On Mon, 31 May 2010 21:00:32 -0400, Tom W8JI wrote: >Something else was going on Jim. Not shielding.
>In short, >the "antenna" used on the analyzer has increasing >sensitivity for a given flux level with frequency. Agreed. But there's more -- my amp, a Titan 425, has an L network output, and there's a stub about three feet of coax from the output. Thanks to antenna switching, I can't get closer than that. So I'm looking to see some additional harmonic suppression, and so far I don't. That is, I don't see suppression when I add the stub. I'm confident of the stub tuning -- it was cut in a 50 ohm system (HP generator and HP analyzer), and tweaked at the 2nd harmonic. More on that below. >More likely you changed the impedance presented to the tank >circuit on the second harmonic. The coax I was changing was between the K3 and the amp. >Changing 80 meter impedance at the 160 meter tank can radically >affect harmonic suppression. Yes, and this is not generally understood. It's a general characteristic of all passive networks. >When we want to measure something, we have to be careful to >actually measure what we think we are measuring. :-) Yes. >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. As it happens, the primary reason that I've been working on this is to study the effect of stub placement, with exactly these effects in mind. :) And I fully understand exactly what you're saying. But the source is not the only boundary condition contributing to the impedance along the line -- the antenna does as well. In this case, it's a vertical about 30-40 ft of RG8 away. It looks like 50 ohms at the fundamental. I haven't measured at the 2nd harmonic, but I'd bet the Z is prtty high. :) If you look at my website, you'll find a first draft of a tutorial on coax and stubs that addresses this issue. Any comments you might have would be appreciated. http://audiosystemsgroup.com/Coax-Stubs.pdf 73, Jim K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
