Identifying toroid cores for RF use - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Radio Wrangler · 8th Mar 2023, 6:11 pm

That's an excellent way to find the conjoint of the ranges "Good for resonated inductors" and "still good enough for transformers". Speedily effisient.

You still need to get down and dirty with Q/loss measurements to split those two ranges.

Be aware that with a bifilar twisted winding you may get transmission line coupling effects at high frequencies, making the top end look more extended than the core is responsible for.

David

regenfreak · 9th Mar 2023, 2:25 am

Measurement of unloaded Q of an inductor is a tricky business. You have to ensure that the signal generator and scope are weakly coupled to the LC tank to have a minimum of -20db insertion losses in both ends. Then the measured Q and L are indicated readings that include the distributed capacitance of the coil windings. The actual L has to be corrected by finding the distributed capacitance or self resonance frequency of the inductor.
The series LC dip method is not foolproof. It can catch you out if you trust the measurements 100%.

GMB · 9th Mar 2023, 10:34 am

I also use an aerial analyser to test torroids but in a much simpler way. I just wind a turn or two through and see what the analyser thinks it is seeing.

EMC ferrites look like resistors. If it shows actual inductance at HF then it is likely to be dust iron. The point here is that it is about loss and a lossy core makes the winding look like a resistor - simple.

Quote:

Toroids are self shielding

Should be - but most people (and commercial companies who should know better) wind them incorrectly!

Radio Wrangler · 9th Mar 2023, 5:54 pm

One of the popular EMC ferrite materials, Fair-Rite number 43 makes good inductors into the hundreds of kHz, is great for transformers to well above 30MHz, and doesn't start getting properly lossy until 100MHz.

Other makers have comparable materials and they often turn up as LOPT cores and deflection yokes (The TV gang are gonna lynch me for revealing that!). And you'll also find most RFI sleeves in plastic moulded onto cables are similar.

David

Cruisin Marine · 9th Mar 2023, 7:27 pm

This is a fine article referring to using a nano VNA

https://pa3a.nl/wp-content/uploads/2...fying-them.pdf

G6Tanuki · 9th Mar 2023, 7:41 pm

If you're going to be using them for any significant amount of RF power, you need to determine when they will saturate.

B-H curves ! https://www.electronics-tutorials.ws...ysteresis.html

I remember one of my competitors used a toroid in a low-frequency RF amp circuit which, when thrashed, heated the toroid beyond its Curie-point.

Malcolm T · 9th Mar 2023, 9:08 pm

I do not own an mfj analyzer so i,m going to have to think about another option.

8th Mar 2023, 6:11 pm	#21
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,864	Re: Identifying toroid cores for RF use That's an excellent way to find the conjoint of the ranges "Good for resonated inductors" and "still good enough for transformers". Speedily effisient. You still need to get down and dirty with Q/loss measurements to split those two ranges. Be aware that with a bifilar twisted winding you may get transmission line coupling effects at high frequencies, making the top end look more extended than the core is responsible for. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

9th Mar 2023, 2:25 am	#22
regenfreak Heptode Join Date: Nov 2018 Location: London SW16, UK. Posts: 655	Re: Identifying toroid cores for RF use Measurement of unloaded Q of an inductor is a tricky business. You have to ensure that the signal generator and scope are weakly coupled to the LC tank to have a minimum of -20db insertion losses in both ends. Then the measured Q and L are indicated readings that include the distributed capacitance of the coil windings. The actual L has to be corrected by finding the distributed capacitance or self resonance frequency of the inductor. The series LC dip method is not foolproof. It can catch you out if you trust the measurements 100%.

9th Mar 2023, 5:54 pm	#24
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,864	Re: Identifying toroid cores for RF use One of the popular EMC ferrite materials, Fair-Rite number 43 makes good inductors into the hundreds of kHz, is great for transformers to well above 30MHz, and doesn't start getting properly lossy until 100MHz. Other makers have comparable materials and they often turn up as LOPT cores and deflection yokes (The TV gang are gonna lynch me for revealing that!). And you'll also find most RFI sleeves in plastic moulded onto cables are similar. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

9th Mar 2023, 7:27 pm	#25
Cruisin Marine Heptode Join Date: Jul 2018 Location: Worthing, West Sussex, UK. Posts: 992	Re: Identifying toroid cores for RF use This is a fine article referring to using a nano VNA https://pa3a.nl/wp-content/uploads/2...fying-them.pdf __________________ "Behind every crowd, there's a silver Moonshine"

9th Mar 2023, 7:41 pm	#26
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,996	Re: Identifying toroid cores for RF use If you're going to be using them for any significant amount of RF power, you need to determine when they will saturate. B-H curves ! https://www.electronics-tutorials.ws...ysteresis.html I remember one of my competitors used a toroid in a low-frequency RF amp circuit which, when thrashed, heated the toroid beyond its Curie-point. __________________ I'm the Operator of my Pocket Calculator. -Kraftwerk.

9th Mar 2023, 9:08 pm	#27
Malcolm T Hexode Join Date: Dec 2018 Location: Lugo, Spain Posts: 483	Re: Identifying toroid cores for RF use I do not own an mfj analyzer so i,m going to have to think about another option.