Spider Coils

Dave Schmarder's Ueberlitz Spider Coils

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Spider Coil Calculations






    cell   name               formula
    ____   ____               _______
     C4    Wire diameter      Input this value
     C5    Number Turns       Input this value 
     C6    Inside Diameter    Input this values
     C7    Coil Depth         (C4*C5) 
     C9    Outside Diameter   (C7*2)+C6
                              [Add a little more wire for connections]
 Length measurements are in inches, inductance in µH.






Feedback E-Mail

Dave, I found your webpage while searching for a formula for determing the number of turns for a given inductance of a spider web coil. The formula on your page is fine but isn't too practical if you want to build a coil to a specific inductance. Well, I used your formula to derive a formula for determing the number of turns required. After some math manipulations I was able to get what I wanted. The formula was checked for accuracy throughout the range of 1uH to 2000uH. The results agreed with the original formula. At first glance, the formula looks overwhelming since it is a quartic equation ( a 4th degree polynomial), but with the "Quartic Equation Solver" available for use at www.akiti.ca/Quad4Deg.html, it is a piece of cake to solve.

The formula for determining the number of turns required for a given inductance of a spider web coil is: w^2x^4 + 4rwx^3 + 4r^2x^2 - 60Lwx - 32Lr = 0

  Where    x = number of turns to be determined
           w = diameter of wire (inches)
           r = inner radius of coi (inches)
           L = desired inductance of coil (uH) 

Enter the values for w, r, and L in the above formula; do the indicated multiplications and the formula will then appear as: Ax^4 + Bx^3 + Cx^2 - Dx - E = 0

A, B, C, D, E represent the results of the multiplications. Now take these values (don't forget that D and E are negative) and plug them into the similarly labeled boxes in the quartic equation solver. Press the "calculate solutions" button and it will spew out the results. Since this is a quartic equation, there will be 4 answers displayed. The correct answer will be obvious since it is a positive number, the other 3 will be either negative or complex.

George Miller, W4DNW (ex DL4ADP)

Various Litz Wire Spider Coils
Spider Coil Examples

Here are some actual measurements of some coils that I have made. I will add to this table as I make more coils. These inductances are a little larger than you usually see. This is because I build the coils a little larger so I can take a little wire off if needed to trim the coil to the circuit. The lengths are in inches and feet

A little note about my Q numbers: These numbers were reached using my unverified HP 4342A Q meter. Since I made the measurements on the same machine using the same methods, I will stand by the numbers inasmuch as they relate to one another. I do not make any claims of these being actual Q values. I have measured a limited number of coils using the old manual method that I used to use before getting the Q meter. They do fall in line with what I have measured on the HP. I do, however find that my numbers are in the ball park with others that have measured the Q of similar coils. So please take them on your own beliefs.

  L µH   Thickness  I.D.   O.D.  Litz Size  Turns   Length  Q-1Mhz   Cself
  154       .125      2.25   3.50   40/44     40      32ft    330   4.7pF
  248       .125      2.25   3.50   40/44     46      40       
  225       .125      1.75   3.50   40/44     56      38ft    340   3.5pF
  248       .125      1.75   3.50   40/44     58      40ft    375   3.7pF
  149       .125      2.25   4.75  100/45     39      33ft    540   6.3pF
  238       .125      1.75   4.25  100/45     55      42ft    620   5.4pF 
  241       .125      2.25   4.75  100/45     49      42ft    620   5.4pF
  146       .125      1.88   4.25  165/46     42      34ft
  200       .125      1.88   4.25  165/46     48      38ft
  220       .125      1.88   4.25  165/46     51      42ft
  232       .125      1.75   4.75  165/46     55      44ft    750   4.0pF
  250       .125      1.88   4.25  165/46     54      45ft  
  250       .125      2.25   4.75  165/46     49      45ft
  155       .125      1.88   6     630/46     40      43ft
  244       .125      1.88   7     630/46     49      62ft    
  150       .125      1.88   6     660/46     39      42ft   1000+  4.7pF
  241       .125      1.88   7     660/46     48      60ft   1000++ 3.0pF 

Spider coil wound for use on the long wave band
Long Wave Coil





We'll see how this works when the band opens again.

Making a spider coil. Measure first!
How I Make My Coils













Spider Web Coil Links

Gollum's Spider Coil
Gollum's 2 coil dx receiver
Gollum's shortwave spider radio
Mike Tuggle's Spider Coil
Formula for calculating spider coils