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Re: primary calculations

>>From Esondrmn-at-aol-dot-com Tue Feb 20 10:46 MST 1996

>I saw your post last week on this and I didn't see anyone respond.  I have a
>saucer shapped primary 14 turns of 3/8 copper tubing wound at 30 degrees.  I
>used the formula for calculating a flat spiral primary and came within 5%.  I
>calculated 86uh and measured 82uh.
>
>Ed Sonderman
>
Ed,
        I just recalculated my new (#2) primary and came up with 92.4uH. I
measured it at 124uH.
      OhOh, I just noticed I'm using diameters not radiauses. Retry: 143.4uH
/ 124uH = 1.16 much better, given a 5% or 10% scope reading error. For 14
turns I measured 101uH. No, let me rephrase that: for 14 turns, I measured a
period of 7.4us using a 13.7nF cap. a function generator and a 10K series
resistor between the generator and the LC. I varied the frequency of the
drive signal to maximize the voltage across the tank circuit. I then
measured the period of the signal with my Oscope. I calculated an inductance
of 101uH from these numbers. Tomorrow night I'll check my Oscope against my
short wave reciever to see if my scope has lost calibration. It's about 5
years over due. I'll also check the numbers with the coil off the ground! It
was about 2" from my damp concrete floor.

        the formula I'm using for a pancake coil is:
                      2         2
                L =  a    x    n
                    --------------
                    8 a   +  11  w


        my coil measures:

                inside diameter:        11.75"
                outside diameter:       31"

                turns:                  15.5

                1/4" copper pipe on a 30 degree slope from horizontal.


        What are the diameters of you coil?

                                jim