Electronics > RF, Microwave, Ham Radio

How to measure VSWR of tiny antennas - how to eliminate effect of VNA cables?

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I find myself working on small devices these days, sometimes only few centimeters across. And in the same context, I would use a PIFA or a chip antenna. Now.. how do I actually connect my VNA in a way that the test cable doesn't become a part of the antenna system? With lower frequencies, say, below 1GHz, I have used ferrite sleeves in the past. I'm kind of at loss (no pun intended) at what to do for 2.4GHz, for example..

You may want to try using the search.  I posted about something similar a while back.  Maybe there is something useful:
Fix link

maybe try a thinner coaxial cable if the size is annoying  if your frequency is only a few GHz then you can use a very thin cable.

If your VNA is close to the antenna then it will effect the antenna (loading), so a long cable is like a requirement towards accurate tests of the antenna, but it might be less realistic of a result then you get if the antenna is mounted inside of a small enclosure or whatever

The important thing is to calibrate the VNA on the antenna feeding point. If you calibrate it with typical SMA kits, it will be inaccurate. If you calibrate at the VNA port and add offset time for the cable length, it won't help either. So I recommend using a 0 ohm resistor (or short with solder), a 50 ohm resistor and open circuit and keeping the VNA and cable as it is while calibrating.

I normally add 0402 resistors to circuit for antenna matching network and so I use a low tolerance 0402 50 ohm resistor (FC0402E50R0BST1) as a calibration load. You can find alternatives for your PCB. You can use ferrites around the cable to decrease it's reception from the surrounding.

I've attached an image. As far as I remember the cable is RG-405 having about 28 pieces of 74270033 around it. The cable is rigid so that I can keep the orientation and bends between calibration and measurements. I keep the styrofoam and VNA fixed as well.

You can use VNA Port Extension feature, but mikkie mouse gadgets like nanovna may not have it. In which case the best way would be to calibrate right on the PCB at the antenna terminals, using a SMT 50 Ohm resistor, as mentioned above.


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