On Thu, Dec 8, 2022 at 06:49 AM, Phil, WF3W wrote:
Is there a way/method/technique for subtracting coax length from readings, as
can be done in Rig-Expert's ANTScope, sftwr?
The AntScope2 software by RigExpert allows the user to add or subtract cable and then make a series of measurements. There is a database of cable types (RG-58, LMRxxx etc.) to choose from. You select the cable type and length and it calculates the approximate impedance at the antenna when measuring at the far cable end (subtraction method). If the measurements were made at the antenna and you want to know what happens if you add cable (addition method) it will calculate the approximate impedance at the cable end. The reason I say approximate is the attenuation and velocity of propagation are not exactly known due to manufacturing tolerances and vary with frequency.
The equations to do the impedance transformation are well known. Attached is an excerpt from the ARRL Radio Amateur's Handbook describing how to do it. The equation shown will work for the "addition method". To do the "subtraction method" you need to rearrange the terms so that ZLoad is the desired calculation. The length of the cable, velocity of propagation, and attenuation for the frequency being used are inputs (some unit conversion required).
Roger
Roger Need
