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    Adding a Feed Line

    Updated

    In the case of a transmitting antenna that has a single feed port, the transmission line used to feed the antenna can be modeled in the Feed Line tabsheet, Fig. 1.

    Fig. 1: Feed Line tabsheet where the transmission line used to feed a transmitting antenna can be modeled.

    There is a list of cable types where real-life transmission lines are available, which include matched loss parameters adjusted to the cable datasheets. The cable types are ordered by part numbers, and they include the manufacturer name.

    For instance, type “RG-8” in the Cable Type option and this part number will be shown for different manufacturers, Fig. 2. The RG-8 Belden 8237 will show a set of K0, K1 and K2 parameters. The constants K0, K1 and K2 have been adjusted so that a matched loss curve is obtained as a function of frequency according to the matched loss vs. frequency table published in the cable datasheet. K0 is related to the DC losses in the transmission line conductors, K1 is related to the skin effect losses which depend on the square root of frequency and K2 is related to dielectric losses which increase linearly with frequency. These losses are then considered in the standard RLGC model of a lossy transmission line.

    The nominal values of the cable characteristic impedance Z0 and velocity factor will also be shown for the chosen part number and manufacturer. After the cable type has been chosen, the operating frequency and input power to the feed line can be set. The frequency can be selected from a list that shows the frequencies that have been set in the Setup tabsheet >.

    Fig. 2: Cable Type option where the type of transmission line can be chosen.

    Then, the length of the cable can be set. The length is entered according to the length unit used for drawing wires in the workspace. Go to Tools > Preferences > in the main menu to change the length unit. While typing the cable length, the length measured in wavelengths (l) and electrical degrees will be shown automatically. As a matter of fact, all the feed line results are calculated automatically by just modifying any of the feed line parameters.

    The load impedance of the feed line can then be chosen. The default option is to consider the antenna input impedance (Zin) as the load impedance of the transmission line, so the antenna input impedance at the chosen frequency will be shown automatically as a load for the line. However, any value for the line load impedance can be entered by choosing the “Custom Load” option. This allows us to use the Feed Line tabsheet as an independent calculator for transmission lines.

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