[TowerTalk] inductance of tubing vs bar or strip

[Steve, W3AHL]

Jim,

A good approximation is to consider a right angle bend as a 1/4 turn coil 
with the same radius of the bend.   But the typical coil inductance 
calculators make too many simplifying assumptions to handle this.  AC power 
system engineering handbooks are a good source of info on this topic.  I'll 
look for a reference next time I'm at the university library.   Seems like 
something every savvy ham should know....?

I did a quick measurement with a 4' piece of #6 copper wire.  Adding a 1" 
radius 90 degree bend increased the impedance about 5 ohms at 10 MHz, which 
would equate to an inductance of about 0.08 uH for the bend.   This was not 
a precision test, but gives an order-of-magnitude estimate at least.

My experience with this relates to PCB trace layout modeling and the effect 
of sharp corners on trace inductance on fast edge rate signals.  I'm retired 
now and don't have access to the EM modeling software we used.

The mechanical stress is only high because of the interaction of the 
magnetic fields with surrounding objects.   The interaction of magnetic 
fields in a right angle bend of the current-carrying conductor not only 
cause high stresses at the bend, but an increase in self-inductance at the 
bend also.

Steve, W3AHL

<snip.......>
> Steve,
>
> Do you have an equation or other reference for the inductance of a bend?
>  Last time I went hunting for such a thing, I couldn't find it, and
> running some FEM models showed that the inductance of a "less than
> quarter turn" bend wasn't all that high. (because the inductance comes
> from the interaction of the magnetic field of one part of the wire with
> the current flow in another part, and for bends, there just isn't much
> interaction).  I wasn't interested in deriving an analytical expression.
>
> There are good reasons to not bend lightning conductors or other pulsed
> high current wires, because while the inductance isn't high, the
> mechanical stresses are.
>
> Jim
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