The wire in the model should have the same diameter as the coax shield and the same insulation as the coax jacket. It should be connected to this element of the antenna model, to which the coax shield will be connected in the real antenna. It should be routed in the way the real coax is expected to be routed. The grounding point is the place in which we can consider the coax shield grounded (where it touches the ground for the first time).
The transmission line object can be routed from the antenna feed point to any direction in your antenna system model. In contrast to the wire discussed before, the idealized transmission line does not have to follow the expected position of the real coax. You need to enter the transmission line parameters: characteristic impedance, length and (in case of the EZNEC) velocity factor and attenuation per unit of length.
In the EZNEC simulator, the transmission line length should be entered as the real cable length between the transmitter output and the antenna feed point and the source power PSC should be equal to the transmitter output power. EZNEC will calculate the PIN based on the entered data.
In the 4nec2 simulator, the transmission length should be entered as the real cable length divided by its velocity factor. The most popular coax cables have the velocity factor equal to 0.66, but there are cables with other values too. You should check this parameter in your cable datasheet. However, the 4nec2 simulator does not handle transmission line attenuation, so entering the transmission line lengths makes sense only in the more complex antenna systems with more than one feed point. In such systems, phase differences between the feed point voltages and currents influence the whole system parameters and performance.
If you have just one feed point, the transmission line length does not matter. You may set it even as zero. In the 4nec2, you enter PIN as source power. In order to find out the value of PIN , you need to know what the coax attenuation will be in the real installation. I use the TLDetails program by AC6LA to calculate the PIN.
For example, let’s assume that you enter the following data in the TLDetails:
The program will then calculate the “Power at load” as equal to about 74 W.
And you can enter now the input power in your 4nec2 model as 74 W. So, the process is a little more complex than in the EZNEC.
But that's not all. If your antenna model has only one feed point, the model can be simplified even more – see the drawing below. Such model will work in every simulator: EZNEC, 4nec2 and MMANA-GAL. PIN should be calculated in the external program like explained above.