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Re: [TowerTalk] coax stubs /bandpass filter

Subject: Re: [TowerTalk] coax stubs /bandpass filter
Reply-to: "Tower and HF antenna construction topics." <>
Date: Tue, 30 Mar 2010 18:56:34 EDT
List-post: <">>
Jim, to further put the numbers in perspective, using the 10 to 1 example,  
the impedance could be between 5 ohms and 500 ohms (50/10 - 50 X 10). The  
nominal 30 dB notch with .05 ohm short would have attenuation from 20 dB to 
40  dB. 
One would be smart to make sure the notch is placed at one of the 500 ohm  
points along the cable. 
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?
If so, the high impedance points would occur every odd quarter wavelength  
(at the suppression frequency) along the cable, right?
Gerald K5GW
In a message dated 3/30/2010 5:43:29 P.M. Central Daylight Time, writes:

On Tue,  30 Mar 2010 18:10:41 EDT, 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  

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. :) 


Jim Brown  K9YC


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