How to measure the gain of an antenna in an anheonic chamber?

Hi
How can one measure the gain of an antenna in an anheonic chamber. Since gain is measure of RF energy in some specific direction reletive to isotrop or λ/2 antenna, it make scnece a gain of xdBi for directive antenna.

What about gain of 5dBi gain (f: 2~7GHZ) for an UWB antenna having azimuthal radiation pattern.

There are 2 method for gain measurment

1)Absolute method
2)comparison method

for more return to balanis or kraus or stuszman measurments chapter.

Unfortunaly i do not have them. Can u briefly explain it.
Thanks

2 method :

1. use a standard Horn antenna to check the TX powe.Since the spec of the horn will mention about its gain vs frequency.

Say, you got -32dBm by using a horn to transmit
And the horn has 8dBi gain.

So, by simple calculation, the 0dBi is expected if your DUT provide 40dBm

Added after 5 minutes:

2. use a monopole antenna to check the TX power

say, your DUT is working at 2.4G. Make a monopole which is best for 2.4 G and then check the TX power in the chamber.

If the power received is 40dBm, then 40dBm is a reference of 0dBi.

Next, put your DUT in the chamber, transmit power and read the receive power,
If the received power is -44dBm, your DUT thus provides -4dBi gain.