[TowerTalk] Radial Connection to tower
Richard (Rick) Karlquist
richard at karlquist.com
Sun Dec 13 14:33:27 EST 2015
On 12/13/2015 11:12 AM, Jim Lux wrote:
> Putting strap, ribbon, or multiple parallel wires (that are close to
> each other) reduces the R but doesn't change the L very much. The
> magnetic fields couple between parallel wires or "filaments" within a
> wide flat strip, so the L stays pretty much the same.
> Strip has lower AC resistance (more surface area) than a round wire of
> the same cross section.
> So if you had multiple wires going out to the ring in different places,
> you'll reduce both L and R (the magnetic field from the wires won't
It's worse that that. The "proximity effect" is such that adding
multiple wires in parallel isn't effective for either resistance or
inductance. Strip only has lower AC resistance when used as
a transmission line like microstrip or stripline. A strip in near
free space (nowhere near a ground plane) suffers from current
crowding at the edges; most of the copper goes to waste. A round
wire is unique in that is has no edges, so the RF current is
forced to be uniformly distributed.
In my consulting practice dealing with RF plasma, I constantly
run into clients with strips (they call them straps, whatever)
flying around their machine as if they are accomplishing
something more than round wire. After cleaning up the rats
nest, the clients are always surprised to find that stuff
works a lot better.
If you want to read about this topic, study inductor design
articles in the Radiotron Designer's Handbook (I think its
free on line now).
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