In article , "Henry
Kolesnik" wrote:

John

I'm from a school that taught me that Q was a physical characteristic of a
coil and I still believe that.

There are two values of "Q" associated with a coil, ignoring that the "Q"
may change with frequency. The first is the "Q" of the coil by itself,
which is what you describe above, but there is also the "loaded" or
"circuit" "Q", which includes the effects of external circuit elements
like the resistors that Patrick has mentioned.

Q = {2 * pi * f)/R Selecting the optimum sizes of wire, number of turns,
and diameter to maximize Q is well documented. However in this Miller TRF
circuit I see no mechanism for changing any physical characterisic of the
coils. There are many ways of reducing Q and I see none in the circuit.

I would have to look but I believe you are correct about the Miller
circuit. I don't know what the "Q" of the Miller coils was, perhaps they
were not all that great, so they didn't have to add any series resistors
to reduce the "Q" at the low end of the band. I don't know, these designs
are not perfect, they are just one way to go.

I assume your neat little one stage mock up works and it's interesting that
L1/T2 and L2 are in seperate shielded cans but T1 is open. I'm curious as
to how RF is coupled from L1/T2 to L2 by T1. I've struggled to try to
simplify this simple circuit further but so far no cigar. Is it possible
for you to tune this to a known stations and then measure the variable
capacitor so that the Ls can be worked out from f = 1/(2 * pi * (LC)^0.5)

It's not my "neat little one stage mock up", it's Robert Casey's, but L1
and L2 are coupled in the same manner as in the W.E., Miller, and the
other similar tuners, by means of a common impedance in the ground end of
L1 and L2, this common impedance is T1 & C2 in Robert's design.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/