look at an SWR (TLI) meter giving an indication calibrated to be proportional to SWR. Best regards, Richard Harrison, KB5WZI |
"Reg Edwards"
But I'm afraid you are wrong. The meter correctly indicates SWR on the feedline when it is located at the antenna end of the line. ______________ PS: In this example the mismatch between the antenna and the line is the source of the reflection that results in system SWR. Convention is to state that the SWR belongs to the antenna, not the line -- although the added stress on components applies only to the line and tx, and not to the antenna. An ideal SWR meter will read that antenna reflection to have the same value when installed at either end of the line, or anywhere along its length (assuming a perfect 50 ohm line, and neglecting line loss). RF |
Reg Edwards wrote:
Jim, I'm sorry you are unable to answer the simple question "Feedline or Antenna?". ---- Reg. It appears you have no interest in understanding and simply wish to throw out straw men, red herrings, and who knows what to complicate a very simple concept. -- Jim Pennino Remove .spam.sux to reply. |
On Tue, 27 Sep 2005 15:21:51 +0000 (UTC),
wrote: Owen Duffy wrote: On Tue, 27 Sep 2005 02:54:31 +0000 (UTC), wrote: Owen Duffy wrote: On Tue, 27 Sep 2005 02:25:11 +0000 (UTC), wrote: SWR is nothing more than a dimensionless impedance ratio. The fundamental definition of SWR flows from the behaviour and properties of RF transmission lines. And power=EI. And it also equals I^2*R and E^2/R. SWR can be expressed in terms of power ratios, current ratios, and impedance ratios. When a transmission line is terminated in an impedance other than its characteristic impedance, there will be both a forward wave and a reflected wave of such magnitude to resolve the conditions that must apply at the termination. Irrelevant. The forward wave and the reflected wave sum at all points along the line having regard for their magnitudes and relative phase to produce a "standing wave". The Standing Wave Ratio (SWR or VSWR) is defined to mean the ratio of the maximum to the minimum of the magnitude of the standing wave voltage pattern along the line. Is is also defined as a current ratio and an impedance ration. The SWR on a lossless line can be calculated knowing the complex characteristic impedance of the line and the complex load impedance. What no waves, just impedences!! Now you are contidicting yourself. The SWR on the line does not depend in any way on some unrelated independent reference resistance as you suggest in your formula. Read it again. The R is the R of the thing at the end of the line. The X is the X of the thing at the end of the line. The X is the impedance of the line. You seem to be suggesting that your redefined SWR is a really good (obscure) way to talk about an impedance (independently of a transmission line) in terms of some standardised reference value, and you can throw away the fundamental meaning of SWR to support your SWR(50) concept. In your terms (independently of a transmission line), for instance, a Z of 60+j10 would be SWR(50)=1.299, and so would an infinite number of other Zs have SWR(50)=1.299... how is that of value. To know Z is 60+j10 is to know more than to know SWR(50)=1.299. The equations given are general and can be derived from first priciples. The Z in the equations is the Z of your reference, i.e. 50 for a 50 Ohm system. SWR is *ALWAYS* relative to some reference impedance. Jim, your comments are full of inconsistencies (like pronumeral X having two different meanings in the same formula, equations described as "general" but which do not allow for a reactance component in your "reference z" which is actually the characteristic impedance of the line in the real world, equations derived from first principles and you state the first principles are "irelevant"). In the absence of logic in your writing, I won't waste anymore time... you have some deeply entrenched misconceptions and seem to have built a large framework of simple views (like power=EI... a DC circuits concept) to support the misconceptions. Owen -- |
Reg Edwards wrote:
Cec, you can make the meter read anything you like just by twiddling the calibration pot. Of what bloody use is that? !**?!! Just answer the obvious question. No If's or But's I did answer the question, Reg. You just didn't like the answer. Let's say we have an SWR meter at point 'x' in the following diagram: XMTR---1WL 50 ohm coax---x---1WL 75 ohm coax---100 ohm load If the meter is calibrated for 50 ohms, it will indicate the SWR on the 50 ohm coax, 2:1, on the source side of the meter. If the meter is calibrated for 75 ohms, it will indicate the SWR on the 75 ohm coax, 1.33:1, on the load side of the meter. An SWR meter samples the magnitude and phase of the voltage, samples the magnitude and phase of the current, assumes it exists in the Z0 environment for which it was calibrated, and accurately reports those results. If the SWR meter is installed in a Z0 environment other than that for which it was calibrated, the instrument is being misused and the operator is at fault, not the instrument. Any instrument can be misused. -- 73, Cecil http://www.qsl.net/w5dxp |
Owen Duffy wrote:
On Tue, 27 Sep 2005 15:21:51 +0000 (UTC), wrote: Owen Duffy wrote: On Tue, 27 Sep 2005 02:54:31 +0000 (UTC), wrote: Owen Duffy wrote: On Tue, 27 Sep 2005 02:25:11 +0000 (UTC), wrote: SWR is nothing more than a dimensionless impedance ratio. The fundamental definition of SWR flows from the behaviour and properties of RF transmission lines. And power=EI. And it also equals I^2*R and E^2/R. SWR can be expressed in terms of power ratios, current ratios, and impedance ratios. When a transmission line is terminated in an impedance other than its characteristic impedance, there will be both a forward wave and a reflected wave of such magnitude to resolve the conditions that must apply at the termination. Irrelevant. The forward wave and the reflected wave sum at all points along the line having regard for their magnitudes and relative phase to produce a "standing wave". The Standing Wave Ratio (SWR or VSWR) is defined to mean the ratio of the maximum to the minimum of the magnitude of the standing wave voltage pattern along the line. Is is also defined as a current ratio and an impedance ration. The SWR on a lossless line can be calculated knowing the complex characteristic impedance of the line and the complex load impedance. What no waves, just impedences!! Now you are contidicting yourself. The SWR on the line does not depend in any way on some unrelated independent reference resistance as you suggest in your formula. Read it again. The R is the R of the thing at the end of the line. The X is the X of the thing at the end of the line. The X is the impedance of the line. You seem to be suggesting that your redefined SWR is a really good (obscure) way to talk about an impedance (independently of a transmission line) in terms of some standardised reference value, and you can throw away the fundamental meaning of SWR to support your SWR(50) concept. In your terms (independently of a transmission line), for instance, a Z of 60+j10 would be SWR(50)=1.299, and so would an infinite number of other Zs have SWR(50)=1.299... how is that of value. To know Z is 60+j10 is to know more than to know SWR(50)=1.299. The equations given are general and can be derived from first priciples. The Z in the equations is the Z of your reference, i.e. 50 for a 50 Ohm system. SWR is *ALWAYS* relative to some reference impedance. Jim, your comments are full of inconsistencies (like pronumeral X having two different meanings in the same formula, equations described as "general" but which do not allow for a reactance component in your "reference z" which is actually the characteristic impedance of the line in the real world, equations derived from first principles and you state the first principles are "irelevant"). The last line is obviously a typo, it should be: The Z is the impedance of the line. Z is an impedance. An impedance is an absolute value. The impedance of RG-8 coax, for example, is approximately 50 Ohms. As I said, if you don't believe the equations, go get some resistors and capacitors and do an experiment. Until you do that you have no case. In the absence of logic in your writing, I won't waste anymore time... you have some deeply entrenched misconceptions and seem to have built a large framework of simple views (like power=EI... a DC circuits concept) to support the misconceptions. Sigh, the power thing was a simple illustration of the fact that a thing can often be represented a number of different ways. How about acceleration is the first derivative of velocity and also the second derivative of position? Do you like this example better? Owen -- -- Jim Pennino Remove .spam.sux to reply. |
Richard Harrison wrote:
You drive an automobile and glance at the speedometer. It is an electrical meter giving an indication proportional to vehicle speed. And if you think it is calibrated in km/hour when it is actually calibrated in miles/hour, your speed reading will be in error and you may get a ticket. This is akin to an SWR meter being calibrated for the wrong Z0. -- 73, Cecil http://www.qsl.net/w5dxp |
There is no mystery about the 'required additional information'.
The nearest the so-called SWR meter ever gets to measuring anything is the "magnitude of the reflection coefficient", MRC, which arises due to the impedance of whatever is presented to the meter's output terminals. (Recall, this impedance is the 4th variable arm of the meter's RF resistance bridge.) This impedance can have an angle anywhere between 90 and -90 degrees. And the MRC can have an angle in any of the 4 quadrants, ie., anywhere between 0 and 360 degrees. But the meter is capable of indicating ONLY the MRC. All the angle information is lost and gone forever. This is equivalent to losing information about the location along the line of the peaks and troughs in the standing wave. That is, of course, if a long line extending back from the input of the meter to the transmitter actually exists. Now, if the line with standing wave exists, the magnitude of the SWR can be calculated from - SWR = (1 + MRC) / (1 - MRC) or the meter scale can be calibrated in terms of SWR. It is frequently thought the SWR can be used to calculate the power lost in the line. But, particularly when the the line is less than 1/4-wavelength long, this is not so. It requires the location of peaks and troughs to be known - which they are not. It is also thought that by rearranging the equation it is possible to calculate the reflection coefficient from the indicated SWR. Wrong again - can't be done, and in any case the reflection coefficient is useless without an angle. So the indicated SWR is not of much use except to provide a topic of conversation. On the other hand, just by recalibrating the meter scale, you can have a valuable, indispensible TLI. By the way, I hear Californian wine makers have been hijacking the names of French grape-growing districts and have been obliged to re-calibrate their bottles. Ah well, back to the Chilian stuff. ---- Reg. |
On Wed, 28 Sep 2005 04:07:21 +0000 (UTC), "Reg Edwards"
wrote: There is no mystery about the 'required additional information'. Hi Reg, Of course, no one thought so except you - until now, and you still have nothing to offer that distinguishes the probe method from the common SWR meter available to every CB operator. By the way, I hear Californian wine makers have been hijacking the names of French grape-growing districts Boy, are you late in taking in your newspaper. This has been going on since the American vineyards saved the French lines from a devastating rust blight decades ago. There is no original French line that has not been re-planted from American root cuttings for half a century or more after the Germans tilled their soil with Stukas. 73's Richard Clark, KB7QHC |
Reg Edwards wrote:
It is frequently thought the SWR can be used to calculate the power lost in the line. But, particularly when the the line is less than 1/4-wavelength long, this is not so. It requires the location of peaks and troughs to be known - which they are not. In my no-tuner system of tuning, the peaks and troughs are known. The purely resistive current maximum point is always located at the balun/choke. -- 73, Cecil http://www.qsl.net/w5dxp |
Richard Clark wrote:
Boy, are you late in taking in your newspaper. This has been going on since the American vineyards saved the French lines from a devastating rust blight decades ago. There is no original French line that has not been re-planted from American root cuttings for half a century or more after the Germans tilled their soil with Stukas. A lot of those American wine-making families had French roots. :-) -- 73, Cecil http://www.qsl.net/w5dxp |
Reg, I think you're tilting at windmills.
======================================= Dave, First I am called Punchinello, and now Don Quixote is implied. Yet you have repeatedly said "Reg is correct". The only thing I have ever asked is to change the NAME. It is the NAME itself which causes ill-educated IEEE members and befuddled university professors to become old wives. They are reduced to CB-ers who perhaps can be forgiven for being fooled just by a NAME. They actually believe the thing measures SWR on a line which does not exist. Or they find a line which does exist but on which it is impossible for the thing to measure anything because it is located in the wrong place. Their contorted imaginations somehow allow them to argue interminably between themselves but without ever coming to sensible conclusions on which they can agree. The evidence of battles about waves, reflections, re-reflections, virtual reflections, conjugate matches, etc, etc, is littered around these newsgroups. And it's all due to a misnomer. Just change the name of the so-called SWR meter and 50 years of bitter warfare will revert once again to blessed peace and an understanding of how things really work. Sack your lawyers. And if anybody should think I take all this seriously then think again. ;o) ---- Reg, G4FGQ |
Reg Edwards wrote:
Or they find a line which does exist but on which it is impossible for the thing to measure anything because it is located in the wrong place. Reg, the SWR meter may be smarter than you think. Here's an experiment for you. The system is lossless. XMTR--a--1WL 50 ohm--b--1WL 75 ohm--c--1WL 92 ohm--d--load An SWR meter calibrated for 50 ohms will read the SWR on the 50 ohm feedline when installed at points a,b,c, or d. An SWR meter calibrated for 75 ohms will read the SWR on the 75 ohm feedline when installed at points a,b,c, or d. An SWR meter calibrated for 92 ohms will read the SWR on the 92 ohm feedline when installed at points a,b,c, or d. Now Reg, you have to admit that an SWR meter that can read the SWR on the 92 ohm feedline when installed at point 'a' is a darned smart meter. :-) -- 73, Cecil, http://www.qsl.net/w5dxp |
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