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| Example Feed System Dialog Box |
cialog Feed System
In the context of this version of cialog, a feed system is a rather simple concept. It is a specified length of a specified type of transmission line. The feed system must be of a single line type. In other words you cannot specify 25 feet of RG-8 which is then connected to 50 feet of RG-213. The current version of cialog does not allow heterogeneous series combinations of transmission lines in a feed system.
The length of the feed system can be specified in either feet or meters.
The transmission line must come from a list of line types which are compiled into the cialog program. If you need a type of cable that is not shown on the list, please contact me, so that it can be added. Note: in some cases, cables with the same electrical characteristics (impedance, velocity factor, pf/ft, etc) have been assigned different name designations by different vendors. Before giving up on the predefined table of transmission lines, check the line characteristics as well as the name. Perhaps the cable you are looking for is really on the list, under a slightly different name.
cialog accepts up to four feed system specifications. Each can be given a user-specified name. A feed system is selected with the Feed System menu. When a feed system is selected, cialog assumes that the specified transmission line is connected to the CIA-HF, between the CIA-HF and the device under test (DUT).
Adding a feed system more than doubles the data managed by cialog. Without a feed system, cialog, at most, uploads four types of data from the CIA-HF - the SWR, resistance, reactance, and impedance, measured at the CIA-HF. With a feed system, it is possible to compute these same four characteristics at the load end of the transmission line. In addition, the loss along the cable can be computed.
The computation of the information at the load is based solely upon the resistance and reactance measured at the CIA-HF. Lossy transmission line equations are used to transform the resistance and reactance at the input of the feed system to resistance and reactance values at the load side of the feed system. The load resistance and reactance, along with the complex impedance of the lossy transmission line, are used to compute the SWR at the load side of the feed system. The impedance at the load is simply the square root of the sum of the squares of resistance and reactance.
Feed systems are specified on the Feed System Dialog Box. Here is a screen capture of that window with all four slots used.
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| Example Feed System Dialog Box |
Although it is not necessary to fill in the Name column, adding a useful reminder of the particular cable will help out when the data is reviewed at some future point. The default length unit is the foot, and fractional lengths are allowed. If you prefer to work in meters, check the Use meters (M) as opposed to feet (FT) control, and the length labels, as well as the values in the fields, will convert to meters.
The transmission lines are characterized by a name, optional numeric designation, characteristic impedance, velocity factor, and capacitance per foot. Associated with each cable choice is loss information at 1, 10, and 100 MHz. Whenever a transmission line computation is made, the loss at a given frequency is computed based upon a logarithmic interpolation of the data at the standard frequency points.
After a desired feed system has been specified, it is used by selecting it on the Feed System menu. Use the Select Feed System popup to select one of the four predefined slots for feed systems, or, select No Feed System if you are not using one.
Once a feed system is selected, cialog assumes that it is always connected to the CIA-HF, between the CIA-HF and the device under test (DUT). cialog will attempt to compute as many characteristics as possible from the uploaded data. As mentioned earlier on this page, all computations at the load side of a transmission line are made using only the resistance and reactance at the input side.
The name, line type, and line length are recorded whenever a feed system is used in calculating load values for a data capture. If you save that data in a file, the name, line type, and line length are saved with the data, and when that data is reloaded into cialog, the saved data will drive the calculations, providing the same results every time. Even if you delete the feed system or change it on the dialog box, the saved data is branded with the feed system specification as it existed at the time of the data capture.
Last update:
Tuesday, April 20, 2004 08:26:57 PM
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