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MFJ-269 Antenna Analyzer experience
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snip Basically what you do is calibrate the instrument at the measurement point, whether that point is the instrument connector or at the end of a length of coax. You attach an open, a short and a known resistance; 50 ohms by default but it is user definable. The instrument than frequency sweeps and stores the results in a user definable calibration file. When you make a measurement of an unknown, you define which calibration file to use and the instrument corrects the readings to display the characteristics at the measurement point. Given that this is a $500 insturment and not a $20,000 labratory instrument there are going to be limits to how accurate all this is. When I inspected antennas, we had two multi-kilobuck "Site Master" instruments from Anritsu, mentioned here, that had a set of calibrated terminations. IIRC, to calibrate the unit(s), we had to connect the terminations, a short, a 50-ohm resistor and a shielded open circuit, one at a time, to the instrument and tell it which one was connected. It swept the frequencies of interest and stored its own baseline behavior over that band of interest. Then, anything connected to it was referenced to that baseline. We could also store a range of sweep frequencies (usually by the name or type of antenna we intended to sweep) and it would recall all the parameters. Automated, repeatable sweep testing is not available (yet) in lower cost instruments. I presume we could have calibrated any given cable, too. (Never required by our test memos.) Sal |
MFJ-269 Antenna Analyzer experience
Sal M. Onella wrote:
. snip Basically what you do is calibrate the instrument at the measurement point, whether that point is the instrument connector or at the end of a length of coax. You attach an open, a short and a known resistance; 50 ohms by default but it is user definable. The instrument than frequency sweeps and stores the results in a user definable calibration file. When you make a measurement of an unknown, you define which calibration file to use and the instrument corrects the readings to display the characteristics at the measurement point. Given that this is a $500 insturment and not a $20,000 labratory instrument there are going to be limits to how accurate all this is. When I inspected antennas, we had two multi-kilobuck "Site Master" instruments from Anritsu, mentioned here, that had a set of calibrated terminations. IIRC, to calibrate the unit(s), we had to connect the terminations, a short, a 50-ohm resistor and a shielded open circuit, one at a time, to the instrument and tell it which one was connected. It swept the frequencies of interest and stored its own baseline behavior over that band of interest. Then, anything connected to it was referenced to that baseline. We could also store a range of sweep frequencies (usually by the name or type of antenna we intended to sweep) and it would recall all the parameters. Automated, repeatable sweep testing is not available (yet) in lower cost instruments. The $600 TenTec/TAPR VNA does open/short/thru/load calibration with sweeps, etc. I don't have the AIM, but I'll bet it does too. These days, it's not a big deal to include it. |
MFJ-269 Antenna Analyzer experience
"Jim Lux" wrote in message ... Sal M. Onella wrote: . snip Basically what you do is calibrate the instrument at the measurement point, whether that point is the instrument connector or at the end of a length of coax. You attach an open, a short and a known resistance; 50 ohms by default but it is user definable. The instrument than frequency sweeps and stores the results in a user definable calibration file. When you make a measurement of an unknown, you define which calibration file to use and the instrument corrects the readings to display the characteristics at the measurement point. Given that this is a $500 insturment and not a $20,000 labratory instrument there are going to be limits to how accurate all this is. When I inspected antennas, we had two multi-kilobuck "Site Master" instruments from Anritsu, mentioned here, that had a set of calibrated terminations. IIRC, to calibrate the unit(s), we had to connect the terminations, a short, a 50-ohm resistor and a shielded open circuit, one at a time, to the instrument and tell it which one was connected. It swept the frequencies of interest and stored its own baseline behavior over that band of interest. Then, anything connected to it was referenced to that baseline. We could also store a range of sweep frequencies (usually by the name or type of antenna we intended to sweep) and it would recall all the parameters. Automated, repeatable sweep testing is not available (yet) in lower cost instruments. The $600 TenTec/TAPR VNA does open/short/thru/load calibration with sweeps, etc. I don't have the AIM, but I'll bet it does too. These days, it's not a big deal to include it. Thanks for bringing me into the present. I have an analyzer already. When I drop it off the roof (inevitable), I will look at the TenTec/TAPR VNA. |
MFJ-269 Antenna Analyzer experience
Roy Lewallen wrote:
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 322 - j105 ohms. But suppose the measurement at the input end was inaccurate by about 5%, and that the actual input end Z was 22 + j21. Then the load Z is 273 - j125, about 15% off in R, 20% in X. . . Thanks Roy, My knowledge of the matter is at the noobie level, but given that loss eventually gets you reading only the Z0 of the cable, it's what I suspected. I hadn't thought about a big mismatch between the cable and antenna, so there's another data point. - 73 de Mike N3LI - |
MFJ-269 Antenna Analyzer experience
In article , Michael Coslo
wrote: My knowledge of the matter is at the noobie level, but given that loss eventually gets you reading only the Z0 of the cable, it's what I suspected. I hadn't thought about a big mismatch between the cable and antenna, so there's another data point. - 73 de Mike N3LI - Hello, and I ran into this issue years ago when trying to measure high ( 10 or 20) VSWR loads (in this case out-of-band shipboard HF antenna feedpoint impedance) connected via a length of transmission line. Accurate determination of load resistance is difficult to ascertain under these conditions. A 1953 AIEE (a progenitor of the IEEE) paper by W.W. Macalpine, "Computation of Impedance and Efficiency of Transmission Lines with High Sanding Wave Ratio" describes the problem. I was, however, able to obtain accurate results when I included the small imaginary part (frequency dependent) of the characteristic impedance that is present in a low-loss line. Outside the high VSWR load issue the imaginary part can be ignored. Sincerely, and 73s from N4GGO, John Wood (Code 5550) e-mail: Naval Research Laboratory 4555 Overlook Avenue, SW Washington, DC 20375-5337 |
MFJ-269 Antenna Analyzer experience
In article , Owen Duffy
wrote: John, If I understand correctly, Macalpine's work was in finding solutions given the computation tools readily available. The approximation of a real transmission line as having Zo that is real can introduce significant error in some line calcs. Improving the model to include an estimated X value based on the loss is a next step that improves calcs. This technique is widely used, and is revealed by Ro equal to the nominal Zo, whilst Xo is a small negative quantity for practical low loss cables at HF. This model produces significantly different results for high line VSWR at lower frequencies. TLLC (http://www.vk1od.net/calc/tl/tllc.php) goes a step further and estimates a value for Ro based on the loss. For example, the Zo used for RG58 at 1MHz is 50.06-j2.31 ohms. This model produces significantly different results for extreme line VSWR at lower frequencies. The more detailed models would be computationally unwieldly with log tables or a slide rule (the tools of Macalpine's day), but TLLC demonstrates that it is a trivial computational matter today, though not to lose sight of the fact that mathematical function libraries are often approximations favouring speed over ultimate accuracy. Again, we must keep in mind that the uncertainty of the data fed into the model (including manufacturing tolerances, and through life component degradation) contributes to uncertainty of the output, and as Roy has mentioned, the sensitivity of results to input data can be higher than people might otherwise expect. Properly implemented, the VNA OSL calibration process and measurement process should overcome the problem of a sufficiently accurate model of the actual transmission line used in the 'fixture', though not the phase stability between calibration and measurement, especially where they are done under very different environmental conditions. Owen Good info, Owen and it should be noted that "VNA" is a vector network analyzer and "OSL" refers to the open, short, and load (usually 50 or 75 ohms) reference standards used to calibrate the VNA over the frequency range of ineterest. The small reactive (imaginary) part of the transmission line characteristic impedance is covered (as are all things to do with RF transmission lines) in detail in R.W.P. King's seminal work, "Transmission Line Theory". King also covers transmission line baluns in depth. 73s, John Wood (Code 5550) e-mail: Naval Research Laboratory 4555 Overlook Avenue, SW Washington, DC 20375-5337 |
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