From Tim, KT8K…
Here’s a test procedure to test the snap-on ferrite cores you buy at hamfests or Radio Shack.
Ferrite chokes are widely used to reduce or suppress RFI in electronic and electrical equipment. Many computer cables, in general, have lumps in them which are built-in ferrite chokes. Of the many different types of ferrite material, 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 snap-on ferrite cores, 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
- Wind 5 turns of 22 ga. hookup wire through the ferrite core to be tested and connect the ends to the center and shield connection on the antenna analyzer.
- Set the frequency to 2 MHz, and measure the reactance. If it is mix 43, the inductive reactance will be approximately 500 ohms.
- 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 snap-on ferrite cores 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.
UPDATE 8/9/17
This post just got some attention on Twitter, so I thought I’d revisit it. As a result, I changed the wording from “ferrite beads” to “snap-on ferrite cores.” That’s really what this procedure tests.
I also got a question from Adam, @MU0WLV. He asks, “is it possible to do this with toroids too? after all they are ‘beads’ just not split ;-)” The answer, of course, is YES! Just wind 5 turns of hookup wire through the core and test away.
Mark Cooper Sr. says
Outstanding information ! Thank you so much for the informative test procedure ! Just what I needed as I have a large box of salvaged snap-on’s as well as slip on sleeves and some ‘donuts’ that I have accumulated and salvaged from random cables at various thrift stores over the years ! Keep this in mind, thrift stores like The Salvation Army, etc., are a GREAT source for very useful ferrite, snap-on’s and sleeves, to ham radio operators like myself !
73, de Mark, WN3SIX, Pennsylvania