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#21
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"Jimmy" wrote in message . com...
This means your shield has RF on it . Also triming cable has nothing to do with SWR. In theory, yes. It would be on the constant VSWR circles, assuming 50 Ohm transmission line! Trimming the cable WILL have something to do with the series reactance that the PA will see, and may affect the impedance match. Slick |
#22
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#23
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My 2 cents (wont even buy a lolipop any more!) From the old VHF,er
magazine, couple of thoughts: 1) Very lossey coax will show a prefect termination impedence even if open, or shorted, if sufficiant length is used (for the coax's attenuation ) . For that reason, still keep a coil of RG-8 size coax (for power dissipation), with RUBBER DIALECTRIC (was origianlly made for attenuators at the Microwave Region) Even at 80 meters, with no load , has very low SWR! At 10 meters, and up, with a UHF connector on one end, and a N connector, on the other, with NO TERMINATION, makes a great dummy load to 2-300 watts!. and, 2) Just because you think you have a great match, can be a random event. to test , place a 1/4 wavelength piece of coax (figuring with velocity factor). If the SWR stays flat, you can assume a good match, if, howerer, the SWR climbs, then you need to check for a proper match ( this will throw the worst condition into your coax , with this extra length, kinda like the opposite of trimming coax for the best match legends of old CB lore)! as info, Jim NN7K "Tom Bruhns" wrote in message m... (Dr. Slick) wrote in message . com... ... What do you mean by "calibrated to the line"? The SWR meter should read zero reflected power when connected to a load whose impedance is equal to the line's. Does it? If not, it's not properly calibrated. Putting it another way, what's the directionality of the bridge? ... Not so much a surprise as a disappointment! A difference of 70 watts incident power is totally unacceptable with only 8 feet of coax length added. But the load presented to the amplifier is totally different in the two cases, most likely. Only if the amplifier's output impedance were the complex conjugate of the line's, and the line were lossless, and the amplifier behaved as a linear time-invariant system would you (should you) expect the power to remain unchanged. The transmitter does not have a mismatch sensor on it for reducing power at high SWR. It naturally does output less wattage as the transconductance is reduced with higher temperatures, but my measurements were done in close proximity to each other. Exactly how would you measure the output impedance (S22) of a PA? Drive it with a small signal (class A, linear mode) in one frequency (F1), and then inject another frequency (F2) into the output, and then measure the reflected F2 power? If i remember correctly, that's how someone was trying to measure S22 at my former place of employment. But all bets are off as soon as you go non-linear, or non-class A. S-parameters are supposed to be all small signal. I don't think you can properly measure the S22 of a class C PA. Tam brought up the suggestion that i try a really long piece of RG-58 from meter to dummy load, to make the Cantenna more like 50 Ohms. Gonna try it. Or just tune the load to zero reflected power after you're sure the meter is properly calibrated...but that can be a catch-22 situation that the really long RG-58 can help with. Be aware, though, that "50-ohm" line seldom is -- it can be off 5 ohms or more. It's a cause of some consternation to those of us involved in calibration of precision RF test equipment. Load-pull techniques are commonly used to characterize RF source impedances. You make known incremental changes to the load, and deduce from the change in output power what the source impedance is, assuming it's a linear time-invariant system. Note that adding length to a mismatched line is one way to make an incremental change to the load... I wouldn't necessarily say that the output impedance of a class C amplifier is meaningless, but it may well not be constant for all loads. and may depend on parameters you'd have trouble controlling from day to day. Cheers, Tom |
#24
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Tom
He is trying to measure the output power of an amplifier. His meter is calibrated for 50 Ohms; so, he needs a 50 Ohm load. Tam/WB2TT |
#26
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#27
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Almost correct.
The transmitter output impedance has no effect whatsoever on the line's SWR. Roy Lewallen, W7EL Dr. Slick wrote: W5DXP wrote in message ... Jimmy wrote: I hope this is a wifes tale that has long been discredited. Trimming cable changes the impedance seen by the transmitter with no change to SWR. However, a 50 ohm SWR meter reading will change as one changes the length of the ladder-line. When the 50 ohm SWR meter reads 1:1, the transmitter is seeing 50 ohms. That's how I tune my dipole. This is very interesting, as it may indicate that the "wife's tale" may have some validity in _some_ cases. For example, if your antenna happens to have an impedance of say, 60 Ohms, and your transmission line happens to NOT be a perfect 50 Ohms (usually the case), and happens to be around 54.8 Ohms or so... then at odd integer multiples of 1/4 wavelength (velocity factor included, of course), you will be right on 50 Ohms. Additionally, as we have mentioned before, PAs rarely have 50 Ohm output impedances anyways (who actually measures this??). So an impedance transform via perfect 50 ohm coax length may indeed get you closer to a conjugate match and higher incident power, which may improve your VSWR (which is based upon the incident and reflected, of course). Slick |
#29
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On Tue, 12 Aug 2003 02:48:49 -0700, Roy Lewallen
wrote: Almost correct. The transmitter output impedance has no effect whatsoever on the line's SWR. Roy Lewallen, W7EL Hi Roy, Entirely incorrect. Transmitter output impedance that does not conform to transmission line Z, when presented with a mismatched load through that line, adds mismatch uncertainty in the form of an indeterminate SWR and indeterminate Power to the load. This has already been demonstrated twice. This has long been documented with NBS/NIST references going back 4 decades. There is nothing mysterious about it at all, and it conforms to the rather simple principles of wave interference so poorly presented by Cecil in months past. The authoritative site: http://www.boulder.nist.gov/div813/index.html Direct reference: "Juroshek, J. R.; A Direct Calibration Method for Measuring Equivalent Source Mismatch; Microwave J., pp. 106-118; October 1997 Obscure references: http://www.boulder.nist.gov/div813/r...00S_n2nNet.pdf "With vector measurements of the generator and meter reflection coefficients Ãg and Ãm, respectively, the power of the incident signal am can be related to the power of the source." http://www.boulder.nist.gov/div813/r...FRad_ARFTG.pdf which describes radiometer calibration (perhaps too exotic for this group) "tests are based on two assumptions. First, the network responds linearly to our signal ( no power compression), and second, the radiometer is sufficiently isolated from the source impedance." ... "One of the assumptions made in deriving eq. (2) was that the output from the radiometer is not dependent on the source impedance. In the construction of the radiometer, two isolators are inserted at the input of the radiometer to isolate the radiometer from the source." ... "The mismatch uncertainty depends strongly on the poorly known correlation between uncertainties in the measurements of different reflection coefficients, and so we use the maximum of the uncertainties obtained by assuming either complete correlation or no correlation whatsoever." "Forthcoming Paper: Influence of Impedance Mismatch Effects on Measurements of Unloaded Q Factors of Transmission Mode Dielectric Resonators" IEEE Transaction on Applied Superconductivity "Analysis of Interconnection Network and Mismatch in the Nose-to-Nose Calibration Automatic RF Techniques Group , June 15-16, 2000 , Boston, MA - June 01, 2000 "We analyze the input networks of the samplers used in the nose-to-nose calibration method. Our model demonstrates that the required input network conditions are satisfied in this method and shows the interconnection errors are limited to measurement uncertainties of input reflection coefficients and adapter S-parameters utilized during the calibration procedure. Further, the input network model fully includes the effects of mismatch reflections, and we use the model to reconcile nose-to-nose waveform correction methods with traditional signal power measurement techniques." As I mentioned, obscure references. However, given the impetus of their discussion is long known (and that I have already provided the original references they rely on), NIST presumes the investigators already have that basis of knowledge. 73's Richard Clark, KB7QHC |
#30
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He sed:
"The transmitter output impedance has no effect whatsoever on the line's SWR." --------- THIS IS ABSOLUTELY CORRECT! 73 Jack |
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