antenna-theory.com

[Mohican]

Hi Guys,

I have a question about dipole antenna characterization with VNA.

Generally, VNA has signal and ground in its output port. Let's assume that left part of dipole (connected to core and gets signal from VNA) so it has regular quarter wavelength current distribution. Then, the right side of dipole, that suppose to be specular reflection of left one, has constant current distribution=0, since it connected to coating, that is grounded.

1. So since half of dipole antenna has a constant potential (viz. ground), does it mean that only half of antenna radiates,in other words dipole act as a monopole?

2. Does the fact that half of the dipole is grounded affect the measured impedance of the antenna?

Regards, Newbie

[admin]

Ground does not have zero current through it

ground is a path for rf current to flow

[R. Fry]

The inner surface of the outer conductor of a coaxial cable carries the same r-f current magnitude as the outer surface of the inner conductor, just 180 degrees out of phase with it.

When a coaxial cable is connected to the feedpoint of a dipole antenna, r-f current flows out of one side of the dipole and into the other side, changing directions during each cycle of the r-f waveform.

[helix]

The VNA coaxial output is "unbalanced" (asymmetric w.r.t. ground) while the dipole structure is "balanced" (symmetric w.r.t. ground).

You will want to use a device called a balun (balanced to unbalanced converter) to measure the dipole input impedance correctly with VNA. The VNA is connected to the unbalanced port and the dipole arms are connected to the two balanced terminals). If you do not use a balun, then in general you will find that the VNA cable (which you generally don't intend to be part of your antenna/radiating structure) will be "hot" (have "antenna mode" RF currents on it) and this will affect your VNA impedance measurement to various degrees, depending on the circumstances.

A basic test to see how "hot" the VNA cable is is to run your hand along its length and see if the antenna impedance (i.e., Smith chart locus) changes. Ferrite (absorber) beads clamped to the VNA cable can help suppress this effect, but the real problem is that the antenna is not fed in a balanced manner.