I have looked at the spectrum of the MFJ-259 up to 1.7 GHz which is as far as the spectrum analyzer will go. The fundamental appears to be very clean. There are 2.23 volts out of the 259 over the entire frequency range and it is very nearly constant....small variation....maybe a couple houndredths of a volt or so. It is very rich in harmonics. The tenth harmonic, about 1.7 GHz, was about 70 microvolts. Maybe someone has an analyzer available to get to 2.4 GHz and let us know what the output is there. The frequency is plenty stable for test purposes. I suspect that a GDO might have the harmonics available but might not be stable enough to be very useful. I have built a small dipole into a connector which I can screw into the 259 and since it is pattery powered, I hope to be able to set it up "out back" and use it as a test signal for the mode S receive. Don't know if there is enough output to be able to do this. More later.......Jim....W5VZF. Dr. Jim Akers Dept. of Electrical and Computer Engineering Miss. State Univ.