[TowerTalk] Braided Ground Strapping

[Tom Rauch]

> 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
> 
> 
> 
> List Sponsor: Are you thinking about installing a tower this summer? Call
> us for information on our fabulous Trylon Titan self-supporting towers -
> up to 96-feet for less than $2000! at 888-833-3104 <A
> HREF="http://www.ChampionRadio.com"> www.ChampionRadio.com</A>
> 
> -----
> FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
> Submissions:              towertalk@contesting.com
> Administrative requests:  towertalk-REQUEST@contesting.com
> Problems:                 owner-towertalk@contesting.com
> 


73, Tom W8JI
W8JI@contesting.com 

List Sponsor: Are you thinking about installing a tower this summer? Call us
for information on our fabulous Trylon Titan self-supporting towers - up to
96-feet for less than $2000! at 888-833-3104 <A HREF="http://www.ChampionRadio.com">
www.ChampionRadio.com</A>

-----
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com