On 3/22/14 9:04 AM, Gary Schafer wrote:
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
Jim Lux
Wide flat strips have low AC *resistance* because of skin effect, but
the inductance isn't much different from a round wire.
AC resistance is *very important* in applications like RF grounds for
commercial broadcast antennas, because resistance = heat = lost power =
lost money, so they use flat ribbon.
But lightning energy peaks around 1 MHz where low AC resistance is
important.
The problem is that the inductance reactance is orders of magnitude
greater than the AC resistance, so when it comes to transient voltages,
it's L you worry about not R.
Compare the AC resistance of a meter of AWG 10 wire with the inductive
reactance..
AWG10 wire has a ac resistance at 1 MHz of 0.03 ohms
A one meter long conductor has an inductance of about 1 uH, and at 1
MHz, that's 6.28 ohms.
So the transient voltage you are seeing is more determined by the 6
ohms/meter from L than the 0.03 ohms/meter of R. Lowering R by going to
a wide flat ribbon doesn't help much, say you drive the resistance down
to 0.01 ohms/meter. The 6.28 still dominates.
The inductance of two parallel inductors is:
(L+M)/2
where L is the inductance of a single inductor and M is the mutual
inductance.
Yes flat strap has mutual inductance across its width but isn't mutual
inductance considerably lower with a flat strap than separate parallel
wires.
Not really. If the current is flowing evenly along the strip it makes
no difference whether it is separate wires or one continuous flat wire.
There's no current flow "cross ways" across the strip, so it makes no
difference if there's insulator in the way.
there is a "proximity effect" in close wound coils which is akin to skin
effect, where the current is carried in the exposed surface of the turn,
as opposed to the sides of the turns. It's the same thing as skin
effect: the current gets pushed to the outside.
But it's still tightly coupled by the magnetic fields, so the inductance
doesn't change much.
If you go from a 1x1 cm bar to a 0.05 x 20 cm strap, the inductance is
reduced by about 30%. The AC resistance, though, drops by 90% for the
same change.
So if you're in a steady state kind of scenario (AM broadcast
transmitter), where the inductance is "tuned out", that 90% change in
loss is worth it.
If you get up into microwave frequencies, the AC resistance starts to
get larger, because skin depth goes as 1/sqrt(f)
go up to 100 MHz, and now that AWG 10 wire a meter long has an AC
resistance of 0.3 ohms. The L is still 1 uH, so the Z is now 628 Ohms
You have to go way down around a few hundred Hz to where the resistance
starts to dominate over the L.
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