• xtgaby

Bandpassfilter for 40 meter band (7MHz) with Amidon T50-2 cores.

Updated: Nov 27, 2020

Important: only for receivers. This filter can not handle transmitted power.


The Kank 3334 transformer from Toko has on one (primary) side 18 turns (5.5 uH) and on the other (secundary) side 3 turns ==> this means a ratio of 18/3 = 6..

The T50-2 core from amidon (50 stands for 0.50 inch and 2 stands for the material) has an AL value of 49 uH.

Al value means that when you use 100 turns on the core, the coil wil have a value of 49 uH.

Material 2 is ideal for the use between 250 KHz and 10 MHz.

Calculation how many turns are required.

- Now we know that to make a coil of 49 uH, we need to have 100 turns on our core.

- Our needed value is 5.5 uH to have the same value as the Toko coil.

- 49 uH/5.5uH = 8.9

- Coil value is dependant of the square of the number of turns.

- 100 x 100 = 10.000 / 8.9 = 1123. Root of 1123 = 34 turns required for 5.5uH.

- number of secundary turns = 34 / 6 (ratio) = approx. 6 turns required.

Calculation value of de capacitor required.

Resonance frequency wanted = 7.1MHz



Due to the fact that SWR was 2 at 7 MHz (about 100 Ohms), I have changed the numer of turns from 6 to 5. This gives a much better SWR and a better insertion loss.

The new results:

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