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Gain and Diractivity of Antenna

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dkumar

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Hi every one!!
I am stuck up with a petty problem of measuring the gain,and directivity of my micro strip antenna.I have the antenna fabricated and i have to test its specifications.

Please if someone can tell me how can i test these specs,I have VNA with which i have already tested S11 plot for my antenna.

Please reply soon
Thanks in advance.

-dk
 

Hi dKumar,

For Antenna Gain Measurement you need two antennas,one is standard antenna of which gain(Gstd) is known and another one is Antenna under test(AUT).
Attached here is Test Setup for Gain Measurement.

Gain(AUT)=Total Loss -RSL- Gstd.

where,
RSL=Recieving signal level of AUT
Total Loss = PathLoss + Cableloss.

regards,
balaguru.
 

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    dkumar

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Hi,

I'm just wondering what is it that you are measuring in the gain test setup.jpg. Doesnt make sense to me.

Added after 3 minutes:

Do you have any access to any Test Chamber ?

You might wan to download from IEEE "Standard Test Procedure for Antennas"
 

In the drawing, he is measuring S21 (total loss) between the antenna under test and a known good antenna. You rotate around the antenna under test and get a relative gain pattern. THEN you substitute a standard gain antenna (one that you know what its gain is) for the antenna under test, aim the two antennas together, and can "calibrate" the absolute antenna gain of the first test.
 

Kohi_boy said:
Hi,

I'm just wondering what is it that you are measuring in the gain test setup.jpg. Doesnt make sense to me.

Added after 3 minutes:

Do you have any access to any Test Chamber ?

You might wan to download from IEEE "Standard Test Procedure for Antennas"
Click to expand...

Antenna Gain and Pattern Measurement must be done in Anechoic Chamber otherwise it'll lead to errors due to reflection.Distance between two Antennas should be greater than or equal to 2D²/λ. D is the largest dimension of the Antenna.RSL should be maximum RSL.
eg for Gain Measurement at 1795MHz
Considering
Distance between two Antennas = 7m.
0dBm transmit power from VNA,
Gstd=12 dBi (Standard Antenna Gain)
RSL=-28dB (Maximum Recieving signal Level of Antenna under test with respect to Standard Gain Antenna).
Cable Loss can be normalized.
Pathloss = 54.42dB
then gain of AUT= 54.42-28-12=14.42dBi.

regards,
Balaguru.
 

Gain of an antenna could be measured with any 3 antennas, cos there are 3 unknow variables, gains of these antennas. You have to measure every kombination of antennas 1-2, 1-3 and 2-3 and you have system of 3 equations with 3 unknow variables. Every equation is in form: Ptx + Gtx - AIR LOSS + Grx = Prx.
 

farin said:
Gain of an antenna could be measured with any 3 antennas, cos there are 3 unknow variables, gains of these antennas. You have to measure every kombination of antennas 1-2, 1-3 and 2-3 and you have system of 3 equations with 3 unknow variables. Every equation is in form: Ptx + Gtx - AIR LOSS + Grx = Prx.
Click to expand...

Yes three antenna method will give accurate results when compared to Standard Gain Antenna Measurement.

Regards,
Balaguru.
 

I agreee with farin and balaguru. I would like to know if reflection causes gain degradation or up gradation i.e gain will increase or decrease. As the chamber facility will not be available in many of the places and the testing will be done at terrace or free space. Please let me know your views on this guys.
Regards,
Pankaj
 

pankajdudulwar said:
I agreee with farin and balaguru. I would like to know if reflection causes gain degradation or up gradation i.e gain will increase or decrease. As the chamber facility will not be available in many of the places and the testing will be done at terrace or free space. Please let me know your views on this guys.
Regards,
Pankaj
Click to expand...

It depends upon reflection of the signal, Gain will increase if orginal and reflection signal are in Phase and decrease if both are out of phase.
 

On top of that,
Polarisation of the reflected wave also plays a part in the gain.
 

How I can tolerate this if I don't have RF shielded chamber. Also as the reflected waves will come after a delay and if at all it is phase it will act as a destructive only if I am wrong please correct me.
 

IF I am testing on the terrace of the building but I have tall building surrounded in all direction then it will affect lot or less. But in LOS no interruption as buildings are 2-3 mts away.
Please let me know
 

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