Hello all I'm having a problem at work we can't figure out. I build and just recently started performance testing our antennas. We have a indoor antenna range for testing with our equipment inside of it. Heres the problem thats been getting worse in the last few weeks. When we shoot a horn to horn calibration with our low band horns and begin testing we get data that claims some of our antennas are below the spec at 500 mhz but when I test previously known good antennas it will sometimes give us a failed test. After a few runs I can run a new calibration and see that the gain at 500 mhz has dropped a few db from the previous baseline calibration. Currently we don't run a amplifier off of our pna that generates the signal because removing the amp cured our drifting problem for a few weeks. After the problems started again we tried the amp again to boost the signal on the receiving end as well as configuring it to boost up the source horn. But it didn't help the problem. We have checked cabling, attenuators and just about anything else in the signal path. Our current signal generator is a Agilent E8362B and we use Orbit antenna software to get our performance data. Oh and yes I do let the equipment warm up for about 2 hours before and calibrations or testing. But the drifting ocurrs anyway. Any suggestions or help would be graciously appreciated. Thanks in advance!!!!