I am using the Agilent E4446A to take a noise floor measurement. The spectrum analyzer is connected to a bow tie antenna via 3 low-noise RF amplifiers made by Mini Circuits. The antenna is contained in a completely enclosed Faraday cage and the amplifiers are being run with batteries (as to not add noise from a power supply). The amplifiers seem to work fine when connected to the spectrum analyzer and antenna by themselves and in pairs. However, when I connect the 3 together I see a huge spike in the noise floor measurement. I am seeing nearly 60dB from the noise floor at approximately 182MHz consistently. My noise floor measurement is taken from 100MHz to 300MHz. I was wondering if the cascade of the 3 amplifiers results in some kind of noise resonances that are causing this "noise spike"? I would like to use the 3 amplifiers for future measurements to increase the level of the smallest signal that I can detect.