For a quick graphic demonstration of what the other posters are
pointing out without having to build and measure a physical circuit,
try putting this circuit into "RFSim99" (a free program), and select
"Use physical model" in the inductor value dialog box. Then change the
unloaded Q of the inductor there to see the effects on performance,
both loss and filter sharpness (-- rejection of unwanted signals).
You'll need to connect "IN" and "OUT" to measurement ports (one of the
available components in RFSim99) that you set to appropriate
impedances.

Such a simulation doesn't address the effects that Roy mentioned of
coupling to external fields and lowering of Q by coupling to external
material. On the other hand, though toroid coils have lower coupling
to externals, their coupling isn't zero, especially when you use a
low-mu core---so be careful how you mount the toroids, too. In a
couple minutes, the simulation can give you some insights to get you
started and save a lot of time soldering, but expect to ultimately
learn even more by actually building and measuring the circuit.

Cheers,
Tom


Ben Jackson wrote:
In a coupling circuit such as:

C1|| C3||
IN----||----o----o----||--OUT
|| | | ||
--- C
--- C L1
C2| C
| |
'----'
|
===
GND

(see, eg, the "38 Special" NorCal 30m QRP tranceiver sch connecting
the NE602 single-ended output to an amplification stage:
http://www.amqrp.org/kits/38spcl/ )

What is the purpose maximizing the Q of L1 by using, say, a hand-
wound toroid vs a molded inductor? Rs of a 1.8uH moulded inductor
might be 1.5 ohms vs .1 ohm for 5 inches of 24ga wire. However the
expected effect in tank bandwidth doesn't seem to matter compared to
the large effect of varying C1 (very narrow for small C1, 5p in the
example).

I can see why a homebrewer would prefer to keep a bag of T37-2 and
some enamelled wire around, but in a kit such as that, wouldn't a
moulded inductor do just as well?

--
Ben Jackson AD7GD

http://www.ben.com/