# Re: primary calculations

```In a message dated 96-02-21 12:48:13 EST, tesla-at-grendel.objinc-dot-com writes:

>>
>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

Jim,

I also calculate 143 uh for your primary.  My primary is 8.0" inside
diameter, 14 turns of 3/8" tubing with 3/8" spacing.  I think the outside
diameter is about 26.5".  It is 6.0" tall.

Ed Sonderman

```