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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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