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If I have an antenna and I wish to measure its impedance. I have two impedance values, Z=125.52-j207.36Ohms and Z=8.14-j39.6Ohms, and I do not know which one is correct/ which one applies to this antenna.

We know its impedance has a resistive and reactive component. We know its impedance will change with frequency due to the reactance changing with frequency.

Impedance triangle

Would it be possible to measure the resistance of the antenna using a standard multimeter and if the resistance measured is close to the resistive component in one of my impedance values, then that impedance value must be the correct one?

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  • \$\begingroup\$ What kind of antenna? At what frequenc-y(ies)? \$\endgroup\$
    – Antonio51
    Commented Mar 1, 2022 at 13:47
  • \$\begingroup\$ Its a Helix antenna (so just a coil of wire) and at 868.4MHz. \$\endgroup\$
    – MRB
    Commented Mar 1, 2022 at 13:58
  • \$\begingroup\$ The only "good" value is what you have measured exactly at the frequency in use and at the "reference point". If you add some piece of "cable" ..., it will change according with the "laws" of "transmission lines" ... see amanogawa.com for "reference" ... \$\endgroup\$
    – Antonio51
    Commented Mar 1, 2022 at 14:20
  • \$\begingroup\$ Some infos about antennas coredump.ch/2017/04/30/lorawan-868mhz-antenna-test-part-2 \$\endgroup\$
    – Antonio51
    Commented Mar 1, 2022 at 14:36

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Would it be possible to measure the resistance of the antenna using a standard multimeter and if the resistance measured is close to the resistive component in one of my impedance values, then that impedance value must be the correct one?

No, that is impossible. The resistance of the antenna (as given by one of your impedance values) is the radiation resistance of the antenna and not the DC resistance of the wire that makes your antenna.

FYI - radiation resistance is the impedance of free-space (377 Ω) that is projected to your electrical terminals by the antenna. Sometimes it is close to 50 Ω and, for something like a monopole it will be about 37 Ω. A dipole will be about 74 Ω and other more specialist antennas might be hundreds of ohms. This radiation resistance is also highly dependent on whether the antenna is tuned to the precise length needed by the radio frequency being received. For instance, if the antenna is "short" i.e. it is less than the optimum length for a particular frequency then radiation resistance will drop dramatically. It's much, much more convoluted and twisted that being equivalent to the DC resistance measured by a multimeter.

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  • \$\begingroup\$ The antenna itself is just a coil of wire so why would its resistance change with frequency? The impedance measurement was obtained from a VNA so in my mind, the resistance value should be fixed and only the reactance changing with frequency? Is this correct? \$\endgroup\$
    – MRB
    Commented Mar 1, 2022 at 13:33
  • \$\begingroup\$ @MRB read Andy's 1st paragraph again. \$\endgroup\$
    – brhans
    Commented Mar 1, 2022 at 14:15
  • \$\begingroup\$ @MRB The DC resistance you would read with a multimeter will be one of two values for most antennas, either very close to zero, or very close to infinity. Antennas generally are either shorted for DC or open for DC, so about all you can do with a standard multimeter is verify that it's whichever of those two it's supposed to be. \$\endgroup\$
    – GodJihyo
    Commented Mar 1, 2022 at 17:23

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