Home 
Search 
Today's Posts 
#12




Roy:
[snip] "Roy Lewallen" wrote in message ... One more thing. I've never seen that conjugate formula for voltage reflection coefficient and can't imagine how it might have been derived. I've got a pretty good collection of texts, and none of them show such a thing. [snip] You are absolutely correct Roy, that formula given by "Slick" is just plain WRONG! rho = (Z  R)/(Z + R) Always has been, always will be. Where does that "Slick" guy get his information? And where does "Slick" get off with all of his "potifications"??? I dunno... *He* thinks "Besser", "Pozar" and ARRL are authoritative sources for transmission line technology!!! Me? I have made a living as a professional Engineer designing transmission equipment over the past four decades, currently more than $4BB gross shipped to world wide markets, where the Zo I used is neither real, nor a constant! And what is more... I have never consulted any of those three authorities referenced by "Slick". I certainly don't think of them as being authoritative, "cream skimmers" perhaps, but not certainly not authoritative. I believe that "Slick" has gotta stop "pontificating" and start reading in better circles... much better!  Peter K1PO Indialantic BytheSea, FL. 
#13




William E. Sabin wrote:
Roy Lewallen wrote: A big deal is being made of the general assumption that Z0 is real. As anyone who has studied transmission lines in any depth knows, Z0 is, in general, complex. It's given simply as Z0 = Sqrt((R + jwL)/(G + jwC)) where R, L, G, and C are series resistance, inductance, shunt conductance, and capacitance per unit length respectively, and w is the radian frequency, omega = 2*pi*f. This formula can be found in virtually any text on transmission lines, and a glance at the formula shows that Z0 is, in general, complex. A good approximation to Z0 is: Z0 = R0 sqrt(1ja/b) where Ro = sqrt(L/C) a is matched loss in nepers per meter. b is propagation constant in radians per meter. The complex value of Z0 gives improved accuracy in calculations of input impedance and losses of coax lines. With Mathcad the complex value is easily calculated and applied to the various complex hyperbolic formulas. Reference: QEX, August 1996 Bill W0IYH The usage of complex conjugate Z0* becomes significant when calculating very large values of VSWR, according to some authors. But for these very large values of standing waves, the concept of VSWR is a useless numbers game anyway. For values of VSWR less that 10:1 the complex Z0 is plenty good enough for good quality coax. W.C. Johnson points out on page 150 that the concept: Pload = Pforward  Preflected is strictly correct only when Z0 is pure resistance. But the calculations of real power into the coax and real power into the load are valid and the difference between the two is the real power loss in the coax. For these calculations the complex value Z0 for moderately lossy coax is useful and adequate. The preoccupation with VSWR values is unfortunate and excruciatingly exact answers involve more nitpicking than is sensible. Bill W0IYH 
#14




Roy Lewallen wrote in message ...
Quite a number of the things we "know" about transmission lines are actually true only if the assumption is made that Z0 is purely real; that is, they're only approximately true, and only at HF and above with decent cable. Among them are the three I've already mentioned, the simplified formula for Z0, the relationship between power components, and the optimum load impedance. Yet another is that the magnitude of the reflection coefficient is always = 1. That would be only into a passive network. As people mainly concerned with RF issues, we have the luxury of being able to use the simplifying approximation without usually introducing significant errors. But whenever we deal with formulas or situations that have to apply outside this range, we have to remember that it's just an approximation and apply the full analysis instead. Tom, Ian, Bill, and most of the others posting on this thread of course know all this very well. We have to know it in order to do our jobs effectively, and all of us have studied and understood the derivation and basis for Z0 calculation. But I hope it'll be of value to some of the readers who might be misled by statements that "authorities" claim that Z0 is purely real. Roy Lewallen, W7EL No one ever said that Zo is always purely real. But many texts do approximate it this way. Even the ARRL "bible". Slick 
#15




So another way for the lurkers to check all this: assume a line Zo =
50j5, and a load Zload = 1+j100. Assume some convenient Vf at the load. Calculate rho = Vr/Vf from the equation quoted below. Now find Vr, and from the line impedance and Vf and Vr, find If and Ir. Add the V terms and I terms to get the net line voltage and current at the load. Does that correspond to the expected load current for the given Zload? If so, fine; if not, where does the difference in current come from? If you assume the line current is correct from your If and Ir calcs, and the load current is correct as the net line voltage = net load voltage, and use Zload to get Iload, does the line power dissipation plus the load power dissipation equal the power fed in from a generator? Try all those calcs after revising the Vr/Vf formula to match what Besser is now teaching, and see if things line up a bit better. The truth is all there to be seen with just a bit of work. Cheers, Tom (yeah, I've done it, as you might guess. And so have a lot of others.) (Dr. Slick) wrote in message . com... Actually, my first posting was right all along, if Zo is always real. From Les Besser's Applied RF Techniques: "For passive circuits, 0=[rho]=1, And strictly speaking: Reflection Coefficient = (ZloadZo*)/(ZloadZo) Where * indicates conjugate. But most of the literature assumes that Zo is real, therefore Zo*=Zo." And then i looked at the trusty ARRL handbook, 1993, page 162, and lo and behold, the reflection coefficient equation doesn't have a term for line reactance, so both this book and Pozar have indeed assumed that the Zo will be purely real. That doesn't mean Zload cannot have reactance (be complex). Try your calculation again, and you will see that you can never have a [rho] (magnitude of R.C.)greater than 1 for a passive network. How could you get more power reflected than what you put in (do you believe in conservation of energy, or do you think you can make energy out of nothing)? If you guys can tell us, we could fix our power problems in CA! But thanks for checking my work, and this is a subtle detail that is good to know. Slick 
#16




(Dr. Slick) wrote in message . com...
(Tom Bruhns) wrote in message om... Fascinating... Please have a look at the following reply I got from Besser... I still wish people would go through the simple math themselves, and make up their own minds what's correct and what isn't. I gather that Slick has made up his own mind, though see no evidence that it's on the basis of the simple calcs from what I believe he already agrees with. Oh, well, not MY problem. (This is twice now, recently, that I've followed up on other people's references and found them to be at best questionable in some way.) I have no problem admitting i am wrong, when i am wrong. But you haven't given me any reason to think so. Well, you may not think I have, but... What is your definition of a conjugate match? When do you think max. power transfer occurs? I'd be happy to answer this more directly after you show us the steps, as I suggested, to get from the basic TEM transmission line relations and the load boundary conditions to Vr/Vf. But for now, I'll let you consider, if you wish, the case where you have a long transmission line with reactive Zo, terminated so you have no reflected wave. Rho = 0. SWR = 1:1. I trust you'll agree you "see" an impedance equal to Zo looking into the sourceend of the line. Now imagine that you have cut this line at some point; you also see Zo looking into that cut, right? (The side with the load attached, that is.) So, can you simply connect those two pieces back up and still see no reflection on the piece on the source side? I _do_ believe that the line can't tell whether the impedance it's connected to is a load right there, or the impedance presented by another length of line, so it should be obvious from that what I believe the line must be connected to, to get rho=0. Cheers, Tom 
#17




"Peter O. Brackett" wrote in message nk.net...
Roy: [snip] "Roy Lewallen" wrote in message ... One more thing. I've never seen that conjugate formula for voltage reflection coefficient and can't imagine how it might have been derived. I've got a pretty good collection of texts, and none of them show such a thing. [snip] You are absolutely correct Roy, that formula given by "Slick" is just plain WRONG! rho = (Z  R)/(Z + R) Always has been, always will be. As long as they are all purely real. Roy disagrees even when he is wrong, because too many people read this NG, and it might make him look bad (i.e., Not the AllKnowing Guru he pretends to be). Where does that "Slick" guy get his information? And where does "Slick" get off with all of his "potifications"??? I dunno... *He* thinks "Besser", "Pozar" and ARRL are authoritative sources for transmission line technology!!! Bwa! HAah! Much, much, MUCH more than you will ever be! Me? I have made a living as a professional Engineer designing transmission equipment over the past four decades, currently more than $4BB gross shipped to world wide markets, where the Zo I used is neither real, nor a constant! I feel sorry for your customers... And what is more... I have never consulted any of those three authorities referenced by "Slick". I certainly don't think of them as being authoritative, "cream skimmers" perhaps, but not certainly not authoritative. Dr. Besser kicks your ass backwards when it comes to RF knowledge. And the ARRL is extremely well known. Pozar not so much, but the guy is out there on the PhD level. I don't give a Sh** who you think is an authority. Look them up, they have way more credentials than either you or I. I believe that "Slick" has gotta stop "pontificating" and start reading in better circles... much better! Much better than the likes of you, then yes, you would certainly be correct! The conjugate formula is correct. If you believe in cancellation of reactance. Why else would the magnitude rho (numerator of Reflection Coefficient) be zero when Zload=Zo*??? Slick 
#18





#19




Seems to be, indeed, though you never know. The lurkers may well have learned a thing or two. The one reference you did post now disavows the form you posted. You've been invited to do some simple math that would show you the truth and you apparently refuse. You are posting any number of ideas contrary to what's easy to show from fundamentals and what's in a large number of published papers and texts and what has been posted here by many contributors recently and over the years. It's been done with both symbolic math and specific examples. Several inconsistencies demonstrate clearly that Vr/Vf does NOT equal (ZloadZo*)/(Zload+Zo), and of course most certainly a (ZloadZo) denominator is going to get you quickly into trouble. The inconsistencies have been pointed out here by me and by others, but apparently you've missed them. I'm sure it's apparent to most lurking where the communications is breaking down. But the formulas you posted above have given me a good laugh tonight, at least! Thanks! Cheers, Tom When a fine engineer, with a good education and a distinguished career, stoops to argue with an anonymous fellow who doesn't have a firm grasp of even the most basic ideas of wave mechanics, the result is bound to be a certain amount of frustration. You might want to ask yourself, Tom, whether Slick is arguing in good faith, or whether he has other motives. 73, Tom Donaly, KA6RUH 
#20




Slick:
[snip] [snip] You are absolutely correct Roy, that formula given by "Slick" is just plain WRONG! rho = (Z  R)/(Z + R) Always has been, always will be. [snip] After consideration, I must agree with Slick. Slick is RIGHT and I was WRONG! Slick please accept my apologies!!! I was wrong, and I admit it! Indeed, the correct formula for the voltage reflection coefficient "rho" when computed using a "reference impedance" R, which is say the, perhaps complex, internal impedance R = r + jx of a generator/source which is loaded by a perhaps complex load impedance Z = ro + j xo must indeed be: rho = (Z  conj(R))/(Z + conj(R)) = (Z  r + jx)/(Z + r  jx) For indeed as Slick pointed out elsewhere in this thread, how else will the reflected voltage equal zero when the load is a conjugate match to the generator. Slick thanks for directing the attention of this "subtlety" to the newsgroup, and again... Slick, please accept my apologies, I was too quick to criticize! Good work, and lots of patience... :) Regards,  Peter K1PO Indialantic BytheSea, FL. 
Reply 
Thread Tools  Search this Thread 
Display Modes  

