CALIBRATING ANALYZER AT END OF COAX
The next method I investigated sounded really promising. Obviously, the analyzer is calibrated for measurements at its SO-239 input. With this method, the a specific piece of coax is used for all measurements, and the analyzer is calibrated at the end of the coax, effectively nulling out the transmission line transformation caused by inserting the coax between the antenna and the analyzer.
After building the 3 terminators required for calibrating the analyzer and selecting a length of coax to be used - in this case, 25 ft. of LMR-240 - the analyzer was calibrated by connecting the terminators to the end of the coax, one at a time, and running the calibration procedure.
The "Load"(in this case ANT') is then measured as shown in Fig.3, by enabling "CAL" and then measuring as usual.
In theory, the results should be the same as when measuring directly at position ANT'.
Unfortunately it seems my analyzer has broken today. Although calibration procedure appears to run OK, when I enable CAL and apply it to measured data, the SWR shows a steady line across the top of the scale. In other words, it appears to be infinite.
As a result, I did not get any results (today) measuring with this method.
Before I show the results using the "Subtract Coax" method (below), I will show the impedance of the load (ANT') as measured today:
Measured directly at ANT':
Measured directly at the load, the minimum level of SWR was 1.70, at 3.676 MHz.
Measured w. 25 ft. LMR-240:
With +25 ft. coax, minimum SWR was 1.78 (+0.08) at 3.692 MHz (+14 kHz).
25 ft. LMR-240, CAL enabled:
CAL Procedured Failed
I will try to do a reset on the analyzer and see if I can get the CAL procedure to work.
As I was unable to calibrate at the far end of the coax, nothing was learned from this, except finding today's stake-in-the-ground for the impedance of ANT' (801), and finding the amount of change when measuring with an additional 25 ft. of LMR-240 coax.
Here is a closer look at the two curves:
Measured directly at ANT':
Measured with 25 ft. LMR-240 Coax:
NEXT, I will use the "Subtract Coax" Method
This method is used by selecting the make and model of coax in AntScope-2 setup menu, and entering the length of the coax. Although I purchased a 25 ft. length of LMR-240 with PL-259 male connectors on each end, the length must be entered in meters, not feet. I entered 7.62m.
Results when entering 7.62m:
The minimum SWR was 1.76 at 3.696 MHz. As seen in the first screen shot above, the SWR at ANT' is actually 1.70 at 3.676 MHz. The level of SWR is very close (+0.06 - about 3.5% higher), and the frequency at which minimum SWR occurs is 3.696 MHz, 20 kHz higher than at the values measured directly at ANT' - (just 1/2 percent).
(Close enough for Government work!)
Results when entering 25 ft.:
As you see, the results are so far off, there is no sense in commenting on this, other than to say that this is proof that the length of coax should not be entered in feet. It must be entered in meters.
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