Re: Ferrite technical considerations for FSL’s

Robert Conboy

Interesting! A Varactor tuned ferrite sleeve loop with a preamp, and a rotator for remote operation. I would have thought that the varactors would be too temperature sensitive, and too low Q - but apparently not. I have a 5-500 pF vacuum variable capacitor for use in a future similar project.

I have gone a different direction with the active part, using an amplifier for regenerative feedback to overcome losses in the ferrite. This enables Q to be dialed up high enough to eliminate the audio almost entirely while looking at carriers and their offsets using an SDR. Gain is in the neighborhood of 50 dB while -3dB bandwidth is about 20 Hz just before oscillation.

 It is also useful for cleaning up extremely weak audio with lower Q settings (that are still higher than attainable without feedback, gain in the neighborhood of 35 dB with -3dB bandwidth about 100 Hz. This bandwidth results in audio that is heard at very low level, (turn up the volume) but noise is even lower.  It is not as low bass heavy as one would expect because the speaker in a tiny  portable radio is incapable of reproducing it. 

Ferrite sheets are an interesting proposition, if they can have high enough permeability to overcome their thinness, with low enough losses to attain good selectivity, and large enough for antenna use. Ferrite designed for EMI suppression is deliberately extremely lossy. I have made the assumption sheet ferrites aren’t suitable, but I have been wrong before.

On Jul 29, 2021, at 5:01 AM, valentino barbi <barbivalentino@...> wrote:

Il giorno gio 29 lug 2021 alle ore 10:02 i2NDT Claudio via <> ha scritto:
this could be another way to make a ferrite sleeve:

using ferrite sheets which are much more light and cheep than ferrite roads...

best DX
i2NDT Claudio

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