Jim K9YC wrote:
>
>Those interconnections, especially analog audio, is one big reason why
>chassis-to-chassis bonding is far better than running individual wires
>to a common point (or bus bar).
>
>And there is nothing about that chassis-to-chassis bonding that is less
>good than individual wires to a common point. Remember that with those
>wires to a common point, we still have a loop to create magnetic
>coupling -- it's formed by the interconnections between the boxes and
>those long wires to the common point.
>
The aim of "bonding" is to equalize the chassis potentials of several
separate items of equipment, so that unwanted voltages induced along the
lengths of the interconnecting cables are effectively shorted out.
For the type of RF equipment that we use, we should be aiming to
equalize the potentials not only at DC and 50/60Hz, but also over the
entire range of RF frequencies from LF through at least 30MHz, and as
far into the VHF region as we possibly can. As Jim also points out, it
is also important to minimize the areas of the loops that are formed
between the signal interconnecting cables and the bonding conductors.
And finally, we need to do all this in a way that will be practicable
for our own particular station layout. We need to do our own thinking
about this, because most of us don't use from 6ft racks any more.
Amateur operating desks tend to have separate items of equipment lined
up side by side, probably with at least one more shelf running over the
top. To make things even more difficult, most of the necessary
connections are at the rear of the equipment.
No single method of bonding completely meets all of those requirements,
so no single method can claim to have all of the advantages.
The best engineering compromise that I have been able to find involves a
strip of metal maybe 6 inches wide and 1/16in thick, laid flat along the
rear of the table-top underneath the rear feet of the transceivers,
amplifiers etc. Connections to the "chassis ground" terminals of
individual pieces of equipment can then be made by short vertical
lengths of wide braid or soft copper strip. The generous dimensions of
the main metal bonding strip meet most of the requirements for low DC
resistance, low AC/RF resistance and low inductance as they vary in
importance across the frequency range. Interconnecting cables can be
made to lie directly on top of the bonding strip, which automatically
minimizes the area of open loops.
Because the resistance and inductance of the 6in wide strip is so low,
this method of bonding every chassis down to the strip should give even
better performance than direct point-to-point bonding from one chassis
to the next. You've spotted the practical difficulty, of course: how to
make the short vertical connections to the chassis terminals at various
unrelated points along the strip.
Whichever bonding method we use, most of us will also throw in a few
compromises... and then the performance of the different bonding methods
will begin to converge. The key point is: do whatever will work best in
your particular shack layout, to equalize the potentials between the
different chassis across the entire frequency range.
73 from Ian GM3SEK
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