Home |
Search |
Today's Posts |
|
#1
|
|||
|
|||
On Thu, 28 Aug 2003 05:58:09 -0500, W5DXP
wrote: Richard Clark wrote: "...the generator impedance is 100+0j ohms, and the line is 5.35 wavelengths long." What does the generator impedance have to do with line losses? Hi Cecil, From Chapman (you following this George?) page 28: "It is reasonable to ask at this point how, for the circuit of Fig. 3-1(b), page 18, on which the above analysis is based, there can be voltage and current waves traveling in both directions on the transmission line when there is only a single signal source. The answer lies in the phenomenon of reflection, which is very familiar in the case of light waves, sound waves, and water waves. Whenever traveling waves of any of these kinds meet an obstacle, i.e. encounter a discontinuous change from the medium in which they have been traveling, they are partially or totally reflected." ... "The reflected voltage and current waves will travel back along the line to the point z=0, and in general will be partially re-reflected there, depending on the boundary conditions established by the source impedance Zs. The detailed analysis of the resulting infinite series of multiple reflections is given in Chapter 8." The Challenge that I have offered more than several here embody such topics and evidence the exact relations portrayed by Chapman (and others already cited, and more not). The Challenge, of course, dashes many dearly held prejudices of the Transmitter "not" having a characteristic source Z of 50 Ohms. Chapman also clearly reveals that this characteristic Z is of importance - only to those interested in accuracy. Those hopes having been dashed is much evidenced by the paucity of comment here; and displayed elsewhere where babble is most abundant in response to lesser dialog (for the sake of enlightening lurkers no less). Clearly those correspondents hold to the adage to choose fights you can win. I would add so do I! The quality of battle is measured in the stature of the corpses littering the field. :-) So, Cecil (George, Peter, et alii), do you have an answer? Care to take a measure at the bench? As Chapman offers, "just like optics." Shirley a man of your erudition can cope with the physical proof of your statements. ;-) The only thing you and others stand to lose is not being able to replicate decades old work. Two resistors and a hank of line is a monumental challenge. 73's Richard Clark, KB7QHC |
#2
|
|||
|
|||
Richard Clark wrote:
So, Cecil (George, Peter, et alii), do you have an answer? Years ago, I had a discussion with Jeff, WA6AHL, here on this newsgroup. I suggested that the impedance looking back into the source might be Vsource/Isource, i.e. the transformed dynamic load line. However, I have never taken a strong stand on source impedance. If reflections are blocked from being incident upon the source, as they are in most Z0-matched systems, the source impedance doesn't matter since there exists nothing to reflect from the source impedance. My basic approach is to achieve a Z0-match and therefore forget about source impedance. -- 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! =----- |
#3
|
|||
|
|||
On Thu, 28 Aug 2003 12:18:44 -0500, W5DXP
wrote: Richard Clark wrote: So, Cecil (George, Peter, et alii), do you have an answer? Years ago, I had a discussion with Jeff, WA6AHL, here on this newsgroup. I suggested that the impedance looking back into the source might be Vsource/Isource, i.e. the transformed dynamic load line. However, I have never taken a strong stand on source impedance. If reflections are blocked from being incident upon the source, as they are in most Z0-matched systems, the source impedance doesn't matter since there exists nothing to reflect from the source impedance. My basic approach is to achieve a Z0-match and therefore forget about source impedance. Hi Cecil, That's all fine and well. It exhibits a rather standard behavior and confirms conventional expectations. I take by this response that you have no interest in the confirmation of interference in both Optical and RF metaphors being visited at the bench. That is fine too. It is a rather tough example to replicate - except when stumbled upon, then we hear cries for exorcism being needed (my cue). My missives simply offer touchstones of clarity in contrast to the murky sea of un-fettered statements. We are presented with fantastic notions that the characteristic source Z of a transmitter is unknowable, and this statement is usually closely allied to the notion that this same "unknowable" Z is actually responsible for reflecting all power arriving at the antenna terminal. Few of those who utter these witless jokes have any response to the straight line "So what is this Z that does all that reflecting?" In their chagrin, they fail even to repeat "it is unknowable...." Absolutely none can venture a guess that it is either: "much less than 50 Ohms," or it is "much more than 50 Ohms." This would be two obvious rejoinders and yet neither is uttered. Such is faith. The universal silence condemns their specious claims absolutely. These absurd notions deserve a hearty laugh, because it invalidates the need for a tuner which is purposely inserted between the source and load to serve that very purpose (and which you describe as your typical habit which is a nearly universal application). But, again, this discussion is generally reserved only for those interested in accuracy. :-) 73's Richard Clark, KB7QHC |
#4
|
|||
|
|||
Richard Clark wrote:
But, again, this discussion is generally reserved only for those interested in accuracy. :-) Like I say, my solution is to block any reflections from being incident upon the source. But I have a question. Since we are discussing coherent sine waves, it seems to me that any reflection from the source impedance will become indistinguishable from the generated wave. In fact, the present convention of generated power equals forward power minus reflected power is designed to overcome that very problem. -- 73, Cecil, W5DXP |
#5
|
|||
|
|||
On Thu, 28 Aug 2003 12:33:53 -0700, W5DXP
wrote: Richard Clark wrote: But, again, this discussion is generally reserved only for those interested in accuracy. :-) Like I say, my solution is to block any reflections from being incident upon the source. But I have a question. Since we are discussing coherent sine waves, it seems to me that any reflection from the source impedance will become indistinguishable from the generated wave. In fact, the present convention of generated power equals forward power minus reflected power is designed to overcome that very problem. Hi Cecil, So you DO want to perform this test? Your presumption of coherency is false unless you engineer the solution. I got there first and made sure that wasn't gonna happen. :-) Any random attempt has only a one in 360 chance of being correct within one degree of coherent. This is simple interference math after all. Most individuals would just notice a 10 degree error which would boost your chances to slightly less than 3% - not very good coherency wise. 73's Richard Clark, KB7QHC |
#6
|
|||
|
|||
On Thu, 28 Aug 2003 12:18:44 -0500, W5DXP
wrote: Richard Clark wrote: So, Cecil (George, Peter, et alii), do you have an answer? Years ago, I had a discussion with Jeff, WA6AHL, here on this newsgroup. I suggested that the impedance looking back into the source might be Vsource/Isource, i.e. the transformed dynamic load line. However, I have never taken a strong stand on source impedance. If reflections are blocked from being incident upon the source, as they are in most Z0-matched systems, the source impedance doesn't matter since there exists nothing to reflect from the source impedance. My basic approach is to achieve a Z0-match and therefore forget about source impedance. Hi Cecil, That's all fine and well. It exhibits a rather standard behavior and confirms conventional expectations. I take by this response that you have no interest in the confirmation of interference in both Optical and RF metaphors being visited at the bench. That is fine too. It is a rather tough example to replicate - except when stumbled upon, then we hear cries for exorcism being needed (my cue). My missives simply offer touchstones of clarity in contrast to the murky sea of un-fettered statements. We are presented with fantastic notions that the characteristic source Z of a transmitter is unknowable, and this statement is usually closely allied to the notion that this same "unknowable" Z is actually responsible for reflecting all power arriving at the antenna terminal. Few of those who utter these witless jokes have any response to the straight line "So what is this Z that does all that reflecting?" In their chagrin, they fail even to repeat "it is unknowable...." Absolutely none can venture a guess that it is either: "much less than 50 Ohms," or it is "much more than 50 Ohms." This would be two obvious rejoinders and yet neither is uttered. Such is faith. The universal silence condemns their specious claims absolutely. These absurd notions deserve a hearty laugh, because it invalidates the need for a tuner which is purposely inserted between the source and load to serve that very purpose (and which you describe as your typical habit which is a nearly universal application). But, again, this discussion is generally reserved only for those interested in accuracy. :-) 73's Richard Clark, KB7QHC |
#7
|
|||
|
|||
On Thu, 28 Aug 2003 12:18:44 -0500, W5DXP
wrote: My basic approach is to achieve a Z0-match and therefore forget about source impedance. Hi Cecil, This is a cavalier attitude if you can afford it. Otherwise, those who so desperately hammer out the last 0.1 dB antenna gain are going to fall to their knees in wrack when they discover that their rig's characteristic Z of, say, 70 Ohms meeting the discontinuity of their low pass filter's 50 Ohms turns that effort into heat behind the antenna jack. I have long since stopped being surprised by those who spin on like whirling dervishes over trivial matters in the face of 10 fold losses in front of them. This, of course, is even more trivial when they gush on about their premium equipment that behind the knobs "efficiently" transforms 20 - 25 Amperes of DC current into 100 Watts RF. Now, that puts perspective to the topic: smoke and reflection coefficient. 73's Richard Clark, KB7QHC |
#8
|
|||
|
|||
Richard Clark wrote:
W5DXP wrote: My basic approach is to achieve a Z0-match and therefore forget about source impedance. This is a cavalier attitude if you can afford it. It's all part of my "Work Smarter, Not Harder" nature. The elimination of reflected energy incident upon the source is extremely rewarding in multiple ways. -- 73, Cecil, W5DXP |
#9
|
|||
|
|||
On Thu, 28 Aug 2003 12:39:25 -0700, W5DXP
wrote: Richard Clark wrote: W5DXP wrote: My basic approach is to achieve a Z0-match and therefore forget about source impedance. This is a cavalier attitude if you can afford it. It's all part of my "Work Smarter, Not Harder" nature. The elimination of reflected energy incident upon the source is extremely rewarding in multiple ways. Hi Cecil, If smarter were hotter, then you could toast bread at 10 feet. Casting back ten watts by burning 20 hardly qualifies for more. 73's Richard Clark, KB7QHC |
#10
|
|||
|
|||
Yes, Richard . . .
Did you mean "Chipman" by chance? That is the author's name . . . -- 73/72, George Amateur Radio W5YR - the Yellow Rose of Texas Fairview, TX 30 mi NE of Dallas in Collin county EM13QE "In the 57th year and it just keeps getting better!" "Richard Clark" wrote in message ... On Thu, 28 Aug 2003 05:58:09 -0500, W5DXP wrote: Richard Clark wrote: "...the generator impedance is 100+0j ohms, and the line is 5.35 wavelengths long." What does the generator impedance have to do with line losses? Hi Cecil, From Chapman (you following this George?) page 28: "It is reasonable to ask at this point how, for the circuit of Fig. 3-1(b), page 18, on which the above analysis is based, there can be voltage and current waves traveling in both directions on the transmission line when there is only a single signal source. The answer lies in the phenomenon of reflection, which is very familiar in the case of light waves, sound waves, and water waves. Whenever traveling waves of any of these kinds meet an obstacle, i.e. encounter a discontinuous change from the medium in which they have been traveling, they are partially or totally reflected." ... "The reflected voltage and current waves will travel back along the line to the point z=0, and in general will be partially re-reflected there, depending on the boundary conditions established by the source impedance Zs. The detailed analysis of the resulting infinite series of multiple reflections is given in Chapter 8." The Challenge that I have offered more than several here embody such topics and evidence the exact relations portrayed by Chapman (and others already cited, and more not). The Challenge, of course, dashes many dearly held prejudices of the Transmitter "not" having a characteristic source Z of 50 Ohms. Chapman also clearly reveals that this characteristic Z is of importance - only to those interested in accuracy. Those hopes having been dashed is much evidenced by the paucity of comment here; and displayed elsewhere where babble is most abundant in response to lesser dialog (for the sake of enlightening lurkers no less). Clearly those correspondents hold to the adage to choose fights you can win. I would add so do I! The quality of battle is measured in the stature of the corpses littering the field. :-) So, Cecil (George, Peter, et alii), do you have an answer? Care to take a measure at the bench? As Chapman offers, "just like optics." Shirley a man of your erudition can cope with the physical proof of your statements. ;-) The only thing you and others stand to lose is not being able to replicate decades old work. Two resistors and a hank of line is a monumental challenge. 73's Richard Clark, KB7QHC |
Reply |
|
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
A Subtle Detail of Reflection Coefficients (but important to know) | Antenna | |||
Re-Normalizing the Smith Chart (Changing the SWR into the same load) | Antenna | |||
Mother Nature's reflection coefficient... | Antenna |