Characteristic Coax Impedance

[wa1fxt]

I am working on an antenna project that requires that I know the Zo of a coax very accurately. The cable is RG-63. The frequency of interest is 145Mhz.

If I use the "Measure Cable" function I get readings of about 124 ohms. But, Z readings above 80Mhz are all over the map. I measured the cable length very accurately at 10.166 feet. The VF reported seems to be OK at 0.851.

Am I doing something wrong? Or, is this some sort of limitation on the AIM?

BTW, this little machine is a wonderful instrument....thanks Bob

73, Bob
WA1FXT

[Bob]

Hi Bob,

To check the impedance I recommend using a good, low inductance resistor at the far end of the stub and comparing the AIM readings to a theoretical model, like that used in "Transmission Details". This free program is available at: AC6LA.com

As an example, I put a 200 ohm terminating resistor in the TLD model and set the length of RG63 to 10.166 feet. I got the following results:

25MHz, Zin=85.2+j26.3
50MHz, Zin=119.4-j56.5
75MHz, Zin=148.8+j57.6
100MHz, Zin=80.2-j9.4
125MHz, Zin=181.4-j38.9
150MHz, Zin=98.4+j42.3

You can double check the electrical length of your stub using the "Distance to Fault" function.

73/ Bob