[Elecraft] Buryflex all the way to K3?

[Jim Brown]

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