[600MRG] coil Q note

jrusgrove at comcast.net jrusgrove at comcast.net
Thu May 17 15:59:28 EDT 2018


Rudy

While the shorted turn (or disk) might not be appropriate for use in a short vertical loading coil application ... might it be okay in other inductor applications that don't require high Q? L and T networks with low transformation ratio come to mind. 

Jay W1VD  
  ----- Original Message ----- 
  From: Rudy Severns 
  To: 600MRG 
  Sent: Thursday, May 17, 2018 1:15 PM
  Subject: [600MRG] coil Q note


  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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