Toroid Rings.

Hello John,

first, is it really 60kHz - ie 60000 Hz, nearer audio than what most would call RF

If so - what is you are trying to build - I can't see what you would want a toroid - if you want a ferrite rod aerial that isn't a good start.

Is the application for fun or for serious stuff, and I'm guessing you want to pick up the MSF signal from Anthorn ?

MK

Hello, Michael, and thanks for the reply.

The request is serious in that I'm trying to learn, but hobby in that I now do electronics as a retired electronics man (with little knowledge of coils and most other things radio!!)

You're right. It's about MSF. To that end I bought one of those Canadian receivers (yet to arrive) but I also use the opportunity to wind coils, try this, try that, try to understand the maths, etc. So I decided to see if I could make a simple TRF radio with a couple of tuned RF / audio stages. If all else fails I still have the bought receiver, but that will teach me nothing about LCR circuits. So you see my quest is a bit of both hobby and serious. I will probably try a crystal stage to see how I get on with that, too, but I kind-a want to make a tuned stage just for the hell of it ... to see if I can make it work. Learn something about ferrites and so on.

I looked at toroids because my reading-up seemed to show they might work, and I get to learn a bit about those parts, their data sheets, etc; I get to learn even if toroids don't work like that!! Self teaching is always full of difficulties, and I'm at that stage with the coils where I need a hand as the Q of my built coils is always too low (B&K 880 LCR meter) with 90 being about the best no matter what I wind them on, or how, or with.

There we are. That's the story.

John

Hi John,

As Michael says, normally a ferrite stick is used, because it can provide some directionality without needing much of an antenna. With a toroid, you may need to experiment more with the antenna. Another reason (practical) is that since this is such a low frequency, it will need winding a lot of turns, and that's not fun with a toroid.

Anyway, long story short, it is a fairly experimental process to build up the toroid. There is no optimum method for selecting toroids, some of it will need to be by experiment, and some of it by what is available out there.

You can search online for the LC formula for your resonant circuit (if this is what you're making), and pick a value for C and look for reasonable values for L. So, for example, if C is about 700pF (this value can be achieved with a few hundred pF fixed capacitor, in parallel with a variable capacitor for trimming), then L is 10mH.

Next step is how to construct such an inductor, and you can seek out the different types of toroid material, and manufacturer recommendations for frequency. For 60kHz, type 'J', type '61' and type '67' would all work (as would some others). The manufacturer will have formulas to work out how many turns, but it is just a ballpark (it varies with batch, and with temperature too). Since you need a lot of turns, the toroid needs to be large too. There is an online calculator here, and it is as accurate as possible (I've confirmed this in the past), but as mentioned expect variation, and you'll need to experimentally check : https://www.changpuak.ch/electronics/amidon_toroid_calculator_2.php

Using that, something like FT-87-J would work, with about 58 turns, will equal 10mH approx. There's other things that will affect Q, but with such a high inductance value (relatively), you don't have much control over that once you've picked the toroid, because it is a lot of turns.

Anyway, it is a niche thing and Farnell won't have a great selection for the toroids for radio applications, very few distributors sell them. Most of the toroid cores from distributors are intended for use as chokes, and they don't specify the nH/turns^2 value (which is what is needed for calculating inductance). For suitable cores, there is a US source (Amidon) but shipping and minimum order costs are extremely high last time I checked (it was not worthwhile for an order less than a couple of hundred $).

So, it may be good to use the online calculator for a few different toroids, and hunt on ebay etc., to see if anyone is selling any of those particular toroids.

If you don't want to wind a toroid, you could use an off-the-shelf 10mH unshielded inductor. It won't have very high Q, but you may not need very high Q anyway. You could always swap it out later if it wasn't performing as well.

Anyway, it sounds like a fun project.

Hi johnsmalley I'm glad you asked this question as it has produced some really knowledgeable answers, which I enjoyed reading. Unfortunately I know little to help answer your question, sorry.

I would be really interested to read about any progress you make - a follow up blog would be great

A toroid won't work like a ferrite rod aerial - put simply there is no way for the magnetic component of the RF to get into the core and generate emf in the coil;.

I was interested in this too - so I've ordered some ferrite rods from Amazon

https://www.amazon.co.uk/gp/product/B074W4RNCV/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

I found this article which is fun:

The Creative Science Centre - by Dr Jonathan P. Hare

I'm thinking of a minimal RF amplifier and using an STM32Hxxxx processor with its 16 bit ADC to do a SDR (Software Defined Radio)  and demodulator.

How far I get will be time limited (as usual).

@John

60kHz is quite a low frequency for LC filters - the Ls need to be big and can be expensive  and/or quite lossy. But active RC filters work very well in this frequency range.

The MSF signal uses on/off modulation with 100ms time slots, so a bandwidth of about 20 to 30Hz at the input to the de-modulator seems about right. That's quite hard to do

with analogue filters but easy in maths - which is why I fancy the ADC and processor for the filtering.

MK

I wasn't sure if John was planning a separate antenna connected to the resonant circuit or not, but the rod acting as antenna + resonant circuit is way better. A microcontroller-based SDR sounds really cool : ) There's another low-frequency that I was interested in recently, which was to do with heart-rate straps (i.e. as used for the gym). They work at around 5kHz apparently, so I was tempted to build up something for that to decode it, but in the end decided to locate a BLE one. I still have the 5kHz strap, so may try to experiment with it one day.

Guys ....

Thanks for the responses.

Just to clear it up, I’m *not* trying to use a toroid core as an antenna. I’ll mess with a ferrite rod for that.

The toroids are for a kind of IF strip but, of course, it’s not IF it’s at the antenna frequency to make a TRF receiver (but at one, fixed frequency = 60KHz). These coils will be tuned with a C to make a tank cct amplified by a BJT or an FET or a chip.

My reading tells me that the best Q is always a compromise, but it is to be had with the largest practical core. Then the fewest possible turns to reduce capacitive losses between windings and to the core. Further, the core material should be designed for the frequency of interest. In addition the skin effect @ frequency of the wire needs to be chosen correctly: I already have Litz wire that should work. (Did I miss anything to reduce losses?)!

I have some 40mm 3E27 cores laying around. They should be ok. Experimentally, 10 turns of .75mm enamelled Cu wire makes about 1.5 mH. I resonated that with the desired C. Signal if fed in with a loop close by and is extracted to a scope with a pickup turn on the core. Or, the inductor (no capacitor) is connected to my B&K 880 LCR meter where Q, D, ESR, Z, DCR, R, theta can be measured. I’ve tried different gauge wires wound in different ways to get an idea of how these things affect the inductor properties for the same value of L - just for learning.

As mentioned, all this is pretty new knowledge to me so I’m likely to have got stuff wrong and missed important points.

John

Thanks. This is mostly further down the line for me. If I can get a clean and amplified 60KHz signal it’ll be an achievement in itself for me.

Hello. I’m afraid I have no idea how to make a blog. I’m an old person. It would be different if I were young. Ha ha.

It's dead easy on E14.

When you are logged in, click the create button at the top right hand of the screen, next to your name, and then select Blog Post.

From there on it's pretty straightforward.

MK