antenna-theory.com

[raisoman]

On the page http://www.antenna-theory.com/measurements/impedance.php it says that

"Before we begin, I'd like to point out that object placed around the antenna will alter its radiation pattern. As a result, its input impedance will be influenced by what is around it - i.e. the environment in which the antenna is tested."

Can anyone explain why that happens? Is it because the signal is being reflected?

Cheers,


Nicolas

[admin]

If there's metal, you can think of this as reflecting the energy and disturbing the electric fields that are required for radiation.

If it's not metal, then two effects happen:
- dielectric loading. Since there the speed of light slows down in materials that are not vacuum/air, the changes in a material, which means your antenna will have a different resonant frequency as a result. i.e. if you design a half-wavelength dipole, you need to know what a wavelength is, which depends on the material
- some materials eat rf energy: they have some conductivity associated with them even though they are not metal. This converts E-fields to heat, absorbing the antennas energy.

[raisoman]

Thanks! That's interesting.

So say I point a directional antenna towards the sky and then point it towards the ground - I would get different impedance measurements in the two cases?

What would typical values be?

If I want to carry out the experiment myself, what equipment would I need? A VNA and a dish antenna?

[admin]

You would want at least a 1-port VNA to measure impedance. You could use any antenna at all, and just touch it or place it near metal and the impedance will change. As on example, see the analysis here, figures 6-8:

http://www.antenna-theory.com/antennas/wideband/log-periodic-dipole.php