> This discussion brings up a good question. What is the
best what
> to make accurate inductor Q measurements? I understand
that
> there are Boonton Electronics Q meters out there. How do
those
> compare with other techniques like using a vector network
> analyzer? I see people talking about Q's of 400 to 800
for
> HF loading coils. How easier or hard is it to measure Q's
in that
> range?
The worse thing about measuring a large coil is the physical
size. Anything a few coil diameters away affects the coil,
as do wire leads used to make connections.
I use two methods.
I have a large test fixture (three feet on a side) made
from double sided PC boards that has BNC connectors
scattered around along with two ceramic vacuum variables. I
can resonate the inductor with very short leads in a
shielded housing that has walls well away from the coil and
measure the parallel or series resistance at resonance and
the amount of capacitance required to resonate the system.
I use test gear for direct readings, either a currently
certified Agilent vector network analyzer or an HP
impedance test set designed specifically for measuring
components. The problem is always calibrating (normalizing
out) test leads on larger components, and not having things
around the inductor affect it.
In my experience we really can't use 3db bandwidth or SWR
bandwidth to closely estimate Q.
The highest inductor Q I've ever measured is just under
1000. I know some programs estimate much higher Q's, I've
just never seen them and don't believe it is possible
outside of a cavity. The best inductors are large
transmitting type edge wound or copper tubing.
73 Tom
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