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#1
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**** Post for FREE via your newsreader at post.usenet.com ****
"Cecil Moore" ...a two foot long section of 50 ohm coax is all the length needed to force the V/I ratio to be 50 ohms at HF... Of course, this is only true (in the practical sense) for that brief interval until any reflections arrive back at the point where the measurements are being made and all hell breaks loose. It is very obviously all tied into the meaning of 'characteristic impedance' - there's no mystery here. Semantics. There is often miscommunication(*) about the distinction between the initial period (before the reflections arrive) and the steady state mess that arises further along the time axis. *These can be easily identified - even defined - as any thread that includes more than about 20 postings by Cecil. ;-) -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= *** Usenet.com - The #1 Usenet Newsgroup Service on The Planet! *** http://www.usenet.com Unlimited Download - 19 Seperate Servers - 90,000 groups - Uncensored -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= |
#2
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Cecil, W5DXP wrote:
"---a two foot long section of 50 ohm coax is all the length needed to force the V/I ratio to be 50 ohms at HF---" At 3 MHz? When power is applied to a transmission line, energy from the power source doesn`t appear everywhere along the line at once. Instead, energy travels away from the source in the form of an EM wave called the "incident wave" arriving at various spots along the line in order and at sequential times.The time it takes to travel through each line segment depends on the four properties of the line, series resistance (R), series inductance (L), shunt capacitance (C), and shunt conductance (G). Source current will start charging the shunt capacitance of the first line segment. It is delayed by the series inductance and resistance of the first segment. Resistance does not directly delay current, but limits current to the capacitace. As the shunt capacitance is charged, the charging current tapers, but the next line segment starts charging through its series inductance and resisitance. This energy travel process continues sequentially throughout the line. The value of current in an infinite line is the line voltage divided by the line`s Zo. In a line with reflection, the current in each direction is the voltage motivating the current in thet direction divided by Zo. Just how short can a transmission line be and still enforce its Zo? A 1/4-wave matching section inverts impedance between its ends by enforcing its Zo. For Zo to equal the square root of L/C, (a resistance), XL must be much greater than R, and XC must be much greater than G. These restrictions impose frequency limits on Zo. And, these restrictions may place a low frequency limit on how short a line can be and still enforce Zo. Best regards, Richard Harrison, KB5WZI |
#3
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"Richard Harrison" wrote
Cecil, W5DXP wrote: "---a two foot long section of 50 ohm coax is all the length needed to force the V/I ratio to be 50 ohms at HF---" At 3 MHz? When power is applied to a transmission line, energy from the power source doesn`t appear everywhere along the line at once. ( much clippage) Just how short can a transmission line be and still enforce its Zo? A 1/4-wave matching section inverts impedance between its ends by enforcing its Zo. For Zo to equal the square root of L/C, (a resistance), XL must be much greater than R, and XC must be much greater than G. These restrictions impose frequency limits on Zo. And, these restrictions may place a low frequency limit on how short a line can be and still enforce Zo. ______________ For a concept of what that length actually is in the real world, recall that Bird Corp and others supply directional wattmeters giving reasonably accurate measurement of forward and reflected power -- leading to an SWR value. The coax sampling sections for RF frequencies at least as low as 540 kHz. is around 9" in length. RF |
#4
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Richard Fry wrote:
"For a concept of what that length actually is in the real world, recall that Bird Corp. and others supply directional wattmeters giving reasonably accurate measurement of forward and reflected power -- leading to SWR value." True, and these work with mismatched loads if you have enough 50-ohm cable connecting the wattmeter. The Bird Model 43 wattmeter is 5.125 inches (13 cm) wide. This is the distance between its input and output connectors. This length of "high precision 50 ohm coaxial air line designed for insertion between the transmitter or load" requires either some more 50-ohm line or a matched load to enforce Zo. IF you were to insert the Model 43 into most 75-ohm transmission systems, the precision 50-ohm meter line of 5.125 inches would not likely enforce the 50-ohm V/I ratio and the meter reading would be in error. At VHF, 1/2-wave of connecting line including the Model 43 wattmeter is ideal, allowing you to insert and withdraw the meter without affecting the match. Best regards, Richard Harrison, KB5WZI |
#5
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"Richard Harrison" wrote
IF you were to insert the Model 43 into most 75-ohm transmission systems, the precision 50-ohm meter line of 5.125 inches would not likely enforce the 50-ohm V/I ratio and the meter reading would be in error. ________________ Yet a 50 ohm RF bridge or network analyzer with a 75 ohm termination applied directly at its output port has no trouble showing the true SWR. These measuring devices are looking at the same transition plane from 50 to 75 ohms as the Bird 43 would see with a 75 ohm load at its output port. If the Model 43 is unable to make an accurate measurement of this, is that not due to reasons other than not having the right 50-ohm V/I ratio in its line section? RF |
#6
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Richard Fry wrote:
"If the Model 43 is unable to make an accurate masuremnt of this, is that not due to reasons other than not having the right V/I ratio in its line section?" Many details of desisn, construction, and application must be chosen and executed right to get accuracy, but the line impedance is essential. Bird can adjust the current sample to exactly equal the voltage sample, both taken from the transmission line at any point. But it must work with a fixed voltage to current ratio. Bird chose 50 onms. For a directional meter, it`s necessary to respond to one direction while rejecting the other. When power is applied to a line, the resulting current is is in phase with the volts. On reflection, the volts and amps in the reflected wave are 180 degrees out of phase. The phase difference of the reflected wave is used by Bird to distinguish it from the incident wave. By selecting and adjusting for equal samples of volts and amps in the forward wave, their total is 2X that of either sample. But, the samples from the reflected wave, being equal but out of phase, cancel. To get the value of the reflected power samples, it is only necessary to reverse the polarity of one of the samples. They are now in phase and the forward power samples are now out of phase and cancel. If some other voltage to current ratio is used for the power samples than that of the design, the samples won`t be exactly equal and cancellation of the undesired direction does not work. Best regards, Richard Harrison, KB5WZI |
#7
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Richard Fry wrote:
The coax sampling sections for RF frequencies at least as low as 540 kHz. is around 9" in length. The guys over on s.p.e said it has something to do with conductor spacing Vs conductor length. They said a 100/1 ratio is plenty long enough for Z0 to assert itself. -- 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! =----- |
#8
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"Richard Harrison" Just how short can a transmission line be and still enforce its Zo? The whole thing is perfectly clear if one imagines applying a step function (rising edge) to any short, even VERY short, length of transmission line. The current in the short line will be equal to V/Zo - at least until the reflections (if any) start arriving back at the input. If the line happen to be terminated with Zo, then no reflections and I=V/Zo is the steady state. The only issue of shortness is that a very short line means very short time until the reflections arrive. The step function makes things a lot easier to understand than RF. It 'enforces' the distinction between the transient period and steady state. -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= *** Usenet.com - The #1 Usenet Newsgroup Service on The Planet! *** http://www.usenet.com Unlimited Download - 19 Seperate Servers - 90,000 groups - Uncensored -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= |
#9
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On Fri, 3 Sep 2004 17:16:48 -0300, "Another Voice" wrote:
**** Post for FREE via your newsreader at post.usenet.com **** "Richard Harrison" Just how short can a transmission line be and still enforce its Zo? The whole thing is perfectly clear if one imagines applying a step function (rising edge) to any short, even VERY short, length of transmission line. The current in the short line will be equal to V/Zo - at least until the reflections (if any) start arriving back at the input. If the line happen to be terminated with Zo, then no reflections and I=V/Zo is the steady state. The only issue of shortness is that a very short line means very short time until the reflections arrive. The step function makes things a lot easier to understand than RF. It 'enforces' the distinction between the transient period and steady state. IMO, the length of the line is irrelevant when using a device such as the Bruene bridge or a Bird 43. Each of those instruments are designed or adjusted to indicate the forward or reflected power, based on three things: 1) ratio of the foward and reflected voltages, the voltage reflection coefficient 2) the scale numbered from 0 to 1, where 0 indicates the reflection is zero, and 1 equals total reflection, but the significant point is that a 3:1 mismatch gives a reflection coefficient of 0.5, which then means that the half-scale reading of 0.5 indicates the 3:1 mismatch, or a 3:1 SWR, and 3) the device is so designed or adjusted so that the voltage ratios indicate the correct value because it's inherent characteristic impedance, Zo, is 50 ohms. Thus, no transmission line is necessary. For example, the device can be connected directly to the antenna terminals, or any other device you desire to determine the mismatch, and power it directly from the signal source--no transmission line is needed on either port for the device to indicate the degree of mismatch. Walt, W2DU |
#10
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"Walter Maxwell" wrote in message ... On Fri, 3 Sep 2004 17:16:48 -0300, "Another Voice" wrote: **** Post for FREE via your newsreader at post.usenet.com **** "Richard Harrison" Just how short can a transmission line be and still enforce its Zo? The whole thing is perfectly clear if one imagines applying a step function (rising edge) to any short, even VERY short, length of transmission line. The current in the short line will be equal to V/Zo - at least until the reflections (if any) start arriving back at the input. If the line happen to be terminated with Zo, then no reflections and I=V/Zo is the steady state. The only issue of shortness is that a very short line means very short time until the reflections arrive. The step function makes things a lot easier to understand than RF. It 'enforces' the distinction between the transient period and steady state. IMO, the length of the line is irrelevant when using a device such as the Bruene bridge or a Bird 43. Each of those instruments are designed or adjusted to indicate the forward or reflected power, based on three things: 1) ratio of the foward and reflected voltages, the voltage reflection coefficient 2) the scale numbered from 0 to 1, where 0 indicates the reflection is zero, and 1 equals total reflection, but the significant point is that a 3:1 mismatch gives a reflection coefficient of 0.5, which then means that the half-scale reading of 0.5 indicates the 3:1 mismatch, or a 3:1 SWR, and 3) the device is so designed or adjusted so that the voltage ratios indicate the correct value because it's inherent characteristic impedance, Zo, is 50 ohms. Thus, no transmission line is necessary. For example, the device can be connected directly to the antenna terminals, or any other device you desire to determine the mismatch, and power it directly from the signal source--no transmission line is needed on either port for the device to indicate the degree of mismatch. Walt, W2DU Walt, I hope people are listening to what you are saying. I built up a Bruene meter in SWCAD using 0% tolerance components and other ideal parts. Works exactly like Bird claims their meter does, except that the error only depends on the PC floating point arithmetic. Transmission line or not makes no difference. BTW, it is kind of neat to see the directional coupler properties, by driving the two sides with different signals, and then being able to separate them. Tam/WB2TT |
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