interference source

what is the best antenna choice to use to pick up an interference source by using spectrum analyzer and why ?

Please be more specific or application

Added after 48 seconds:

send us some scenario, case etc. It help us to understand the problem and guide you

Ok , i am working in a WiMax operator and as you know we have to pay money to communications commission to rent a frequency band , but as you know that anyone can use it in illegal way so we have to give CC information about the interference source . this is the story

It is not only the antenna, you can have a amplifier too.
You can select any antenna( based on frequency of interest).
Have the cascaded calculations then determine the signal strength (RSSI)

i will bring an amplifier but i think that the amplifier will see what antenna can see, so we have to use antenna with high gain . am i right .

which is better ? using log-period antenna , horn antenna or flat antenna ?

It is not important exactly what type of antenna you get. What is important would be that the antenna is directional, so that you can scan it about and find the true location of the source. You want it to
1) have high gain in one direction
2) have low sidelobes off-axis
3) receive very little from the backside
4) be small enough to move around/transport

I would be tempted to try a horn, maybe a dielectrically loaded horn to make it small.

spectrum analyzer is normally 50ohm input impedance, so select the corresponding frequency band antenna with 50 ohm source

To cover the frequency range log period antenna is best choice. Know the frequency range of your interest then find the suitable antenna operating in the frequency range.

can you (Mr. kspalla) tell me ,why you prefer log period antenna ?

Log periodic antenna is basically wide band, unidirectional, narrow beam etc.
This helps you not only identifying the frequency but the direction too.
Wide band nature assist you to catch the energy on the wide spectrum. Other wise you need to keep changing the antenna for each range of measurement.

what do you mean by unidirectional ?

Its antenna property.
Better you read antennas specifications and understand.