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Old 15th Aug 2020, 7:40 pm   #1
PsychMan
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Default Ferrite Rod Antenna - TRF Radios

Hi Folks,

Ive been mucking about with a few TRFs lately. I built an active ferrite rod antenna as per Paul Sherwins circuit published on here some time ago. It works well for the superhets ive tried it on so far, but has been useless on the 2 TRFs I have tried it with.

I suppose the antenna impedance is different and this is a poor match. Any suggestions on what the TRF sets are looking for that its not getting out of my little antenna? I tried it with both a set that has a capacitor trimmer on the antenna input (tried bypassing that also), and also into a TRF without such a trimmer.

Cheers
Adam
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Old 15th Aug 2020, 7:55 pm   #2
G6Tanuki
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Default Re: Ferrite Rod Antenna - TRF Radios

I suspect that the problem is when the regenerative-detector is on the edge of oscillating (or actually oscillating if you're trying to receive CW/SSB transmissions) there's enough leakage radiating from the detector back into the RF--stage's ferrite-rod to cause the classic "You're hearing your own local-oscillator" problem.

This was the kind of issue that led to the "Fremodyne" and various superheterodyne-superregenerative receivers in days-gone-by.
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Old 17th Aug 2020, 8:46 am   #3
PsychMan
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Default Re: Ferrite Rod Antenna - TRF Radios

Interesting, so this may not work at all.

Any suggestions on what might be a decent compact antenna for a TRF?
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Old 17th Aug 2020, 9:53 am   #4
paulsherwin
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Default Re: Ferrite Rod Antenna - TRF Radios

At a guess, the different loading presented by the TRF is allowing the ferrite rod amp to oscillate. That design does have a tendency to oscillate.

I've never tried to use one with a TRF, though there's no reason why it shouldn't work in principle.
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Old 20th Aug 2020, 9:42 am   #5
PsychMan
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Default Re: Ferrite Rod Antenna - TRF Radios

Interesting, I suppose some more experimentation is needed.

The best results so far have been obtained by tuning a ferrite rod coil with a capacitor, and connecting that across the grid of V1 and ground (with no amplifier). Not as neat as sending something into the antenna input though. I might just go back to trying long lengths of wire and perhaps coil it up to keep the size down.
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