Have a gsm antenna near me and need to measure a power

Hi,
where are you from? Use nokia gsm phone with enabled network monitor.

Is the another way to monitor the power?

Hi, contact your local radio frequency reglation authorities.

Using a Spectrum Analyzer (zero span mode, trigger = video, RBW = 1MHz) connected to a calibrated dipole antenna.
Have to search for the Base Station TX channel (925MHz to 960MHz or 1805MHz to 1880MHz).

Usually power sent by a cell phone is difficult to measure because:
1) TX level is function of power received from local cell.
2) you don't know the pattern of the antenna.
3) power is impulsively modulated.
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I suggest to perform an approximate measurement; measure absorbed DC current, the TX power cannot be higher than absorbed DC power.
Also you know that efficiency may be into 25 to 50 % range, so you'll be able to give a rough estimate of TX power with a uncertainty bar of 3 dB

Do you mean the antenna of Base station ,or mobile handset ?

For this particular application to measure the radiated power, using the Spectrum Analyzer and a calibrated vertical dipole antenna, is more than enough.
It will give the radiated power in that particular location. For a better accuracy shall be done multiple measurements (and average), at different wavelengths distance.

How to enable network monitor use nokia gsm phone?Thanks.

To enable network monitor on nokia cell phone you need

some program like logomenager and data cable.

Put nokia network monitor in google search and you will find

a lot of useful information about that issue.