[SOLVED] Is there any effect of touching the cable of VNA while making antenna measurement?

[nmobin27]

Hello everyone

Would you please tell me if there is any effect of touching the cable of VNA while making antenna measurement. If the effect exist how can I minimize that?

Thank you

 

[FvM]

The cable effect can be expected for all kind of antennas that generate sheat currents, e.g. monopoles without suitable ground plane. In simple terms the cable becomes part of antenna. If your measurement setup involves a balun (symmetrical antenna input), the hand effect might also indicate a bad balun.

The hand effect can be usually suppressed by ferrite tubes acting as sheat current filters or more specifically by "bazooka" λ/4 bandstops. https://www.edaboard.com/threads/129541/

 

[D.A.(Tony)Stewart]

Also keep in mind, you must have an anechoic chamber or open field without near field reflections to test. Otherwise the deep return loss will be influenced not only by hand on cable but moving people near every λ/2 boundary. ( excellent for motion sensing detector). As FvM indicated , baluns can improve performance .A CM choke can raise CM impedance and lower CM sheath current effects of a hand contact.

However if this is a mobile device antenna, some replica of a hand/body ought be included when tested.

 

[nmobin27]

Thank you for your reply. Please let me explain my case. I am measuring a ungrounded dipole antenna. Without touching the cable, I found that the S11 curve seems to have some ripple at most of the points (frequency sweep 10 KHz - 6 GHz). However, while I touch the cable the S11 curve seems smoother and the resonant frequency shifts a little bit. Would you please explain this point (why the curve becomes smooth)?

Currently in my lab there is no balun. Hence, is there any other alternative?

Thank you again.

 

[D.A.(Tony)Stewart]

Yes that is normal for lack of a ground plane. Since your load is not a dummy 50 Ohm there are many reflections and standing waves on the cable. Since it is also a radiator, reflected signal includes both radiated and conducted signals. The stray reflections are variable lengths so RL will have lots if ripple. Putting hand on cable shunts much of the stray radiated signals getting on the cable , thus what you see gets closer to standing wave effects more expected from conducted standing waves but with a common mode load capacitance ( your hand ) which shifts the resonant point lower slightly.

You want to tune the antenna in its representative environment. All physical proximity effects come into play. Also the dielectric constant of your hand is possibly 3x air so it alters the resonance a few percent from ground shunting.

 

[biff44]

Absolutely!!!! In fact, unless you go to great pains to restrict RF cable currents, the antenna gain and pattern will be very incorrect after measuring with a VNA.

The problem is exacerbated if the antenna, and more importantly the ground plane, is electrically "small". In this case there are lots of currents running in the ground return, which leak right onto the outside of the flexible cable. The flexible cable then acts like a 2nd antenna!

There is not much you can do about it! Get a big box of clamp on ferrite beads and use them on the cable. If the antenna has some sort of a ground choke where the connector is...that helps a lot, although it will be narrowband.

And the worst possible case? A small antenna hooked to a flexible cable with your hand holding the cable!

One way around this problem is to have an onboard source (a small battery operated VCO) driving the antenna. That way at least you have the right ground plane area and no wires hanging off. Of course, you would make such a measurement with a spectrum analyzer instead.