Dave, Thanks for the fascinating story. I always wondered about the
purpose of those holes in TV tuners. No wonder, the secrete of the hole
was kept inside a very tight group of "ins".
73, Lou KE1F
On 6/24/2021 21:40, David Gilbert wrote:
This is all very true ... especially the higher in frequency you go
but even at HF.
One of the early jobs in my career was as the tuning diode Product
Engineer for Motorola Semiconductor, and one of my first business
trips was to the biggest maker of mechanical TV tuners in the U.S. I
was trying to persuade them to convert to electronic tuning so we
could sell them tuning diodes. I got in a meeting with their two
design engineers, who were almost three times older than me and at
least three times smarter. They pulled out some of their mechanical
tuners and explained how they worked and what some of the advantages
were versus electronic tuning (and there are some).
The thing that really got my attention was when they explained how
they managed the coupling between the RF, Mixer, and Oscillator
stages. They would literally adjust some of the coupling by directing
the flow of the eddy currents in the walls of the stage shielding ...
which of course was all "grounded". One of the guys pointed to a
couple of punched out slots in one of the side walls and told me he
did that to make more of the eddy current go where he wanted it to go.
These guys were artists as much as they were engineers, but it
certainly taught me that, as you say, "where there is metal there is
resistance, and where there is current there is voltage" ... and I
will add that where there is current there are fields. Fields that can
couple to other bits of metal.
73,
Dave AB7E
On 6/24/2021 12:49 PM, Hal Massey wrote:
Fixing a common misconception about grounding…
Grounding does not get all the devices at the same potential. It just
minimizes the potential difference among them. “Where there is metal
there is resistance and if there is any current there will be a
voltage (period).
This is one of the biggest urban myths around and the source of many
difficulties for lay folks trying to understand grounding. -BSEE and
MSEE here.
A practical application of this for hams is to keep the runs short
and low inductance too!
73
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