Ferrite tests

Purpose of this article:

a) Pick three random but common ferrites to characterise.

b) Determine what test method best indicates how well a ferrite may work at a given frequency for EMC reduction.

c) Demonstrate how the number of turns effects impedance vs frequency.

d) Demonstrate how adding ferrites in series (when combined with using the appropriate number of turns) works.

The ferrites used are:

1) The small grey clamp on is marked as TDK-ZCAT-1325-0530 (Very common on Ebay about £1 each)

2) The centre one is unmarked but I think it's a Wurth product. (Very common on Ebay about £1 each )

3) The end right item is marked as TDK-ZCAT-2132-1130 (Unknown source)

Search term "ferrite clip on" will bring up results on Ebay etc.

The Grey TDK plots are first. The screenshots are a bit hard to see so it might be better to right click your mouse and save them for closer inspection, or view the page on a smart phone where zooming is easier.  Each turn lowers the frequency and the impedance gets higher.  Another alternative is to view the webpage as a pdf file that shows the graphics better.

And here's the TDK marked as TDK-ZCAT-2132-1130. Each turn lowers the frequency and the impedance gets higher.

Here's a table of results.

Grey TDK ZCAT-1325-0530 (6mm bore size)

FREQ OF HIGHEST IMPEDANCE WITH 1 TURN = 114Mhz 467 Ohms

FREQ OF HIGHEST IMPEDANCE WITH 2 TURN = 34Mhz 516 Ohms

FREQ OF HIGHEST IMPEDANCE WITH 3 TURN = 21Mhz 975 Ohms

FREQ OF HIGHEST IMPEDANCE WITH 4 TURN = 12Mhz 1599 Ohms

Possibly an unmarked Wurth Electronics product. (13mm bore size)

FREQ OF HIGHEST IMPEDANCE WITH 1 TURN = 121Mhz 604 Ohms

FREQ OF HIGHEST IMPEDANCE WITH 2 TURN = 70Mhz 535 Ohms

FREQ OF HIGHEST IMPEDANCE WITH 3 TURN = 34Mhz 903 Ohms

Black  TDK ZCAT 2132-1130 (11mm bore size)

FREQ OF HIGHEST IMPEDANCE WITH 1 TURN = 114Mhz 528 Ohms

FREQ OF HIGHEST IMPEDANCE WITH 2 TURN = 36Mhz 447 Ohms

FREQ OF HIGHEST IMPEDANCE WITH 3 TURN = 20Mhz 875 Ohms

Theory in to practice ? I recently had cause to use what I believe to be an unmarked Wurth Electronics clip on ferrite on the coax of my 70Mhz Slim Jim antenna to cure common mode currents giving me SWR problems. In practice, optimum reduction occurred when I was able to insert two turns of coax through the clamp on to shift the peak to 70Mhz,  and two of them in series. So that would in effect make it : Ferrite 1 (535 Ohms) + Ferrite 2 (535 Ohms) = 1070 Ohms in series. It appears that the correct interpretation of the S11 analysis is to use the parallel R figure on the graphs.

Another brief analysis using a 2 port S21 set up puzzled me a little.   The test was simply a feed wire going through the ferrite, either straight through (equals 1 turn) or more. The plots were almost flat across the frequency range with barely any variation, but the apparent RF attenuation did vary according to the number of turns as expected.  But there's a major problem here. The S21 through line plots would make you believe that you would get the same attenuation from say 1Mhz - 200Mhz, and yet practice has definitely shown that you must choose the amount of turns through the ferrite in order to make it effective at the frequency that you desire. So I do not recommend this method at all. For the sake of completeness I have included some S21 results from 1 - 200Mhz at the end of the page.

Conclusion: my preference is to measure the RF choking performance as an S11 measurement and follow the reactive impedance, since this appears to correlate well with practical EMC suppression results etc.  I have also taken a look at the manufacturers data sheets and in my opinion they are totally meaningless to  be useful. I don't even know why they bother to write them ! Also, it is important to remember that an increase in power throughput can seriously degrade the impedance of a ferrite ring. This test concentrates upon the performance of cores at very low signal levels. But once you start increasing power to the sort of levels that maybe transmitter power output like a tranceiver from several Watts upwards then it's different. These small ferrites will saturate and the impedance levels / attenuation falls off a cliff edge. So really these tests are about killing noise on receive. For transmitter problems, you need much larger physical size cores.

RF ATTENUATION USING THROUGH LINE S21 METHOD

GREY TDK ZCAT-1325-0530 (6mm bore size)

0.5db - 1 turn

16db - 2 turns

22db - 3 turns

Possibly an unmarked Wurth Electronics product. (13mm bore size)

7db  - 1 turn

13db - 2 turns

18db - 3 turns