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Author Topic: removing choke lowers noise level ???  (Read 419 times)

KB7TT

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removing choke lowers noise level ???
« on: December 06, 2020, 10:17:26 AM »

I am working with a friend who is trying to isolate and remove noise at his Qth.  He removed a DX engineering feedline choke from his coax on the 80M dipole and his noise went down one s-unit.  Any ideas how that is possible?
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K6AER

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Re: removing choke lowers noise level ???
« Reply #1 on: December 06, 2020, 12:14:59 PM »

You would have to make that determination over several days going back and forth. I have seen noise levels vary 4 "S" units from the same source over a 4 hour period.

To quote my high school math teacher.."one data point does not make a graph".
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WB6BYU

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Re: removing choke lowers noise level ???
« Reply #2 on: December 06, 2020, 02:20:10 PM »

It certainly is possible that removing a choke will reduce common
mode current, depending on where it is placed in the feedline, along
with the various line lengths and ground connections.  So if the
noise is being picked up via common mode current, that would reduce
the noise level.

Here is an example:  suppose the coax is 1/4 wavelength from the
antenna to a well-grounded station.  Because one end of the coax
shield is grounded and the coax is 1/4 wavelength, the shield will
look like an open circuit at the feedpoint, and there will be little
common mode current.

Now insert a choke at the radio.  With a perfect common mode
choke the shield now looks like a quarter wave open at the far
end, which will have a low impedance and a significant amount
of common mode current can flow on it.

That is simply an example that such a condition is possible.
Whether or not that is the explanation for the specific behavior
in your case depends on a lot of other factors.

W9IQ

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Re: removing choke lowers noise level ???
« Reply #3 on: December 06, 2020, 03:18:34 PM »

When a common mode choke is poorly engineered, the designer relies on choking impedance rather than choking resistance. The dominant reactance can easily be reduced or cancelled by the impedance of the outer braid of the feedline. A worst case example of this is an ugly balun (coiled coax) - all reactance with little or no resistance.

If you want the choke to perform consistently, regardless of the application, go for the highest choking real resistance you can attain. This will always involve at least one ferrite toroid or its equivalent with the proper number of turns.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

KH6AQ

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Re: removing choke lowers noise level ???
« Reply #4 on: December 07, 2020, 08:36:15 AM »

Where is this choke located, near the shack or at the antenna?

If near the shack, is there a choke at the antenna?

What is this choke (how is it constructed or what is its model number)?
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