According to DR. Tom Van Doren, the return current path would start
following the 'U' shape around 1 kHz.

I attend one of his courses on GROUNDING AND SHIELDING where he had a short
piece of coaxial cable. On one end of the cable he connected an oscillator
and the other end was a resistor. Then he shorted the shield at both ends
with a wire. With a clamp-on current probe he measured the current through
the shorting wire. At dc all of the current went through the shorting wire.
As the frequency was increased less and less current flowed through the
wire. At about 1kHz there was a slight decrease in current and at 10kHz
there was a significant decrease in current.

The coaxial cable provided the least impedance path even though it was 4
foot long and the shorting wire was 2 inches.




At 01:16 PM 12/2/2004, you wrote:
>Hi Jack... (safety tip... don't say that on an airliner)
>
>If I remember correctly... the RF return currents will always follow the
>path of least impedance...  :) Wherever that path is... on the ground plane,
>or power plane or even if it's radiated through the air I would imagine...
>
>I must admit though, I do not remember at what freq it stops following under
>the 'U' shape... I would guess it would be around a megahertz...
>
>Best regards,
>
>Bill Brooks - KG6VVP
>PCB Design Engineer , C.I.D.+, C.I.I.
>Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
>e-mail:[log in to unmask]
>http://www.dtwc.com
>http://pcbwizards.com
>
>
>-----Original Message-----
>From: Jack C. Olson [mailto:[log in to unmask]]
>Sent: Thursday, December 02, 2004 10:55 AM
>To: [log in to unmask]
>Subject: [DC] Return Current Path vs. Frequency
>
>Return Current Path vs. Frequency
>
>-=-=-=-
>Easy question:
>
>Imagine a "U"-shaped trace above a solid ground plane,
>with a connection at each end.
>
>Inject a signal through the "U", and vary the frequency.
>
>At what frequency will you see the return current
>follow the shape of the U rather than the shortest path?
>
>-=-=-=-
>Bonus question:
>
>What if I replace the ground plane with a power plane?
>(I say it makes no difference. )
>
>thanks,
>Jack (the "arguing with a new engineer" guy)
>
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Donald Kyle C.I.D.+
281-285-7528 voice
281-285-8593 fax
[log in to unmask]
Schlumberger
Sugar Land Product Center (SPC)
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Sugar Land, Texas 77478

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