> TO BE FLEXED." part. I had never considered braided ground straps to be
> inferior to solid copper wire for grounding. I had also never considered
> that there might be a measurable impedance in braided ground strapping
> when compared to solid wire. I am curious to hear what others thoughts
> are on the matter. Does it really make a noticeable difference to use
> solid wire as opposed to braided strapping?
>From Electronic Designers Handbook page 8-25, in the section on
transmission line..
" When the outer conductor is not solid but is braided to give
greater flexability, the attenuation in decibels per unit length due to
resistance of the outer conductor is multiplied by a factor of
approximately 2.75."
That is for a dense, clean, braided conductor that is compacted by
the jacket. I have other textbooks that quote even larger resistance
changes.
If you loosen the braid, so the conductors do not lay in pressure
contact, the resistance skyrockets. My own measurements of
braid from RG-8 cable, when removed from the cable, show clean
fresh braid from RG-8 heats and discolors with only 8 amperes of
30 MHz RF. The temperature rise is about the same as a #16 solid
wire!
I visited an amplifier manufacturer and saw a prototype with RG-58
braid from the ten meter tap of the tank to the switch. I made an off-
hand comment to the effect "that outta get hot fast". The engineer
snickered, fired it up, and then had his smile fade as the braid
actually smoked and melted. He stuck in some #10 solid wire, and
it ran cool as can be.
The tarnished braid of coax that has been wet inside is by far the
primary loss mechanism in a failed cable.
Knowing what a few amperes of RF can do to braiding that is not
clean...and with pressure between the contact points on weaves....I
sure would hate to depend on it for lightning or RF grounds.
NASA and others prohibit the use of braiding in lightning grounds,
and you'll never see it in BC stations where the ground is involved
in lightning or RF applications unless it is way overkill size and very
short.
One thing that doesn't hurt the system quite as much are parallel
lays of wire that are not woven. In that case you loose only a little
bit of effective surface area for RF, because of the stand-to-strand
air gap at the surface. The current pushes to the outside edge of
the individual conductors, so each individual tiny conductor has
only a fraction of its cross section carrying current...and only the
conductors on the outside carry any current.
That's the reason Litz wire starts to fall apart at 100KHz, and by 1
MHz or so is ineffective.
People forget the current migrates to the outside edges of
conductors, away from the wire core. Keep that in mind, and you
can picture the problems when the conductor weaves in and out or
when you rough up the surface by stranding or weaving the small
wires to make a large conductor.
Smooth and wide is best by far for RF, unless you have no other
choice. That's while the foil is UNDER the braid of low loss coax,
and why hardline has solid center conductors and shields.
>
> Also, does anyone have any thoughts on using a flat copper common buss, as
> opposed to copper tubing?
>
> ----------------------------------
> Kevin Hemsley
> kev@ida.net
> KB7TYA
>
>
>
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73, Tom W8JI
W8JI@contesting.com
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