Antenna and Frequency

Suppose I purchase an antenna from a manufacturer and he tells me that the antenna works in the 2400 MHz to 2500 MHz. Can I use this antenna to measure signals at (for example) 2375 MHz? what Will it work? What are the issues that we will come across?

Thank you.

Short answer: probably yes.
Longer answer: if the antenna is passive (i.e. does not contain a LNA or other circuitry that might effectively filter the incoming signal) then you need to realise that 2375MHz is only about 1% away from the lower end of the stated frequency range. When you compare that to the top end of the range (4%) then there is practically no difference. As most practical components (capacitor, inductors etc.) have a 5% tolerance (you can get components with tighter tolerances but the prices go up accordingly) then the frequency difference you are talkign about is negligible.
Even if the antenna does have active components, to get even 3dB rolloff in that short frequency difference, you would need a very sharp cutoff filter indeed.
Susan

Antennas like these can be usually used a bit beyond the specified frequency band.
As you want to use such antenna to MEASURE signal level, you must calibrate its gain at this or any required frequency. Antenna manufacturers often do not serve a measured gain or radiation pattern.
Also matching to 50-Ohm line is important, so you will have to measure it, too.
If an antenna is only needed to confirm that a transmitter operates, a piece of wire may work but for MEASURING the power level, also such piece of wire must be calibrated under "standard" conditions. For this please read a good antenna handbook for more details.