> I do have an HP 3590D spectrum analyzer sitting in the > shack, and put a > simple loop on it to look at 2nd harmonics from my Ten Tec > 425 amp and 160M > vertical. It was about -44 dBc when I started, and I got > it down to the rated > -50 dBc by doing nothing more than changing coax jumpers.
Something else was going on Jim. Not shielding. We cannot accurately measure harmonic suppression with a probe or loop unless the probe or loop is somehow frequency compensated for sensitivity as frequency changes. In short, the "antenna" used on the analyzer has increasing sensitivity for a given flux level with frequency. As I recall a very small loop open terminal voltage is about 8*10-6th times F times loop effective area. So your loop has 6dB more sensitivity as frequency doubles, although it could in practice be much more or even somewhat less because of other effects that might unintentionally compensate or enhance the change. More likely you changed the impedance presented to the tank circuit on the second harmonic. Changing 80 meter impedance at the 160 meter tank can radically affect harmonic suppression. Harmonic suppression will be less with a low impedance terminating a pi network on the harmonic, and greater with a higher impedance on the second harmonic. Just changing the velocity factor (electrical length) of the cable a small amount can change the level of the harmonic significantly. When we want to measure something, we have to be careful to actually measure what we think we are measuring. :-) Case in point, I have a 2nd harmonic stub on my 160 antennas. It is in a calculated sweet spot 1/4 wavelength on 80 meters from my amplifier pi-network loading capacitor. This makes maximum possible 80 meter Z across the load cap on the 160 amplifier, so the loading cap looks like a more effective short on 80. By altering nothing but distance of the stub from the amp to 1/2 wave on 80 meters on the 160 feeder, the harmonic suppression decreases 10-15 dB. Even the length of the cable to the matching system affects the harmonics, and each matching system is different! Changing cable electrical lengths will change harmonic levels significantly, even without a harmonic suppression stub in the system. 73, Tom ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
