From Tim, KT8K…

Here’s a test procedure to test the ferrite-bead chokes you buy at hamfests or Radio Shack.

Ferrite chokes are widely used to reduce or supporess RFI in electronic and electrical equipment. Many computer cables, in general, have lumps in them which are built-in ferrite beads. Of the many split-bead ferrite choke types, mix 43 is generally accepted as the best choice for stopping RF from getting through your power and audio cables. Other mixes are not much help in amateur radio applications.

The cool thing about this technique, I think, is that now I can pop some batteries into my MFJ259, sling it over my shoulder, and actually evaluate the chokes I consider buying at swaps before I spend my money. In the past I just bought them and installed them and hoped that they were helping. Using this procedure I can be sure of what I’m getting.

The following procedure is adapted from and courtesy of Alan Applegate, K0BG. Read this article for more details.

To test ferrite beads, you will need an antenna analyzer, such as the MFJ-259, and enough hookup wire–size 22 is ideal–to make 5 passes through the bead plus enough to connect the ends to the analyzer.

Test Procedure

1. Wind 5 turns of 22 ga. hookup wire through the bead to be tested and connect the ends to the center and shield connection on the antenna analyzer.
2. Set the frequency to 2 Mhz, and measure the reactance. If it is mix 43, the inductive reactance will be approximately 500 ohms.
3. Push the mode button three times to bring up the inductance menu. If your bead was made with mix 43, the inductance will be about 40 μH. A lower reading may indicate that the ferrite is not mix 43, and it may not work properly. If the reading is higher, that’s fine to a point, but double these figures may indicate mix 77 (ui=1800), which is better suited for 160-meter suppression.

Note: the 259B hasn’t enough range to check mix 43 at much more than 2.5 Mhz unless you reduce the number of turns.

A note from KB6NU…

I just ran down to the shack, and pulled out my Autek VA1 Vector Analyzer to perform this test. The results were quite interesting. Testing four of the five ferrite bead chokes that Tim KT8K bought for me at a hamfest about a year ago, I got the following results:

Bead	Inductance (μH)	Reactance (Ω)
1	34.5	432
2	35.6	448
3	37.1	465
4	34.5	430

These values aren’t too far off, so I’d say I got a decent batch.