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#41
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Richard Harrison wrote:
I don`t have a dog in this fight. Neither do I, Richard. The ARRL Antenna Book seems to assume that the loss in a line is proportional to forward power plus reflected power. If reflected power equals zero, then the losses are matched line losses. I posted an example through Google to which nobody responded so I will re-post it he ************************************************** ******************** Here's an example. Assume 100w is delivered to the load for both the matched and unmatched conditions. Assume 3dB matched line loss in the transmission line. Assume an SWR of 5.83:1 (rho=0.707) at the load for the mismatched condition. Forward 200w------------3dB loss-------------Matched Load 100w Forward 400w------------3dB loss-------------Mismatched Load 100w Reflected 50w both Forward 200w directions Reflected 100w The ARRL equations give an additional loss of 5.44dB. ************************************************** ******************** It is obvious (to me) that if there are reflections at the load, then more power must be sourced in order to deliver the same amount of power to the load. But I am pretty simple-minded about obvious things. :-) -- 73, Cecil http://www.qsl.net/w5dxp |
#42
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Cecil, W5DXP wrote:
"Assume 3 dB matched line loss in the transmission line. Assume an SWR of 5.83:1 (rho=0.707) at the load for the mismatched condition." Using 100 watts forward and 50 watts reflected into the "Antenna Book" graph gives an SWR of 5 or 6. If I had my Bird SWR sliderule at hand, I could be more precise without lifting a calculator. According to the "additional line loss graph", this SWR adds about 2 dB to a matched loss of 3 dB for a total loss of about 5 dB with such a mismatch. Best regards, Richard Harrison, KB5WZI |
#43
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Reg Edwards wrote:
For example, in a large number of cases the single output quantity is related to line loss, such as insertion loss in dB, or load power in watts, or transmission efficiency in percent, or percent of input power lost in the line itself. But before this can be calculated it is essential to calculate input impedance Rin+jXin for given attenuation in dB or nepers, given phase shift in radians and given terminating impedance Rt+jXt. Then include generator impedance Rg+jXg and internal generator volts. Having done this you are half-way through. Well Reg, I have a question about transmission line losses expressed in dB. As you know, furnishing line losses in dB implies a power ratio. For a matched line, transmission line losses in dB will be based on the ratio of power not delivered to the load Vs the power being sourced. For instance, if 100 watts is being sourced and 95 watts are accepted by the load, then 5 watts of the 100 watts are line losses, or ~0.21 dB. But to what power are the transmission losses referenced when reflected power is allowed to be incident upon the source. For instance, assume that at the source, we are reading 100 watts forward power and 85 watts reflected power. That means the source is supplying a NET power of 15 watts to the system. If 10 watts of the total NET power of 15 watts is being accepted by the load (implying 5 watts of line losses) does that mean the power ratio for line losses is 33%, or 1.25 dB of losses? Note from the matched line example above where 5 watts of line losses are 0.21 dB to the mismatched line example where 5 watts of line losses are 1.25 dB (for the same forward power of 100w), the same magnitude of loss appears much higher as a dB value referenced to NET source power. -- 73, Cecil http://www.qsl.net/w5dxp |
#44
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On Wed, 01 Dec 2004 10:17:05 -0600, Cecil Moore
wrote: As you know, furnishing line losses in dB implies a power ratio. dB is dimensionless. Such generalizations forced into logic become naive paradoxes: Note from the matched line example above where 5 watts of line losses are 0.21 dB to the mismatched line example where 5 watts of line losses are 1.25 dB (for the same forward power of 100w), the same magnitude of loss appears much higher as a dB value referenced to NET source power. with boundary conditions being violated with the substitution of Mismatch Loss for Dissipative Loss - and done poorly too. When I had you thank Bart for his teachings, you should have examined the consequences of his lesson not simply his correction of your error. |
#45
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Richard Clark wrote:
Cecil Moore wrote: As you know, furnishing line losses in dB implies a power ratio. dB is dimensionless. Such generalizations forced into logic become naive paradoxes: Uhhhhhhh Richard, a power ratio *IS* dimensionless. Any ratio of like quantities is dimensionless. dB is proportional to the log of the ratio of two quantities. That ratio, e.g. of two powers, is dimensionless. Note from the matched line example above where 5 watts of line losses are 0.21 dB to the mismatched line example where 5 watts of line losses are 1.25 dB (for the same forward power of 100w), the same magnitude of loss appears much higher as a dB value referenced to NET source power. with boundary conditions being violated with the substitution of Mismatch Loss for Dissipative Loss - and done poorly too. I certainly didn't intend to do that. The dissipative loss in each of the examples was 5 watts. The NET source power was 100 watts in the matched case and 15 watts in the mismatched case. When I had you thank Bart for his teachings, you should have examined the consequences of his lesson not simply his correction of your error. I'm asking an honest question, Richard. Why can't you provide an honest answer? -- 73, Cecil http://www.qsl.net/w5dxp |
#46
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On Wed, 01 Dec 2004 12:11:24 -0600, Cecil Moore
wrote: I certainly didn't intend to do that. The dog ate your homework? Shoot the dog, that excuse is too common. |
#47
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Richard Clark wrote:
When I had you thank Bart for his teachings, you should have examined the consequences of his lesson not simply his correction of your error. dB was not mentioned at all in Bart's posting so your comment appears to be just an ad hominem shot in the dark. My news- server indicates that Bart has not posted to this thread. Let me ask my question again. Given the dB equation for power, 10*log(P1/P2), when we say the transmission line losses are 1 dB, what is P2 in the denominator if it is not NET source power? There has to be a standard. I just am not sure what that standard is. My question is: What is the standard if not NET source power? The magnitude of power available for being lost in transmission line losses certainly cannot exceed the forward power minus the reflected power, i.e. cannot exceed the NET source power. It seems to follow that P2 in the equation must necessarily be NET power so that transmission line losses are a percentage of the total power available to be lost in transmission line. What am I missing? In the previous examples, the 5 watt loss in the matched case was 5% of the NET source power of 100w. In the mismatched case, the 5 watt loss was 33% of the NET source power of 15w. It seems to follow that the line loss in the matched case is about 0.2 dB of the 100w of power available to be lost while the line loss in the mismatched case is about 1.2 dB of the 15w of power available to be lost. I'm still trying to explain the difference between Bob's dB loss and the ARRL's dB loss. So an obvious question arose: Are they using the same P2 in the denominator of the dB equation? -- 73, Cecil http://www.qsl.net/w5dxp |
#48
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On Wed, 01 Dec 2004 13:12:39 -0600, Cecil Moore
wrote: What am I missing? ... It seems to follow Violation of initial conditions do not allow anything to follow but distorted logic. And the example is poorly done (incorrect). You are missing coming to terms with Bart's lesson - still. |
#49
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Cecil asked,
But to what power are the transmission losses referenced when reflected power is allowed to be incident upon the source. =================================== Cecil, when there are several different power levels at different places in a circuit, it is entirely up to you how you reference one to another in dB. It's just a matter of convenience with the particular problem in hand. But things are certain to become confused and indeterminate when your imagination allows two or more different power levels at the same place at the same time. For example: the so called forward and reflected power levels. Imagination is assisted by the existence of meters which claim to measure these imaginary forward and reflected powers. Actually they do nothing of the sort. They even purport to make measurements on non-existent transmission lines. Reflected volts - yes! Reflected current - yes! Reflected power - NO! (PS : You can't deal with reflections without involving Distance, Velocity, Place and Time.) ---- Reg. |
#50
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Richard Clark wrote:
You are missing coming to terms with Bart's lesson - still. It's a pretty simple question that, so far, no one has answered, not even Bart. dB is always a comparison of one thing to another. To what is the line loss in watts being compared? -- 73, Cecil http://www.qsl.net/w5dxp |
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