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






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