Measuring Toroid Attenuation

Posted by Fred C (W6BSD) on Jan 31 2020

I needed to remove some RF noise from an air filter in the next room. When the fan turns on, the background noise raises my radio S meter by 2 S units. Like most of you, I tend to collect ferrite cores from random places. Unfortunately, the ferrite cores I gather from hamfest or other projects are rarely marked or color-coded. Testing each toroid on the device causing problems would be finicky, and I would not be able to tell which one is working best.

I am going to show you two ways to test the ferrite cores you have collected over the years.

Using an antenna analyzer

Toroid test using an antenna analyzer
Toroid test using an antenna analyzer

The easiest way to do it is to use a BNC to Banana adapter.

Set the antenna analyzer to the frequency of the noise you are trying to block.

Install a carbon1 50Ω resistor on the banana leads and measure the SWR2. Since your load a resistive and 50Ω, the meter should measure a standing wave ratio of 1.1 / 1.

Install 1 foot to 1.5 foot of wire across the two leads, in parallel with the resistor; the SWR should get high, 10 / 1 or more. The antenna analyzer should also read 0Ω resistance.

Now wrap 6 to 10 turns of that wire around the toroid you want to test. If the SWR goes back to 1.1 / 1 or close, and the resistance is close to 50Ω, you have found the toroid that will choke your RFI3 or EMI4.

If the SWR does not go down, the toroid you are currently testing will not block the RFI on that frequency. Try a different toroid.

You can swipe a range of frequencies to see all the frequencies for which the toroid will efficiently block the RFI.

Using a Vector Network Analyzer

A VNA is a costly piece of equipment that a few of us are lucky enough to own. Now you can buy on Amazon or eBay a NanoVNA for around $50. These inexpensive VNA are not the high precision equipment you will find in labs, but they are good enough for the for the measurements we are taking as ham radio experimenters.

Wrap 6 to 10 turns of electrical wire around your toroid. Plug your contraption, as shown in the picture below. Then measure the attenuation at the frequencies you want to block.

You can set your NanoVNA to display the trace 2, and the format should be log mag. The blue line on the NanoVNA display will indicate how much attenuation you are getting with the toroid tested.

Example #1:

The following example shows I am only getting less than 5dB on 40 meters, 8dB attenuation on 20 meters, and above 10dB on 15meter (gray line).

Yellow Toroid
Yellow Toroid

Example #2:

The following example shows better results, 30dB on 40 meters, 32 on 20meter, and 34dB on 15meter (gray line).

Type 43 Toroid
Type 43 Toroid

  1. The resistor must be noninductive 

  2. Standing Wave Ratio 

  3. Radio-Frequency Interference 

  4. Electromagnetic Interference 

 RFI      Ferrite