[TowerTalk] coax stubs /bandpass filter

[Jim Brown]

On Tue, 30 Mar 2010 18:56:34 EDT, TexasRF at aol.com wrote:

>Wonder how you go about determining where that is? Could you assume the  
>plate load capacitor is a point of minimum impedance and go from there?

I see it as far more complex than that. The output of a tube stage is 
essentially the output Z of the tube transformed by the output network, 
where the output network is the plate tank and everything between it, the 
coax connector, and the transmission line. And, because it's a passive 
network, the TX end of the transmission line is part of the filter! And, of 
course, the transformation that the network provides is different at every 
frequency. 

>If so, the high impedance points would occur every odd quarter wavelength  
>(at the suppression frequency) along the cable, right?

Well, they certainly vary with that spacing, but it's hard to know where 
"zero" is. :)  

Another point. Any filter that you build from discrete components presents 
equivalent problems with respect to analysis. It's convenient to design a 
filter assuming fixed values of source and load impedance, but that is a 
LONG way from the real world. You could easily measure 60 dB of attenuation 
from a 50 ohm generator into 50 ohms resistive, but if the source is 30 -j10 
and the load is really 500 - j300 you'll get a LOT less attenuation! 

73,

Jim Brown K9YC