This is a bit unusual but I am trying to diy a simple high impedance antenna running in 2.45GHz, for some explorations in ultra-narrow band applications in WiFi. And I have heard in ham radio community, folks use end fed half dipole to design a high impedance antenna.

I cut a 6.1cm wire and soldered it onto the signal port of an SMA as my antenna. I calibrated my VNA and I measured the impedance. The VNA showed an impedance of 5+j60 ohms. A photo here is attached with my antenna and set up with the VNA.enter image description here I know the characteristics impedance of an sma is 50 ohms but looking at the connection with my 6cm wire it should still make a half wavelength dipole and the impedance at 2.45GHz should be high (supposedly). Anything concept am I missing here?

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    $\begingroup$ The outside of the coax is part of the antenna and you get something like an (vertical) off-center fed sleeve dipole. $\endgroup$
    – Raonoke
    Commented May 31 at 10:54
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    $\begingroup$ If your picture is the way you are testing it, I would agree with Raonoke; You need to choke the feedpoint of the antenna. On top of that, be aware of nearby reflective objects that could cause your impedance to drop. $\endgroup$ Commented May 31 at 14:00
  • $\begingroup$ If you short-circuit the SMA connector with a knife blade, just where the wire is soldered, do you see a nice short circuit on the Smith chart? See this answer for some relevant suggestions. $\endgroup$
    – tomnexus
    Commented May 31 at 17:13
  • $\begingroup$ Aside from checking the short mentioned by tomnexus, I shall add an RF choke to my antenna. So the calibration plane shall be the VNA->Coax->RF choke, and then I do the ECAL for calibration? $\endgroup$ Commented Jun 1 at 3:48
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    $\begingroup$ My money would still be on the calibration being wrong, or the reference plane being somewhere you don't expect. When you're back in the lab, short the SMA and see how that looks. Then, two more important tests to do every time you test an antenna: 1. Run your hand down the coax from the antenna to the VNA. If the trace moves, the coax is part of the antenna. 2. Wave the antenna around a bit, to see how much of the reflected power comes from the antenna alone, and how much from the environment. . When you've tried all this, please update your question and I'll consolidate this into an answer. $\endgroup$
    – tomnexus
    Commented Jun 3 at 4:06


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