Meaning of a calibrated antenna !

It means an antenna with a known gain at a particular frequency. It is calibrated by knowing all the parameters in your loop. I.e., signal generator output power, cable loss, transmitting antenna gain, pathloss, antenna-to-be-calibrated, cable loss, received power. You know everything in your loop except the antenna-to-be-calibrated gain. You then use simple math to find the unknown antenna's gain. Each frequency will have its own values around the loop, so the unknown antenna will have to be calibrated at all the frequencies that you want.

Yep. but the details are important. Sometimes the antenna is calibrated every GHz with a point on a data sheet. Sometimes you get a "generic" calibration of all antennas manufactured of the same model number, sometimes the manufacturer actually measured each individual antenna made, and shipped a serialized data sheet graph with the antenna.

Often times you do your own calibration...possibly using other calibrated antennas, or using the "THREE ANTENNA CALIBRATION METHOD"

a good cheap way to do your own "calibrated antenna" is to use a monopole or dipole antenna...whose gain is well known theoretically and in practice tracks the theory pretty well. They do sell dipole antenna kits with the length of the antenna elements adjustable...so you just tune the dipole to your desired frequency, and then you KNOW the gain of the dipole. You perform a path loss test from your transmit antenna (uncallibrated) to the dipole, and measure the received power. THEN you replace the dipole with your unknown antenna you want to test, measure the recieved power, and can then calculate its gain.

I have 2 identical dipole antennas working @ 2 GHz, i have used the method of 2 antennas to measure their gain. I have calculated the path loss after placing the receiving antenna in the far field of the transmitting one. I'm using a signal generator (i've given 0 dBm to the transmitting antenna) and spectrum analyzer. the result of the measurement test was a 4.17 dB as a gain (i know that the dipole antenna gain is 2.15 dBi). I'm conducting this test in a room plenty of chairs and tables. I've doubts that this problem is linked to these scattrers. What do you think? did you tested something like that?

I would say that your calibration is off somewhere if you're getting 4.17dB gain for a 2.15dB dipole. It's possible that you're getting some multipath impacts on your readings - depends on the multipath. This measurement should be done on a certified antenna range or anechoic chamber.

Yes, there are a lot of mutipath impacts as i mentioned earlier. Unfortunately, there's no anechoic chamber for the measures. I'm going to redo measurements in free space. By the way, what's the minimum distance from the ground should i place my antennae? Is there a forumla? if yes, from which book or paper?

The antenna range that I used had units at about 4 feet off the ground. You might want to experiment with heights from 3 feet to 6 feet to see if there is any significant differences. IIRC, the transmitter end of the range was 10 meters away.

Thank you SLK001, i think there's a formula to give the height of antenna on the mast.

did you measure the output power of the source with the spectrum analyzer to confirm 0 dBm. Did you carefully calibrate for cable losses.

No, i did not measure the output power of the source by the SA, but from the signal generator i regulate the power to 6 dBm because from data sheet of the cable and the measurement that i've done at 2 GHz i determined the cable loss to be 6dBm. Also i've measured the reflexion coefficient |S(1,1)| by the VNA at the antenna input to ensure there's a litte power loss. Each time i repeat the measurements of the gain of two identical dipoles i got 4 dB . I know that antenna measurements are sensitive but i have no anechoic chamber and i have to approach reasonable results for gain and radiation pattern.

You can't do it the way you did and expect good results. Basically, you have just "guessed" at your cable losses and I believe that you over-estimated the losses (thus the higher than expected gain). You need to actually measure the losses and at every frequency you want to measure. Once you do this, your gains should be closer to what you expect.

The standard that I used for setting up my measurements was TIA-603: Land Mobile FM or PM – Communications Equipment – Measurement and Performance Standards.

This set of measurements have numerous ways to go wrong. MI Technologies has a "blue book" that describes how to make measurements. And it is free. You might run a copy (*.pdf) down. It is very detailed.

To estimate a cable loss correctly,i've connected only the cable between the signal generator and the spectrum analyzer and measured losses at my desired frequency. I've tested also the impedance mismatch for each antenna using a vector analyzer (|S(1,1)|) . By the fact, in which unit i'm realy measuring the gain dBi, dBd or dB? if in dB as mentionned in some litterature , i think it is not correct.

In your loop calculations, use 2.15dBi for the gain of your transmitting dipole antenna. This will result in the calculations giving you your unknown antenna gain, also in dBi. Since 2.15dBi = 0dBd, you have your choice as to which one to use.

Measurements and values in dB and the various subscripted forms (dBi, dBl, dBc, dBm, dBsm, and the list goes on) all have very precise and useful meanings. They are also very easily confused. dB is basically a ratio comparison between two numbers that is then converted using a logarithm. If no subscript is provided, two (arbitrary or unspecified) numbers are compared. If there is a subscript then the comparison is to a specific value denoted by the subscript (and some other value). There is also the confusion between power and voltage values to keep straight.

dB's are useful to save writing many zeros and particularly good if one wants to confuse a non-RF engineer. They are also a shortcut to make multiplying easier.

Thank you for your responses and i'm going to do my best using your advices.