AN-SOF includes an embedded transmission line calculator. A transmission line to feed a transmitting antenna can be set using this feature. You can choose from almost 170 different types of feed lines, including actual coaxial lines from different manufacturers. The matched line attenuation response of each manufactured transmission line has been obtained from the datasheets and embedded in the calculator. The input impedance of a transmitting antenna can be used as a load for the transmission line, but the calculator can also be used separately by setting a custom load impedance.
Feed Line Parameters
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. Type “RG-8” in the Cable Type option and this part number will be shown for different manufacturers.
For instance, 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, cable length, input power to the feed line and load impedance can be defined.
Feed Line Results
After defining the feed line parameters, the following results will be obtained:
It is the “true” characteristic impedance of the transmission line obtained from the RLGC model via the K0, K1 and K2 constants. The real part of Z0 may differ somewhat from the nominal Z0 depending on frequency and losses in the line. An imaginary part will always appear in Z0 due to the non-zero losses. So, note that the true characteristic Z0 will be generally different from the “Nominal Z0” (Z0 in the cable datasheet).
True Velocity Factor
It is the velocity factor obtained from the RLGC model of the transmission line, where the wavenumber (and wavelength inside the line) is affected by losses and the velocity factor will be modified relative to its nominal value accordingly. Therefore, the true velocity factor is a function of frequency and losses in the line.
Any cable datasheet contains a table of matched loss values expressed in dB/100feet or dB/100m as a function of frequency. These values correspond to the attenuation of the line when it is matched (the line has a load impedance equal to Z0). So the Matched Loss value that is shown in the Results panel is the attenuation of the line corresponding to the selected frequency.
Total Matched Loss
It is the matched loss that would be obtained for the specified length of the cable, so the Total Matched Loss equals the Matched Loss (dB/100feet or dB/100m) multiplied by the cable length.
At Line Input
The input impedance of the transmission line will be shown as well as the reflection coefficient (Rho), VSWR and return loss. This is the impedance at the line end opposite to the end where the load/antenna is connected.
At Line Load / Antenna
The load impedance connected to the transmission line will be shown as well as the reflection coefficient (Rho), VSWR and return loss at this line end. The load impedance will be the antenna input impedance if the “Antenna Zin” option was selected as a parameter for the feed line.
Power at Load / Antenna
It is the power in Watts consumed at the line load or effectively delivered to the antenna port. This power will be less than the input power specified as an input parameter for the feed line if the transmission line has losses. However, the power at load/antenna will be equal to the input power in the case of a lossless transmission line.
Power Lost in Line
It is the total power lost along the transmission line in Watts.
Total Line Loss
It is the total transmission line loss expressed in decibels. It will be different from the Total Matched Loss if the transmission line is not matched.
Total Loss – Matched Loss
It is the difference in decibels between the total loss obtained and the total loss that would be obtained if the line were matched. It can be interpreted as an additional loss due to a VSWR different from 1.
It is the total power in Watts radiated by the antenna when it is fed using the power at the load/antenna end of the transmission line. The radiated power will be different from the power delivered by the feed line if the antenna itself has its own losses. The radiated power will be shown if the option “Antenna Zin” was selected as a line parameter.
It is the total power lost in the antenna structure. It will be shown if the option “Antenna Zin” was selected as a line parameter.
It is defined as the ratio of the antenna radiated power to the antenna input power (the power delivered by the feed line). It is expressed as a percentage as it is usual. It will be shown if the option “Antenna Zin” was selected as a line parameter.
Total System Loss
It is the total loss obtained in the antenna and feed line as a whole system.