Common Mode Choke
Common-mode chokes are useful for the prevention of electromagnetic interference (EMI) and radio frequency interference (RFI). They pass differential currents, while blocking common-mode currents.
The main ferrite Toroid I used to make chokes, is the Jaycar LO1234. These are available locally for an acceptable price. The size of the Toroid is 25 x 15 x 10 and a pack of 4 will set you back about A$7.00.
To determine the AL value of the LO1234 I wound 10 turns of 0.2 mm wire onto the Toroid and measured the inductance with an AADE L/C Meter. The inductance measured was 102.5 µH. I use the mini ring core calculator from DL5SWB to calculate the AL value. The AL value of the LO1234 is approx. 1000 nH/N2. Pic1 shows a test setup of two LO1238.
Note: I've measured 6 LO1234 and the measured inductances where 100µH ± 15%, a tolerance of about 30%.
I then used my VNA to measure the attenuation for a single Toroid. Table 1 shows the measured attenuation of one LO1234. Jaycar has a bigger brother, LO1238 (Material L15 size 35 x 21 x 13). Owen, VK1OD has a nice write up on a 1 : 1 Guanella Balun using the LO1238.
I experimented with four LO1234 Toroids and looped RG-58C/U three times through. Measuring and adding another set of 4 improved the performance by approx. 3 dB and as I had a few more of the LO1234, I added another set of four onto the coax for a total of 12 Toroids. Pic 2 shows the choke in its full glory (really three individual chokes).
Picture 1
The next table, Table 2 shows a few JAYCAR toroids and the measured/calculated values.
