Post by Bob on Aug 26, 2011 9:56:57 GMT -5
Custom calibration can be used to optimize the calibration data for a specified frequency range and a specified frequency step size.
Canceling a Filter:
Note that the filter becomes an issue only when you want to calibrate out the effect of the filter.
When the Zmag and Theta change rapidly, the Standard cal may not be accurate enough for the in-between points since it takes data on one MHz intervals. By using Custom Cal, you can have cal points on smaller intervals (only a few KHz if desired).
This is useful, for example, when using a High Pass filter to reject broadcast band interference on 160 meters. There is an example of this in the AIM manual. In this case it is important to use Custom cal with a small step size, perhaps around 10 KHz.
Antenna Feed Point Impedance:
The same principle applies to calibrating a transmission line at the far end so you can measure the impedance at the feed point of an antenna. The transmission line acts like a filter with phase changes versus frequency. These phase changes are canceled after the cable has been properly calibrated so they don't affect the feed point data. When measuring an antenna this way, it's better to use a line that is not a multiple of quarter waves long. By using a cable that is not a quarter wave long (or a multiple of a quarter wave), you avoid the extreme impedance values that occur in the band you are interested in when calibrating the open and short cal loads. Using cal data that is better behaved will make the desired scan data more accurate. This condition on the cable length is recommended but it is not essential. In some cases it may not be practical to change the cable length. If you suspect the cable is causing a problem because it is "resonant" in the band of interest, try adding in a few meters to the cable to shift the resonant frequency just for this test. When using Custom cal with a relatively small step size, the resonant cable will probably not be a problem. If the effect does occur, it will show up as a small glitch in the Zmag and Theta data. If the data varies smoothly across the band, the resonant cable is not a problem. If you see a glitch, only the data in the neighborhood of the glitch will be affected.
Custom cal accounts for the impedance, the length, the velocity factor and the loss in the cable so all of these parameters are canceled out.
Basic Impedance Measurement:
If you are measuring the input impedance of a filter itself, or the impedance of an antenna as it appears at the near end (transmitter end) of the coax, then either Standard Cal or Custom Cal can be used.
Interpolation:
Both Standard cal and Custom cal use interpolation to calculate the data in between the cal frequencies. When doing a scan, the time required is the same for both Standard and Custom cal and the time is not affected by the frequency step size used for Custom cal.
The cal file can have up to 30,000 points. Obviously this would take a long time to calibrate but a few thousand points may be reasonable for some applications. After the calibration is done, the actual Scan can be done between any frequencies that are in the calibrated range. The step size can be different too. For example, you can cal from 1 MHz to 30 MHz with high resolution and then do a scan from 7.0 to 7.3 MHz with any frequency step size to focus on 40 meters. The scan limits are still set the same way, regardless of whether Standard cal or Custom cal is used. The program does a quick check to make sure the limits are within the cal range, but that is transparent to the user. It will show a message if the limits are not in the Custom cal range.
Cal Range:
Standard cal starts at 1 MHz and goes up in 1 MHz increments. For frequencies below 1 MHz, the data is extrapolated backward. For this reason, it is recommended to use Custom cal when scanning below 0.5 MHz, although that is not imperative. Generally, the Standard cal data can be used down to about 50 KHz for less critical applications. With Custom cal, the calibrated Start frequency can be as low as 5 KHz.
--73/Bob
Canceling a Filter:
Note that the filter becomes an issue only when you want to calibrate out the effect of the filter.
When the Zmag and Theta change rapidly, the Standard cal may not be accurate enough for the in-between points since it takes data on one MHz intervals. By using Custom Cal, you can have cal points on smaller intervals (only a few KHz if desired).
This is useful, for example, when using a High Pass filter to reject broadcast band interference on 160 meters. There is an example of this in the AIM manual. In this case it is important to use Custom cal with a small step size, perhaps around 10 KHz.
Antenna Feed Point Impedance:
The same principle applies to calibrating a transmission line at the far end so you can measure the impedance at the feed point of an antenna. The transmission line acts like a filter with phase changes versus frequency. These phase changes are canceled after the cable has been properly calibrated so they don't affect the feed point data. When measuring an antenna this way, it's better to use a line that is not a multiple of quarter waves long. By using a cable that is not a quarter wave long (or a multiple of a quarter wave), you avoid the extreme impedance values that occur in the band you are interested in when calibrating the open and short cal loads. Using cal data that is better behaved will make the desired scan data more accurate. This condition on the cable length is recommended but it is not essential. In some cases it may not be practical to change the cable length. If you suspect the cable is causing a problem because it is "resonant" in the band of interest, try adding in a few meters to the cable to shift the resonant frequency just for this test. When using Custom cal with a relatively small step size, the resonant cable will probably not be a problem. If the effect does occur, it will show up as a small glitch in the Zmag and Theta data. If the data varies smoothly across the band, the resonant cable is not a problem. If you see a glitch, only the data in the neighborhood of the glitch will be affected.
Custom cal accounts for the impedance, the length, the velocity factor and the loss in the cable so all of these parameters are canceled out.
Basic Impedance Measurement:
If you are measuring the input impedance of a filter itself, or the impedance of an antenna as it appears at the near end (transmitter end) of the coax, then either Standard Cal or Custom Cal can be used.
Interpolation:
Both Standard cal and Custom cal use interpolation to calculate the data in between the cal frequencies. When doing a scan, the time required is the same for both Standard and Custom cal and the time is not affected by the frequency step size used for Custom cal.
The cal file can have up to 30,000 points. Obviously this would take a long time to calibrate but a few thousand points may be reasonable for some applications. After the calibration is done, the actual Scan can be done between any frequencies that are in the calibrated range. The step size can be different too. For example, you can cal from 1 MHz to 30 MHz with high resolution and then do a scan from 7.0 to 7.3 MHz with any frequency step size to focus on 40 meters. The scan limits are still set the same way, regardless of whether Standard cal or Custom cal is used. The program does a quick check to make sure the limits are within the cal range, but that is transparent to the user. It will show a message if the limits are not in the Custom cal range.
Cal Range:
Standard cal starts at 1 MHz and goes up in 1 MHz increments. For frequencies below 1 MHz, the data is extrapolated backward. For this reason, it is recommended to use Custom cal when scanning below 0.5 MHz, although that is not imperative. Generally, the Standard cal data can be used down to about 50 KHz for less critical applications. With Custom cal, the calibrated Start frequency can be as low as 5 KHz.
--73/Bob