RE: [SI-LIST] : Heat sink and radiated emissions

From: [email protected]
Date: Fri Jul 28 2000 - 14:36:02 PDT

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Your item #2 below is not valid.

a. Assuming the plastic heat sink has aluminum particles inside, the same
loss should be present in the plastic heat sink. On top of that, the
plastic heat sink also has dielectric loss, which the aluminum heat sink
does not have.
b. At 1.5 GHz, the surface resistivity, Rs, of aluminum is about 0.013 ohm
per square. The heat sink behaves as an antenna in free space, so the
impedance of the heat sink is probably in the range of 10 ohms to 100 ohms
depending on the frequency. The Rs of aluminum is not significant at this
frequency.

George Tang

-----Original Message-----
From: Barry Ma [mailto:[email protected]]
Sent: Friday, July 28, 2000 8:53 AM
To: [email protected]
Cc: [email protected]; [email protected]
Subject: RE: [SI-LIST] : Heat sink and radiated emissions

Actually, there are three possibilities, which may contribute to the fact
that the plastic/metal heat sink radiates as well as aluminum heat sink at
frequencies above 1.5 GHz:

1. The plastic moves close to metal due to capacitance between metal
particles.
2. The aluminum moves to poor conductor due to lower conductivity at GHz.
3. Both move toward each other and become closer.

If we can compare to radiation results of ceramic heat sink and no heat sink
above 1.5 GHz, we may determine which plays a major role.

Thanks.
Best Regards,
Barry Ma <[email protected]>
ANRITSU www.anritsu.com
Morgan Hill, CA 95037
Tel. 408-778-2000 x 4465
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