Problem with antenna measurement setup...

[ifelix]

antenna resonance measurement

Hi all,

I'm in trouble with a problem on antenna measure of resonance and gain.
It's since a long time I'm trying to resolve my doubt but it's still present.
I'm dealing with antennas for GSM, WLAN applications. Small Printed antennas on PCB or also ceramic chip antennas. When I've to realize the tuning of the resonance and I want to measure it with a VNA a problem arises: I've to connect the antenna to the VNA. I use a minicoax (RG174 for example); I solder the central wire of the coax to antenna feed point and the shield of the coax to ground near the feed point. The other end of the minicoax is connected to VNA.
But when I do the measure of S11 I find that the cable interferes with the antenna in the sense that if I rotate and move the cable the resonance of the antenna will vary. Someone told me that this phenomena is due to return currents from antenna on the shield of the cable and also for scattering from the cable of the radiated field of the antenna. But someone also told me that the return currents are present only if the antenna that I connect to the coax is a balanced one(like a dipole) and so I have a balanced to unbalanced connection and there is an unbalance of the currents.

Now my question is: when I want to measure with the VNA the antenna resonance have I to make use of a λ/4 sleeve balun between antenna and coax always or can I connect directly my coax to VNA and soldering the other end to the antenna feed point if my antenna is a chip one or a IFA or a simple quarter wave monopole?

Thankx in adv.
Bye!

 

[vfone]

vna measure resonant antenna

RG174 is a cheap coaxial cable. The shielding is approximately 80% compared to good coax cables where the shielding is approx 98%. Also sometimes its impedance is changing when appear small mechanical stress on the cable.
For printed monopoles, PIFA or IFA types antennas, and for most of ceramic chip antennas you don’t need to use a balun.

 

[Azulykit]

Just another comment, I would expect to see gains around -5 dBi on a good day(even when the specifications claim more). This is always an issue when antennas are electrically small.

If you run your hand up and down the coax and there are strong crrents present you can often see disturbances in s11.

 

[ghb]

i would suggest

1. ground the cable better, it does in fact sound like you have current on the outer conductor of the cable. this is common and partly due to currents from the inside of the outer conductor having a path to the outside.

2. if the problem persists, from the analyzer connect a cable with a ferrite core at far end closest to the connector. then connect your aut to that. this will remove currents from running after the ferrite.

this won't be sufficient for all scenarios, but for yours this should work.