FreeLists / si-list / [SI-LIST] Re: Inductance variation.

["Steve Rogers" <SRogers@xxxxxxxxxxxxx>]

Heres my input...

Putting metal things near a coil is generally not a good idea. The time =
varying fields from the coil induce currents in the metal which will =
flow to oppose the current in the coil (good old lenz law). The effect =
of this (for conventional round solenoids) is to reduce the inductance =
of the coil. However, this is not the whole story. You must also =
consider the electric field effects. In practice you will also increase =
the capacitance to ground from the coils turns. If you think of this in =
very simple terms you have a coil with lots of capacitors in parallel =
with it to ground. At some frequency you will see a parallel resonance =
of the coil inductance and this stray capacitance. The reactance =
increase at high frequency you are seeing could be the coil resonance =
with the stray capacitance.=20

Theres also the fact that in reality the humble coil actually exhibits =
transmission line behaviour. You can see this from the multiple =
resonances you get from it...... I will let you investigate this =
yourself



-----Original Message-----
From: GEORGE VARGHESE [mailto:rfengg@xxxxxxxxx]
Sent: 07 March 2003 11:39
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Inductance variation.



Hi All,
I have one more very interesting observation regarding inductors at RF =
when in the presence of a conducting shield.=20
=20
When I put a high valued inductor close to a copper shield and observe =
the reactance on a VNA , the reactance seems to decrease at lower =
frequencies and the reactance increases at higher frequncies when the =
shield is put near the inductor. What I mean  is that ,  the presence of =
the copper shield causes the reactance to decrease at the lower =
frequencies (20Mhz) and increase at higher frequencies (100Mhz) . Can =
anybody  tell me the reason for this?


Regards,
George.
=20



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