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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 18:10:41 EDT
List-post: <">>
Chris, the attenuation of these stubs is limited only by the loss in the  
coax used to make the stub.
If you think about it, you are trying to create a dead short at the  
frequency of the trap. A dead short will allow no energy to pass beyond the  
and reflects all of that energy back to where it came from.
If the stub coax had zero dB loss, then the short would be zero ohms. BUT,  
coax does have loss and that loss prevents a perfect short.
If the notch has a depth of 30 dB, that means that 1/1000 of the power  
remains after taking the notch into account. That basically means the short in  
this case (assuming a 50 ohm line) is like 50/1000 or .05 ohms. I will 
leave it  up to you figure out what the dB loss would have to be in this case.
This is a very simplistic analysis as no adjustments were made for the  
mismatch losses incurred in the example. 
Gerald K5GW
In a message dated 3/30/2010 10:44:28 A.M. Central Daylight Time, writes:


Great info on stubs, and very timely given that  Field Day is only 2 1/2 
months away.

A couple of quick  questions:

Does a higher "Q" coax yield more attenuation, all other  things being
equal? I know the notchwidth would be reduced, but was curious  about
notch depth.

Also, we've used stubs in the past at Field Day,  but still get some 
de-sensing and "cliks".

Do you have any ideas on  multiple stubs to attenuate the same
frequency, and what levels of  attenuation might be achieved with more
than one stub?

Finally, in  your excellent first draft of  "Some Q&A about coax and stubs 
for  your HF station"
in the section on "Q: What's the best way to measure a  stub" you only
give the physical test layout. A couple of pointers on the  test
procedure itself could be helpful.

Thanks for the great  work!


Chris AI4MI

--- On Tue, 3/30/10, jimlux  <> wrote:

From: jimlux  <>
Subject: Re: [TowerTalk] coax stubs /bandpass  filter
To: "Tower and HF antenna construction topics."  <>
Date: Tuesday, March 30, 2010, 9:01  AM

Along a similar note..

The stub is just a moderate Q  resonator that's easy to fabricate.  Why 
not put a lumped LC shunt  trap across the feedline?

Too hard to get the Q low enough?
Doesn't  have the nice harmonics property? (a coax stub for 40 is also a 
stub for  15)


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