[TowerTalk] coax stubs /bandpass filter

[Jim Brown]

On Tue, 30 Mar 2010 18:10:41 EDT, TexasRF at aol.com wrote:

>This is a very simplistic analysis as no adjustments were made for the  
>mismatch losses incurred in the example. 

Your analysis and description is excellent as far as it goes. But let 
me take it a bit further (and closer to reality). 

The other part of the problem is, "what is the impedance on the 
transmission line at the point where the stub is connected, AT THE 
FREQUENCY THE STUB IS SUPPRESSING?" That is NOT an easy question to 
answer, but it has a LOT to do with how effective that short circuit 
is.  

Let's take the example of a line that has a mismatch at the antenna of 
perhaps 10:1 at the harmonic, and a mismatch at the transmitter of 
perhaps 20:1. These are VERY realistic assumptions -- both the 
transmitter and the antenna provide a fairly low SWR at the operating 
frequency, but almost never do so at the harmonic (unless the antenna 
happens to be something like a log periodic). 

At various points along that line, the impedance varies from a few ohms 
to several hundred ohms, and at all points other than the peak and 
null, has both resistive and reactive components. Without knowing the 
impedance at both ends and the electrical length of the line, there's 
no way to know where the peaks and dips in the impedance fall. If you 
happen to pick a point on the line for your stub where the impedance is 
low, it does almost nothing. If you happen to pick a point on the line 
where the impedance is high, it does a LOT. And anywhere in between, 
the results vary from not very much to a lot. :) 

73,

Jim Brown K9YC