Posted by
Jim Brown-10 on
Aug 25, 2020; 5:28am
URL: http://elecraft.85.s1.nabble.com/K3-going-expeditionary-tp7664289p7664300.html
On 8/24/2020 8:17 PM, Clay Autery wrote:
> Side bonus, was that the 12VDC ground plane was so good afterwards, that
> I have been able to take out a lot of the home run grounds from a bunch
> of lights, et al... ofsetting weight gain by some bit.
Important fundamental concept, Clay, that I learned in an IEEE EMC
workshop. DC follows the path of least RESISTANCE. AC follows the path
of least IMPEDANCE. If there is a transmission line with, for example, a
chassis in parallel with the transmission line return, above about 1
kHz, all the return current will be in the transmission line, NOT in the
lower resistance chassis.
That's because the return patch through the chassis includes a lot of
INDUCTIVE REACTANCE, which increases in proportion to frequency, while
the path through the transmission line, if matched, never rises about Zo
of the line. The whole point of using the transmission line is to reduce
noise and crosstalk. It's only effect on DC (as compared to the chassis
path) is to slightly reduce the DCR of the return path by adding the
transmission line return in parallel. With coax, of course, the shield
is the return. Wtih 2-wire line, one conductor is the "hot" lead, the
other the return. AND -- 2-wire line and coax are no different in this
regard! The KPA500, for example, uses twisted pair for wiring from the
input coax to the amp input stage.
73, Jim K9YC
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