[AMPS] Smallest practical 1.5KW 20uH coil

[Radio WC6W]

Hi Dick,

>I'd go with the stacked T200-2 cores - Before winding, you can give 
>yourself a bigger insulation margin of safety by wrapping the core stack

>with a couple of layers of Scotch/3M #25 fiberglas tape (can someone 
>confirm that #25 is the right designation?) .

   Thanks for the tape recommendation.
 
>Don't know how you calculated it, but loss at 1.8-2.0 MHz should be
almost 
>unmeasurably small - anything over roughly 15W loss (1% or 0.04 dB at
1.5 
>kW) would cause the toroid to run quite hot.  But in fact literally 
>thousands of commercial amps have for decades used approximately the
toroid 
>inductor you describe without problems.

  I used the (ancient) formula from the Micrometals catalog:  B = E*10^8
/ (4.44 * A * N * f)

  Where B is the max. flux density in Gauss, E is the RMS voltage across
the coil, A is the cross sectional area of the core in cm^2, N is the
number of turns and f is the frequency.

  Then, with an E of 1500 volts (half of the RF, since the toroid is half
of the inductance -- and ignoring the fact that the part is located at
the low impedance point of the Pi-net, therefore likely seeing a bit less
voltage in circuit.)    A=5.08 with 4 stacked T-200 cores   N=20 and F =
2 MHz ...   B calculates to 166 gauss.

  The core loss factor, for type 2 material (red), from the table on page
9 of Micrometals Catalog #3, Rev. D, Page 13, is about 800 mw per cm^3. 
Since the total core volume is 65.6 cc, the (calculated) core power loss
is 52.5 watts.

  SInce, as you noted, this method does appear to work in practice, I
wonder if there is something (greatly) wrong with my assumption of half
the RF voltage across this coil?  

73,
  Marv  WC6W









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