Antenna's Far-field measurement in Near field Anechoic chamber

[m.hamza]

Hello All,

I want to know does testing the far-field measurement of an antenna in a near-field anechoic chamber gets effected or not? For example if i am getting a gain of 8dB in simulated far-field results, will the measured gain be effected if it's tested in a near-field anechoic chamber instead of far-field anechoic chamber.

 

[D.A.(Tony)Stewart]

Anechoic is the key to prevent nulls and peaks and is effective at microwave.
What are the lab specs? If ok, what is the problem?

 

[FvM]

In simple words, near field is the range where Friis equation and the calculated far field gain is no longer valid. https://en.wikipedia.org/wiki/Friis_transmission_equation

The deviation from far field gain depends on the characteristics of both, the antenna under test and the measurement antenna.

 

[D.A.(Tony)Stewart]

Peak gain should be very accurate using the standard calibrated E field meter and I assume the separation R is much greater than lambda and no standing waves from possible reflections.

Although it depends on your ratio of R/Lambda what you are calling Near Field.
I use Near Field to locate EMI problems with a shorted loop <<Lambda.
You might be considering 2m distance with <10mm wavelength as "near field" but is actually in between near & far.

 

[m.hamza]

actually while testing my antenna in a near field anechoic chamber, the far-field gain of the antenna has been measured to be 2.5dB as compared to simulated far-field gain of 8.7 dBi. can we contribute this anomaly to the near-field anechoic chamber? Is it required to test our antenna in far-field anechoic chamber to get results reasonably similar to the similated one?

 

[D.A.(Tony)Stewart]

with isotropic antenna for calibration to isotropic E-Field meter, I suspect it is your antenna losses at fault, if lab is calibrated,

Does it have divirsity bandwidth consistent with gain? Where could unexpected losses be?

Did you measure Return Loss?

 

[m.hamza]

VSWR is below 2 for the complete band

 

[biff44]

there ARE ways to do a reasonable far field measurement in a near field chamber. There are plenty of published papers on the method.

 

[D.A.(Tony)Stewart]

Antenna gain of 3 dBi has a beamwidth of a hemi-sphere with 100% efficiency and 9 dBi is 1/8th of a sphere beamwidth at half power points.

What was your beamwidth for XY?

 

[E Kafeman]

Assume the reference antenna data is specified only in far field. Same data can then not be used in near field => system can not be calibrated.
All antennas have less directive gain in near field compared to far field and it is very complex to measure near field gain as radiating wave not is plane and antenna polarization is almost chaotic for both reference and measured antenna.

Distance of far field region is also depending on size of antenna aperture. High antenna gain for measured antenna increases size of near field region. Assume a parabolic antenna, 1 meter dia. and 10 GHz. 10*lambda is then 300mm but at that distance is it not any planar wave. In this case starts far field region maybe at 1000*lambda away. It is min distance to get reliable gain measurements.
It is not totally impossible to measure near field to get far field data. An example of exception is a stargate that measure at relative short distance and then with aid of very complex calculations can describe antenna far field behavior. https://www.cst.com/Applications/Ar...ing-And-Measurement-Of-Wideband-Horn-Antennas
It requires a special type of anechoic chamber and is an expensive system. Advantage is that it is a fast method as it measure a complete circle for each position instead of rotating a slow turntable.

Antenna efficiency, in opposite to gain, can be measured with good result in near field but measurement tools must then be calibrated for near field use.
Anechoic chamber are problematic to use for near field as its anechoic properties only are valid for planar waves in a specific place inside chamber (quite zone).

Among working methods are Wheeler cap. It is a relative simple method: https://www.rfcafe.com/references/articles/Efficiency-Measurement-Antenna-Wheeler-Cap.htm
Measurement in an echoic reverberation chamber is rather popular as it is a fast method. https://www.bluetest.se have developed methods and hardware. Main idea is to rotate/reflect all waves inside a box with reflecting walls and measure an average sum of resulting fields.
Alternatively to an anechoic chamber can far field gain measurements be done at an empty parking lot with aid of an gated VNA (gated to avoid ground reflections). Even if there are a lot of surrounding man-made RF noise outdoor is probably result better then using near field measurement to find far field properties.

 

[m.hamza]

@ E.Kafeman it really helped in solving the dilemma of my antenna's result. I'll be testing it in Far-field anechoic chamber and will share in the results