[Amps] Pi-L Network Question

[Larry Benko]

Jim,

I have often wondered this very thing and recently built several filters 
and have made several measurements.  The effects of coupling between the 
coils are primarily stopband attenuation.  Small amounts of coupling can 
both degrade or enhance the stopband attenuation depending on the 
polarity of the coupling and the filter topology. Large amounts of 
coupling destroy the filter characteristics.  The measurements I made 
were on air core cylindrical, air core toroidal, and iron powder 
toroidal inductors which you might find useful.  If you have a PI 
network figured out I can send you an analysis file. This is only about 
a 5 minute job if you are familiar with LTSpice.

See:
http://www.w0qe.com/Technical_Topics/coupling_between_coils.html

for the coupling data.

73,
Larry, W0QE

On 6/18/2012 9:56 AM, Jim Garland wrote:
> I've been noodling around various tank circuit possibilities for a 160m
> monoband amplifier. A pi-L network isn't very practical for an 8877 because
> the relatively high plate impedance mandates very large values of C1 and C2.
> For an 8877, a simple pi-network is preferred. On the other hand, a Pi-L
> makes a lot of sense for, e.g., three 3CX800a7s or GU-74Bs,  because the
> combined plate impedance is under 1000 ohms and the required capacitance and
> inductance values are quite reasonable.
>
> I know that good design practice is to orient L1 and L2 in a Pi-L at right
> angles to each other (or to use a self-shielding toroid for L2), to minimize
> mutual inductance between the two coils. However,  I don't really understand
> why this is so important. In principle, it would be very convenient to use a
> single coil whose inductance is L1+L2, with a tap for connecting C2 at the
> junction point between L1 and L2. I'm sure that the mutual inductance would
> invalidate this approach, but I don't understand why. Can somebody who's
> thought about this issue clarify it for me. Thanks!
> 73,
> Jim W8ZR
>
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