how to measure antenna gain

Hi, eveyone,


I have the following questions. Could you let me know right methods ? Thank you very much in advanced !

1) If I have two identical antennas and hope to measure their gain (dBi) through S21.

I connect the two antennas at the two ports ( port1 and port2) through the two identical cables, and set the frequency at f ( MHZ) and the distance d ( meter ) between the two antennas. After I get the S21 ( dB) or S12( dB) from the network analyzer, I use the following formula to calculate antenna gain .

Gain( antena1) =Gain( antenna2) = 1/2( Pr ( dB)-Pt(dB)+20log(d) +20log(f)-27.56)
In the above formula, I use S21 ( dB) to replace Pr(dB)-Pt(dB).


2) If I have three different antennas (A, B, C), and A is the AUT and B is a reference antenna ( its gain is known). In order to get the gain of the A , I connect A and B to Port2 as a receiver respectively ( two times) , C is connected to Port1 as a transmitter . After I get S21 ( A and C ) and S21 ( B and C ) from the network analyser. I use the following formular to calculate AUT gain or A antenna gain

Gain( A)=Gain(B)+ S21( A and C)-S21( B and C) .


I am not sure whether the above methods are right or worng. It will be greatly appreciated if anyone knows well these methods and put more details .

Thank you very much.

Ethen.

Probably your methods are not bad but I would recommend a simple one.

- Use for transmission at Port 1 antenna A (any type but the same polarization with AUT)
- Use for reference a calibrated dipole at Port 2 (same polarization with AUT)
- Measure S21 with reference dipole antenna
- Replace the reference dipole with AUT
- Measure S21 with AUT
- The difference between the two S21 is the gain in dBd of your AUT
- For example if S21 = -20dB with dipole, and S21 = -18 dB with AUT means the gain of AUT is +2dBd

An important thing to mention is, if the pattern of the AUT should be approximately omni directional, you have to measure S21 with AUT rotated at different angles, and do the average of S21 before comparing with the S21 of the dipole.

Thank you very much , Vfone!
You gave very clear explains, especailly addressing polarization !

How about the first method that is used to measure the gain of the two identical antennas. Can I use S21 to replace Pr(dB) -Pt(dB) in that formula.
where Pr is the received power by the antenna at port2, and Pt is transmitted power by the antenna at port1.

I paste several sentences from an analyzer manual

*********************************************
b2/a1 Src Port 1 is the ratio of the wave quantities b2 and a1 and corresponds to the Sparameter S21 (forward transmission coefficient).

a1 Src Port 1 is the wave transmitted at test port 1. In a standard S-parameter measurement,
this wave is fed to the input port (port 1) of the DUT (forward measurement).


b2 Src Port 1 is the wave received at test port 2. In a standard S-parameter measurement, this
wave is transmitted at port 2 of the DUT (forward measurement).

What's happening if you can't use a reference dipole antenna ( say at 40GHz) ?

Your formula derivates from the Free Space Path Loss formula:
Free Space Path Loss(dB) = 27.6(dB) ? 20*LOG[Frequency(MHz)] ? 20*LOG[Distance(m)]

where in your case Free Space Path Loss = S21 - (2 * Antenna_Gain)

so,
S21 - (2 * Antenna_Gain) = 27.6 ? 20*LOG(f) ? 20*LOG(d)

doing the math we get:
Antenna_Gain = ? [S21 - 27.6 + 20LOG(f) + 20LOG(d)]

@melc
At that frequency you can use for reference a calibrated horn antenna with a specified gain at the test frequency.

Vfone, thank you very much. You mean that I can use S21 to replace 10*log(pr/pt).

I have meeting a very stuff problem as attached. I connect two identical monopole antennas ( that have no ground plane) to the two test ports through two cables ( with calibration ) , I don't know why b2/a1 is much larger than 0 , but S21 is much less than 0. My understanding is that b2/a1 should be less 0 after DUT ( two antennas in free space). Thank you very much ,

Added after 5 minutes:

I try to attach a file

Hi ethen

Another way for understanding the calculation




pl

If you don?t have proper ground plane for the monopole antennas there is a big chance to get RF emissions from the coax cable, due to the standing waves that could appear. These emissions would influence the radiated test.

Hi, vfone and pl. thank you very much !

Let's me think through your messages for a while.


It looks that S21 is always too much down from our setup measurement system. So, the gain of AUT is almostly "-" . Even so I measure gain of the two identical microstrip fed patch antennas ( IE3d /samples/ar_ele1.geo), the measured gain is also "-" in the broadside direction. The details are below:

The size of the chamble used is about 60 x60X100 cm^3. the patch antennas are fed through caxial cables, f=1.977 GHz, d=60cm, s21=-40 dB.
So according to two antenna method, the measured gain is calculated below
2*gain =
-40+20*log10(0.60)+20*log10(1977)-27.56 =-6.0768 (dB) or -6.0768 dBi.

What are problems that cause these wrong results ?


Thank you and looking forward to hearing you

The only explanation regarding the gain that you get could be the low dimensions of your chamber. For precise antenna gain measurements you need to use a bigger chamber (at least 3m length).
Try the dipole substitution method that I mentioned before, and maybe get better results using your small chamber.

Vfone, thank you for your important points.

Yes, a bigger chamble should be better, I will try to measure them in a bigger space and to see what is happening . But I am still liitle confused , for the dimension of chamble is enough for min. far-field distance ( d= 2*D^2/wavelength) in the two cases mentioned before .

Do you think that the problem is more probably due to the size of the pyramid absorber ? the pyramid absorbers in the small chamble look smaller. Could you give me an approximation to elevate a right size of pyramid absorber for given frequency ranges ?

Hi ethen
you have to look at D of both antenna Tx and Rx not only the patch

plasma

Dear vfone,

could you give me a reference where to find such a formula? A book, an article... I'd need it for a citation in my thesis

Many thanks

There can be odd effects in such measurements. You can measure the two antennas multiple times with different distances between two antennas, and then do it again where one antenna is a known standard gain horn. When you plot the gain difference vs distance, it will take out any ripples, and give you a better idea of antenna gain.