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RF coil temperature stability. Air core VS ferrite or powered metal

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neazoi

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Hi,
Is an air coil better in terms of temperature stability, as compared to a ferrite or powdered toroid?
 

Hi,

I know you talk about the core..

But coating plastics have big influence on temperature stability.
Two component plastics are worse.

--> try to use coils without coating.

Klaus
 

Hi,

I know you talk about the core..

But coating plastics have big influence on temperature stability.
Two component plastics are worse.

--> try to use coils without coating.

Klaus

This was useful!
That is probably why when I wound coils using enameled coil wire, without a metal core (wound onto a coffee straw), they still presented some temperature instability (much lower than ferrite cored coils though).
 

For a very stable VFO you need a ceramic former with the coil turns sintered on to it. This is so the turns cannot move because they are "stuck" to the former. As copper expands with rising temperature, the whole coil "grows" as the temperature increases. Plastics exhibit a terrible temperature coefficient. You could look up to see what glass is like and wind you coil on a glass tube.
FWIW The VFO had a mechanical digital counter as the digital display. This was so that you could freely tune like a conventional receiver, then transfer the mechanical reading to the digital knobs and switch to digital and the station would still be there+- 20HZ.
Frank

- - - Updated - - -

Just been thinking, if you physically match the diameter against the length. You should be able to get the temperature coefficient to be zero. if the coil is short and fat, the increase in diameter will increase the inductance as the temperature rises, if its long and narrow the inductance will decrease as the coil gets longer. From the depths of my memory, I think it might be about 5:1 (length to diam).
Frank
 
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    neazoi

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For a very stable VFO you need a ceramic former with the coil turns sintered on to it. This is so the turns cannot move because they are "stuck" to the former. As copper expands with rising temperature, the whole coil "grows" as the temperature increases. Plastics exhibit a terrible temperature coefficient. You could look up to see what glass is like and wind you coil on a glass tube.
FWIW The VFO had a mechanical digital counter as the digital display. This was so that you could freely tune like a conventional receiver, then transfer the mechanical reading to the digital knobs and switch to digital and the station would still be there+- 20HZ.
Frank

- - - Updated - - -

Just been thinking, if you physically match the diameter against the length. You should be able to get the temperature coefficient to be zero. if the coil is short and fat, the increase in diameter will increase the inductance as the temperature rises, if its long and narrow the inductance will decrease as the coil gets longer. From the depths of my memory, I think it might be about 5:1 (length to diam).
Frank

Thanks Frank, the 5:1 (length to diam) ratio is really a good starting point for me. I will try this as my starting point of winding the coils.
I would use some borosilicate glass tubes I have, which have a very low thermal expansion, but I would like to avoid exotic materials.
On the other side, I am thinking that a simple oven might better worth the effort than using exotic materials and "weird" construction techniques.
 

Hi,
Is an air coil better in terms of temperature stability, as compared to a ferrite or powdered toroid?

Air cored coils will be physically large, which in itself creates some problems of absolute mechanical rigidity. But its a well proven system if physical size does not deter you. The screen around an air coil needs to be quite large if it is not going to upset things because the coil radiates (and readily picks up) stray magnetic fields. A small can around a large coil acts as a shorted turn which can dramatically reduce circuit Q and cause stability problems.

None of this was a real worry back in the days of valves and huge tuning capacitors.

If you need something really small, modern day powdered iron toroids now have excellent temperature stability, particularly mix 6 and mix 7 in the RF grade materials.
The toroid does not magnetically radiate, so it does not need a metal screen, and temperature stability is at least as good as the very best capacitors.

Ferrite is truly evil for oscillator stability.
Its very useful stuff to have for other applications but definitely NOT for a temperature stable oscillator.
 
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