Simple Tuning of Traps Traps with Toroidal Coils

Traps with Toroidal Coils

 In the range up to 150 Wtts RF Traps with toroid-coils are simple to build and to tune. A problem could be the parallel capacitor for high Voltage. Experiments show in a load-test that a Voltage of 2-2,5 KV for the C is sufficient for proper working. If you cannot get such capacitors you can use 4-5 C's with 500 V in series. In a test 3 x 100 pF/500 V in a 10-m-trap could handle 100 Wtts. For the tests I used the toroid-types T94-6, T94-2 and T106-2. The table shows the calculated number of turns, in practice the real number is a little bit lower (nice for tuning...).
 The example shows a toroidal trap.  9 1/2 turns of 1,5-mm-CuL and 3x100 pF/500 V in series gives a renonant frequency of  28,4 MHz. This trap handles 100 W RF, but better is a voltage of 2 - 2,5 KV for the capacitor.
 For the calculating we need the Thomson-formula f (Frequency)  in Hertz, L (Inductance) in Henry, C (Capacity) in Farad
 We need f in MHz, L in uH and C in pF Here are the modified formula for our purposes:
 The AL is a specification of the toroid and gives the Inductance/100 turns in uH on the core. N means  the number of the turns.

The calculated number of turns for the Amateur-bands

 Band Inductivity Capacity Toroid Turns 10 m 0,95 uH 33 pF T94-6 (yellow) 11        1,5-mm-CuL 12 m 0,78 uH 47 pF T94-6 (yellow) 10        1,5-mm-CuL 15 m 1,2 uH 47 pF T94-6 (yellow) 13        1,0-mm-CuL 17 m 1,65 uH 47 pF T94-6 (yellow) 15        1,0-mm-CuL 20 m 1,85 uH 68 pF T94-6 (yellow) 16        1,0-mm-CuL 30 m 2,47 uH 100 pF T94-2 (red) 17,5     1,0-mm-CuL 40 m 5,1 uH 100 pF T94-2 (red) 24,5     1,0-mm-CuL 80 m CW 10,05 uH 200 pF T106-2 (red) 27        1,0-mm-CuL 80 m SSB 9,25 uH 200 pF T106-2 (red) 26        1,0-mm-CuL
 16 1/2 turns close spaced give an inductivity of 2,8 uH 16 1/2 turns wide spaced give an inductivity of 2,5 uH
 The upper picture show how the inductivity can be influenced by changing the distance between the turns of the coil. Closer turns give a higher inductivity, wider a lower. Tuning is no problem! But how can we measure the frequency of a toroidal trap? Look for the DK7ZB-method as described on the other page::
 Here the complete trap for 10,12 MHz: 16 1/2 turns on a T96-2 (red) and a capacitor 100 pF, 7 KV