[600MRG] coil Q note

[Rudy Severns]

There's a point I'd like to make regarding coil Q. 

 

We define Q as the ratio of the series reactance XL to the series resistance
Rs: Q=XL/Rs.  When you insert a loop or disk the inductance falls, i.e. XL
gets smaller.  But the winding has not changed so we still have at least Rs
plus the losses in the loop/disk.  It's also possible the field in the coil
is disturbed so Rs is increased??  We could make the loop/disk super
conducting but we still have at least the original Rs so Q must fall because
XL/Rs is smaller. 

 

If lower inductance were the only effect in the last experiment where L went
from 836 uH down to 792 uH, a ratio of 0.92, would give Q=674.   But I saw
Q=480 so there is an additional loss factor.  The loss in the disk is an
obvious addition but might not account for all of it.  It's possible the
fields in the winding is also altered.  That would be hard to figure out
analytically but I have some EM modeling software which could plot the
fields.  Unfortunately it will be this winter before I can look into that.

 

It could be argued that the experimental coil has quite a high Q (by
design!).   Lower Q coils may not be so sensitive????  Again that's an
experiment for another day. 

 

In the meantime I think the experiments show we may want to be a bit
cautious using this technique. 

 

73, Rudy

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