Total posts : 45366
[quote=Ermi Roos]Your new data indicates that your antenna has an RF bandwidth of about 10 kHz. I, on the other hand, calculated that it is about 5 kHz. …. A 10 kHz bandwidth at 1610 kHz means that the antenna Q is 161. The total antenna resistance is about 11 ohms. This means that the capacitive reactance of the antenna is 11 X 161 = 1771 ohms. The antenna capacitance must be about 1/[(1771)(2pi)(1610)(10^3)] = 56 pF.[/quote]
Probably the difference arises from our definitions of r-f bandwidth. I gave mine in my post of yesterday.
NEC calculates the Xc of this system with no loading coil to be about 3136 ohms. Using your equation above shows that a radiator capacitance of 31.5 pF is needed to produce that Xc. When 3136 ohms of inductive reactance is added to the system, NEC shows the VSWR and impedance data in the plots I linked to yesterday.
[quote]Could it be that your program has a built-in assumption that the antenna has a capacitive hat, and is adding an extra 30 pF to the antenna capacitance calculation?[/quote]
No chance of that at all. The calculations done by NEC are based only on the model parameters defined in its input data. My model consists of a 3-m, linear, vertical radiator broken up into 50 conductor segments. The segment at the bottom contains an 8.65 ohm resistance in series with it (the r-f ground connection). The next one up contains the r-f source (the transmitter), and the one above that contains an impedance of 2 +j 3136 ohms (the loading coil). The rest of the segments have no added sources or loads. No other structure is defined in the model.