A few years ago someone told me that if you use a cable which has a length of an even half-wavelength multiple for the frequency being tested, then the impedance measured at the end of that cable will be the same as if it were measured at the feedpoint. For example (assuming you account for velocity factor) a 2-meter wavelength antenna should be measured with a cable that is in even 1-meter increments.

...but then I began to question just what it means to calibrate a VNA with respect to cable length from the VNA to the DUT, since I would think that the calibration including the feedline to set the reference plane would take care of that detail. Now I wonder if there is value to a \$\frac{\lambda}{2}\$-multiple cable length.

There are a few possible ways that I can think of to calibrate:

1. Calibrate at the VNA's SMA port and tie the VNA directly to the DUT. This may be impractical if proximity effects DUT behavior (like an antenna).

2. Use an arbitrary length of feedline (that is not based on wavelength) and calibrate the VNA with a reference plane at the end of that feedline.

3. Calibrate the VNA at its SMA port and use an even \$ \frac{\lambda}{2} \$-multiple-length of feedline to connect to the DUT.

4. (Obviously this 4th item isn't a good idea because it will rotate around the Smith chart by an unknown angle, but I have included for completeness: Calibrate the VNA at its SMA port and use an arbitrary length of feedline.)

Questions:

• Except for round-trip time delays added by the cable length, are #2 and #3 effectively the same in terms of impedance? Otherwise stated: if you make an "good" impedance match having measured with method #2 will that match be the same as if you had measured with method #3?

• If they are not the same, then why?

• Are there any benefits to using a \$ \frac{\lambda}{2} \$-multiple-length of feedline with a VNA?

(In our specific application we are measuring the feedpoint of a huge helical antenna in the 2m and 70cm bands for a satcom project. Helical antennas are in the usually 140 to 170-ohm range depending on spacing and other details.)

A few years ago someone told me that if you use a cable which has a length of an even half-wavelength multiple for the frequency being tested, then the impedance measured at the end of that cable will be the same as if it were measured at the feedpoint. For example (assuming you account for velocity factor) a 2-meter wavelength antenna should be measured with a cable that is in even 1-meter increments.

...but then I began to question just what it means to calibrate a VNA with respect to cable length from the VNA to the DUT, since I would think that the calibration including the feedline to set the reference plane would take care of that detail. Now I wonder if there is value to a \$\frac{\lambda}{2}\$-multiple cable length.

There are a few possible ways that I can think of to calibrate:

1. Calibrate at the VNA's SMA port and tie the VNA directly to the DUT. This may be impractical if proximity effects DUT behavior (like an antenna).

2. Use an arbitrary length of feedline (that is not based on wavelength) and calibrate the VNA with a reference plane at the end of that feedline.

3. Calibrate the VNA at its SMA port and use an even \$ \frac{\lambda}{2} \$-multiple-length of feedline to connect to the DUT.

4. (Obviously this 4th item isn't a good idea because it will rotate around the Smith chart by an unknown angle, but I have included for completeness: Calibrate the VNA at its SMA port and use an arbitrary length of feedline.)

Questions:

• Except for round-trip time delays added by the cable length, are #2 and #3 effectively the same in terms of impedance? Otherwise stated: if you make an "good" impedance match having measured with method #2 will that match be the same as if you had measured with method #3?

• If they are not the same, then why?

• Are there any benefits to using a \$ \frac{\lambda}{2} \$-multiple-length of feedline with a VNA?

(In our specific application we are measuring the feedpoint of a huge helical antenna in the 2m and 70cm bands for a student satcom project. Helical antennas are in the usually 140 to 170-ohm range depending on spacing and other details.)