### Post by jimworth on Apr 15, 2010 12:50:29 GMT -5

I am trying to pick an optimum length for an open wire feedline attached to a 67' doublet used on the 7 bands from 40 through 10 meters. I have found the Rotate Smith Chart and Theta Rotation functions for Markers in the AIM4170 program useful for this purpose. After scanning with the current feedline length, these functions allow me to see what would happen on the frequencies of interest (Markers) if I change the feedline length. This is a very nice feature of the AIM4170 program.

However, there are a few things that seem harder than they could be. To simulate adding 4' to the feedline, I first calculate how many degrees of wavelength that represents on each of the 7 bands, taking into account the velocity factor of the feedline. Then, for each band, the program requires a few steps to get to the Marker Data window to see the new impedance values for that band. Since the feedline Zo is much higher than 50 ohms and must be entered for the rotation to be done properly, the displayed SWR is not the one I'm interested in - SWR(50 ohms) vs. SWR(450 ohms). So, I have to plug the complex impedance displayed into a calculator to get SWR(50).

Following the above procedure for 7 bands and several feedline lengths is quite tedious. It would be nice to have a function in the AIM4170 program that would calculate the complex impedance for each marker frequency given a feedline length delta in feet or meters and the velocity factor. It would also be helpful to have it show the 50 ohm SWR in addition to the Zo of the line.

I realize there may be an easier way to do this than the procedure above. Any suggestions would be appreciated.

73,

Jim AD4J

However, there are a few things that seem harder than they could be. To simulate adding 4' to the feedline, I first calculate how many degrees of wavelength that represents on each of the 7 bands, taking into account the velocity factor of the feedline. Then, for each band, the program requires a few steps to get to the Marker Data window to see the new impedance values for that band. Since the feedline Zo is much higher than 50 ohms and must be entered for the rotation to be done properly, the displayed SWR is not the one I'm interested in - SWR(50 ohms) vs. SWR(450 ohms). So, I have to plug the complex impedance displayed into a calculator to get SWR(50).

Following the above procedure for 7 bands and several feedline lengths is quite tedious. It would be nice to have a function in the AIM4170 program that would calculate the complex impedance for each marker frequency given a feedline length delta in feet or meters and the velocity factor. It would also be helpful to have it show the 50 ohm SWR in addition to the Zo of the line.

I realize there may be an easier way to do this than the procedure above. Any suggestions would be appreciated.

73,

Jim AD4J