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
short
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.
73,
Gerald K5GW
In a message dated 3/30/2010 10:44:28 A.M. Central Daylight Time,
cpwendling@yahoo.com writes:
Jim,
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!
73,
Chris AI4MI
--- On Tue, 3/30/10, jimlux <jimlux@earthlink.net> wrote:
From: jimlux <jimlux@earthlink.net>
Subject: Re: [TowerTalk] coax stubs /bandpass filter
To: "Tower and HF antenna construction topics." <towertalk@contesting.com>
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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