Types of ferrite suitable for 30MHz toroid

[suebutcher]

What type of ferrite toroid do I need for a small tuned 10-30MHz RF transformer? I haven't a clue about the material codes they use.

[kellys_eye]

Visit the amidon website where they have some excellent documentation on all their ferrite and dust iro products - an invaluable resource for anyone using (or contemplating) torroids.

I'd post a direct link but can't work out how on this tablet!

[Ed_Dinning]

Hi Sue, there are also similar tables in the Magnetics and Fairite web pages.
I believe I also saw some useful data in the Jaycar cat.

Ed

[Radio Wrangler]

For an UNtuned transformer at 30MHz, I'd use Fair-Rite (==Amidon) FT43 material, but for a TUNED transformer I'd avoid ferrite as dust-iron offers better stability and Q

Amidon buy their ferrite cores from Fair-Rite inc. of Wallkill, Noo Joisy, and their dust iron cores from Micrometals inc. of Anaheim California. Mix-2 (painted red) or mix-6 (yellow) should do you most probably.

David

Why a toroid? A 'normal' coil !(or two) would do.

[G8HQP Dave]

'Normal' coils can couple quite well to their surroundings. This is not always wanted.

[kalee20]

If you want low external field, or low pick-up FROM an external field, I'd use a toroid made from almost any low-frequency material. Ferroxcube 3C90 or 3F3 come to mind; Siemens (Epcos) N30, N47 likewise. And consider the transformer untuned.

As Radio Wrangler says, ferrite has lousy stability. Q may not be very high, but losses will be pretty low at 30MHz because the operating flux density will be very low.

Then, in parallel with the primary of your untuned transformer, connect a stable, screened, high-Q coil of the inductance you want. As long as the inductance of the transformer is much higher than needed (and it won't take many turns to make this true), the external inductance will dominate and the exact inductance of the transformer will hardly matter.

[suebutcher]

I'm going for a toroid because that's what's specified for the circuit. It's the EA160, a budget SSB HF receiver project published in Electronics Australia in 1970, and I suppose toroids for RF tuning saved space and money. It's one of those radios I built with my Dad that didn't work, and I'm curious to know why. I know that's crazy, but I love electrical puzzles.

[Radio Wrangler]

It's not crazy at all. I think it's laudable curiosity.

In the middle sixties I built a Lasky's 'Skyrover 6' transistor portable with 5 shortwave bands on it. Performance was completely underwhelming. At home, only a few miles from the transmitters, main broadcast stations on medium wave were just discernible. Disgusted, I set out to learn how to design a better one. I'd still like to know what was wrong with that thing. Unfortunately, at the time, I knew no-one with the knowledge to help. It got boiled-down for parts.

David

[suebutcher]

Same kind of disappointment here, except our two big radio construction failures got passed on for other people to try, and I gave up on radio completely for a while.

BTW, the part list says "Q2" as the type of ferrite, made by Ducon (Plessey). Is that actually a ferrite or iron dust material?

If you know the number of turns, the frequency and capacitance then the inductance can be worked out. Using that the inductance per turn (squared) can be calculated (called Al or aL or the like). Then find a core with something similar.

[Dave757]

Hi Sue,

Try www.toroids.info
I've always found this a very useful resource for both identifying
suitable toroids, and calculating the number of turns required.

Kind regards
Dave

[suebutcher]

Ta very much. Should be OK now. There are so many different types of ferrite, it's possible that the original radio failed because we bought RFI toroids instead of low loss RF toroids. They all looked the same in those days. BTW, Q2 ferrite material is the same as Neosid F25.

[Radio Wrangler]

For any particular ferrite, the frequency range can be divided up into bands for different applications.

At low frequencies a material simply doesn't have high enough permeability to easily make the amount of inductance needed to make useful impedances. So it isn't much use.

At higher frequencies, it's easier to make useful inductors and the Q is good. Usually cores for inductors are gapped and it's the air gap which sets the aL value. The ferrite is behaving as a low-loss magnetic conductor.

At higher frequencies comfortable numbers of turns make enough inductance for rather high impedances and the core is starting to get lossy. The Q of any inductor is starting to get too low for making resonators of good enough quality. But used as a transformer the lossiness of the core still gibes a useful efficiency and the inductance values are good for transformers. (This means conventional, untuned transformers. Tuned transformers are really resonated inductors with some magnetic coupling)

Finally, at higher-still frequencies, the core losses have grown large. Inductors would have trivial Q, transformers would be too lossy, but that lossiness is still useful, the ferrite is now an absorber. It makes good anti-emc cores.

These bands of applications don't have abrupt boundaries, there is a lot of overlap depending on individual applications. Different materials have the boundaries at different frequencies.

FT43 for example, makes good inductors up to hundreds of kHz though it isn't usually made in gapped cores. It makes good transformers (untuned) to 30 MHz, but by 100MHz losses make it into a good anti-EMC material.

David

[Terry_VK5TM]

The cross for Neosid F25 is Fair Rite 67 material.

Cross reference table here:

http://palomar-engineers.com/ferrite...ross-reference

Terry VK5TM