Re: BL2 choking Impedence measurements?

Posted by Jim Brown-10 on Aug 28, 2010; 7:44pm
URL: http://elecraft.85.s1.nabble.com/BL2-choking-Impedence-measurements-tp4686857p5473397.html

On Sat, 28 Aug 2010 11:59:21 -0700 (PDT), Wes Stewart wrote:

>Equally silly (and applicable here) is measuring a device in a 50-ohm
>environment and trying to determine attenuation, when in actual operation,
>neither the source or load impedance is known.

Exactly. It is the 50 ohm environment that is the source of much of the
measurement error. Measuring S21, by one means or another, is an effective
solution, and the one I've used.

Measuring shield current, as David has suggested, is an indication of the
effectiveness of a choke IN A PARTICULAR CIRCUIT -- for example, a feedline
onto which it is inserted, the antenna to which the feedline is connected,
and the termination of the feedline at the receiver. The effectiveness of a
choke in any given application depends upon the common mode impedances
within that particular circuit.

These are two VERY different problems, and two very different measurements.
Both are useful. Measuring the choke on the bench tells us about the
properties of the choke. Knowing that, we can optimize the choke for a
given frequency range and use, AND we can insert the properties of a choke
into a model and, if the model knows enough about the circuit of interest,
predict the effectiveness of the choke in reducing common mode current.

There is, of course, another set of unknowns. They relate to the source(s)
of the noise, their location, polarization, directivity, proximity to the
feedline and to the antenna, the directivity of the antenna, etc. The choke
will have little effect on the component of noise picked up on the antenna
-- it can suppress noise picked up only on the feedline. In general, the
choke will be most effective in suppressing noise that is much closer to
the feedline than to the antenna, and/or is rejected by antenna
directivity.

73, Jim K9YC


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