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#22
August 24th 09, 09:12 PM
posted to rec.radio.amateur.antenna
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MFJ-269 Antenna Analyzer experience
Michael Coslo wrote:
wrote: The 4170 makes this a lot easier as you can measure the feedline actual parameters as well as calibrate out their effects. This is a dumb question on my part, but what you are saying is that the mitigating effects that the cable has on the VSWR, making it look better in general, can not only be calculated and "calibrated out", but that the actual SWR of your antenna at the feedpoint is then given? As you get closer to 1.1:1 at the actual antenna, would accuracy then suffer? If feedline loss can bring an antenna that is not near that to a level approaching that, wouldn't it mean that teh calibration is somewhere in the noise? Like I say, this could be a really stoopid question. - 73 de Mike N3LI - Not at all. Imagine that you have a very lossy line. You'll read very nearly the cable Z0 regardless of what's at the other end. Extreme changes in far-end impedance will make very little difference at the input end, so it's impossible to tell with any accuracy what's at the far end by looking at the near-end impedance. Another case to consider is one where the Z0 of the cable is very different than the Z of the load. In that case, a tiny change in line Z0, length, or loss changes the input Z for a given load Z. It can be impossible to measure the line length, impedance, or loss with sufficient accuracy to find the far end impedance with decent accuracy. This doesn't mean you can't get measurements good enough for amateur or even professional use. But on the other hand, your measurements can be total garbage in spite of your cable measurements if you fail to realize just how sensitive the result can be to small errors. A careful experimenter will do a sensitivity analysis which tells how large an error in results is caused by an error in measuring the feedline or in the input impedance measurement, then the probable measurement errors are estimated to determine the probable error in the calculated result. While a mathematical sensitivity analysis is the rigorous way to do this, something like N6BV's TLW program is just fine for most amateur purposes. Or, if you're using one of the instruments that does the calculation for you, try telling it the line is a few percent longer or shorter, or has a Z0 or loss a few percent different from what it said or you measured. See how much it changes the result. If the change is small, no problem. But if it's large, it means that extreme care and maybe some other techniques have to be used to get a good measurement. Let me give an example, done with TLW. Suppose we're measuring the impedance of an antenna at 30 MHz through 100 feet of RG-8x. TLW gives these nominal values for RG-8x: Z0 - 50.2 - j0.47 Velocity factor - 0.8 Loss - 1.926 dB/100' And suppose that these are exactly what our measurement of the cable said. We measure 21 + j20 at the input end, and conclude that the impedance of the antenna is 374 - j84 ohms. But suppose the measurement at the input end was inaccurate by about 5%, and that the actual input end Z was 21 + j20. Then the load Z is 322 - j105, about 15% off in R, 25% in Z. Or maybe the cable measurement was off by just 1%, and the cable is really 101 and not 100 feet long. In that case, the antenna Z is really 129 + j166 ohms. We're even on the other side of resonance from where we thought. Or maybe the velocity factor was rounded a bit and it's really closer to 0.81 than 0.8. How much difference would that small error make? Well, the antenna Z would be 53 - j120 ohms with our input measurement of 21 + j20! So, what's the real antenna impedance? 374 - j84, 322 - j105, 129 + j166, or 53 - j120? You're fooling yourself if you think you really know. It's easy to get lulled into believing that just because we read a value to six decimal places, it's accurate. But you're usually doing very well to get within a few percent in spite of all those digits. And when that few percent results in a much bigger error in calculated results, it's even more important to realize the limitations of your accuracy. Roy Lewallen, W7EL |
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