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Near-field to far-fiel

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Newbie level 6
Mar 23, 2011
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I would like to know, how is the correct distance between the antenna under test and the probe using the planar near-field technique.
Someone could help me to understand more about this.



Please look into a textbook. The near-field zone of an antenna is defined from the ratio of its aperture size to wavelength.
Large antennas (with apertures many times larger than the wavelength) in fact shape their main lobe of radiation pattern quite far on axis.
Satellite communication antennas (like Intelsat) form their main beam up to hundreds of miles away.
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Your example of satellite communication antennas reminds me of transducers used in medical imaging, which has a physical dimensional several acoustic wavelength long. Their functional region is fresnel region, as opposed to the Fraunhofer region for sat comm.

Wait, I know that: The near-field zone of an antenna is defined from the ratio of its aperture size to wavelength.
But I have 2 problems with this definition.

1) This is true for an aperture antenna, but what is the aperture size for another kind of antenna (i.e. patch) ?

2) In order to compute the far-field from the near-field, it is necessary the use of a probe, so is correct that the distance between the antenna under test and the probe is less than that ratio but I don't want that there is a strong coupling between these antennas. So, what is the common distance to use ?

I would handle a patch antenna like a dipole; its "near-field" zone is also related to its size vers. wavelength.

To "sense" the near-field zone is easy with a probe or a dipole connected to a detector while the antenna is fed with a reasonable power like < 0.1 W at operating frequency.
You will see that close to the tested antenna the indicator (mA meter) will indicate a standing wave, oscillations with maxima and minima separated by a half wavelength. The farther from the antenna you go, the indicated power or intensity will decrease but also will cease to oscillate.

Be aware that all surrounding object also cause standing waves. The best environment is a "free space" like an outdoor range ( ground reflections do matter), or an anechoic chamber (a costly thing). Your body and hand also create nice standing waves.

Learn by doing stuff, this is the beast way to sense electromagnetic field.

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