# Choosing toroid for FM dipole choke

After several failed attempts to buy simple rabbit ears for FM tuner I decided to make my own. Now, I have zero knowledge in RF circuits. While finding the formulas for pole length wasn't hard, I could not find much on toroid chokes. Or rather, I found too much of it.

So, I decided to look at manufacturer's datasheets (e.g. here) and found this: "43 Material is used for EMI/RFI suppression in the 20 MHz to 250 MHz range". Hurray! In another place: "Common ferrite types used for HF baluns are 43, 31 and 61". So I go to digikey and none of those materials there, even though there are thousands of toroids and dozens of core materials.

Question: What common ferrite materials are suitable for a choke with 3-4 turns of micro coax, for 88~108 MHz range?

UPDATE:

From the document reference kindly provided by @jonk it seems ferrite cores #31, #43 and #61 are indeed good fits for this application. Here are some examples from different (not Amidon) manufacturer:

FT82-43 RFI Suppression 5 - 500 MHz, 21.0 x 13.2 x 6.35 mm
FT82-61 RFI Suppression 20 - 500 MHz, 21.0 x 13.2 x 6.35 mm

UPDATE:

I removed images irrelevant to the question.

Also, there seems to be a confusion about my goal. Here is an explanation of what I am trying to do: from many different sources like this one it is recommended to have RF choke (also called feedline choke) added to coax cable to suppress signal pickup by coax braid. Since I am making antenna from scratch anyway, why not make it right, right?

• Comments are not for extended discussion; this conversation has been moved to chat. – Dave Tweed Jun 28 '18 at 21:14

If you want to choose a ferrite material suitable for the application you MUST look at its complex permeability. This is material 43 (from Fair-rite): -

The picture tells you why it is classed as a good suppression material for frequencies of 20 MHz to 250 MHz - of importance is how the $\mu ''s$ graph peaks in this area and how the $\mu 's$ graph starts to fall. This means it is very lossy in the region above 10 MHz and is precisely what you want for a suppressor. But not what you really want for a balun at 100 MHz. Yeah it will be adequate but looking further afield at Fair-rite's other materials you can find material 68: -

Look at how the $\mu ''s$ graph is virtually zero in the FM band - it will make a superb balun.

So, in short, go to Fair-rite's web page and review "Inductive Materials" tab of the table presented: -

As you can see, material 68 is recommended for frequencies less than 400 MHz. If you chose the "suppression materials" tab you won't find material 68 but you will find material 43.

As to whether you can get these components is down to who you are prepared to deal with and whether you can get samples at a reasonable cost. In the past I've obtained freebies from Fair-rite and found them to be very amenable but there are other suppliers like Ferroxcube and Siemens who also make similar materials.

Don't expect sensible help from ebay!

• Thanks a lot for simple to read yet detailed answer. Just a small question. When you say "superb balun", do you mean actual auto- or transformer type? Because what I am trying to make is RF choke, to suppress currents generated by coax shield. It is my understanding that low µ" is great for the balun, but it actually should be high for the choke. – Maple Jun 28 '18 at 11:15
• @Maple sorry I thought you were winding a few turns of coax around a toroid to make a balun. High µ and high frequency performance do not go hand-in-hand because of eddy current losses. The base material is, in effect, "watered down" with bigger gaps (to use a simple anology) and µ falls but eddy current losses also fall. So, inevitably you end up with low µ for high frequency performance. Inductance is still largely proportional to turns squared so all is not lost; a 2 turn inductor on a µ of 100 is virtually the same as 6 turns on a µ of 10. – Andy aka Jun 28 '18 at 12:33
• However, if you look at material 43 it has a µ's (aka µ) of exactly the same as material 68 at 100 MHz! – Andy aka Jun 28 '18 at 12:35