Talbot Andrew wrote:
snip snip
Now for the interesting bit. When it came to measure the unloaded Q of
each coil, the original one was a fair bit higher at Qu = 140, compared
with the Litz wound coil with Qu = 95. Both coils were the same
diameter, same inductance, and roughly the same length in total. So
why was the one made of plain wire better ? Self capacitance ? Q
was only measured at 1 MHz, perhaps I should try measuring gain at a
lower frequency.
Agreed it is interesting. My suggestion is that the Litz wire (with end
wires all bonded together) forms many loops from end to end, and this
gives additional modes of current flow, especially in the high impedance
unloaded situation. In the unloaded case a single wire has no loop
except via the resonating test capacitor. In the loaded case the
external shunting of the load is in parallel with the coil, so reduces
any internal loop effect that may be present in the Litz wire. When the
coil is loaded (which is the only useful application) the Litz wire
hopefully has the lowest series RF resistance for current (that carries
on into the load, which is different to the unloaded test situation).
You did not report in your findings if the Litz wire coil ran cooler
than the single wire coil, but I am presuming it did.
73, Bob ZL2CA
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