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Old 20th Oct 2019, 7:07 pm   #1
Join Date: Jun 2018
Location: Cambridge, Cambridgeshire, UK.
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Default Ferrite antenna circuit design- expert help needed

I thought ferrite antenna principles were straight forward. A parallel resonant circuit is coupled to a low impedance coupling coil and then into the mixer but I'm hitting some problems
Here's the mission... I have been using a untuned frame antenna with one of my valve radios but I would like to assemble a display cabinet of other valve radios and the frame is a bit inconvenient so I decided to make a ferrite antenna system using a ferrite rod recovered from an old transistor radio.

First however I proved the principle by building a single stage amplifier (see thumbnail) and feeding it with a signal derived from my Roberts R200 transistor radio. I connected a link to the base of its mixer transistor which is effectively across the coupling coil MW/LW switch. So the radio in it's OFF condition just provides the tuned RF signal. This works very well.

Now to repeat the excercise with the discrete ferrite rod and a suitable capacitor. The rod carries MW and LW coils The MW coil measures 560uH The LW coil measures 5.3mH The associated coupling coils of each main winding are too low for my inductance meter to measure but if I measure the primary MW in series with it's coupling coil it measures 639uH.
I found an air spaced variable capacitor that measures 350pf max.
The capacitor was placed in parallel with the MW primary coil and the coupling coil connected to the input of my amplifier which was connected in turn to the valve radio. All LW coil connections were unterminated.
The result is that the capacitor reaches a definite resonant position with a good peak in reception on the valve radio. However when the rod is rotated there is no reception null as there was when using my Roberts R200. And the performance is not comparible with that obtained with the R200. So clearly this rod is not working as it should. I have tried all possible connection options such as reversing connections and grounding one end of the primary coil. I have also repeated the excercise with another rod recovered from another radio. But the results are the same. ie no null as the rod is rotated.

Can anyone explain what's going on and how to solve this?

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Old 20th Oct 2019, 7:11 pm   #2
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Default Re: Ferrite antenna circuit design- expert help needed

here is the amplifier
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Old 20th Oct 2019, 7:52 pm   #3
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Default Re: Ferrite antenna circuit design- expert help needed

There is nothing wrong with the design in principle. I suspect there is a basic problem with the construction, or an inductance/capacitance problem.

Lash up a quick MW coil without the LW coils and associated switching. Does the behaviour remain the same?
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Old 20th Oct 2019, 10:17 pm   #4
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Location: Surbiton, SW London, UK.
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Default Re: Ferrite antenna circuit design- expert help needed

You won't obtain enough gain with that circuit. You should consider the sets own aerial
for it's signal coupling of an external antenna, often this can be done with a transistor
sets car aerial socket, which also uses a screened coax.
I often have built this circuit which can offer up to 40dB of useful gain;
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Old 21st Oct 2019, 12:24 am   #5
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Default Re: Ferrite antenna circuit design- expert help needed

You would need to ensure that the cable that led from this arrangement to you radio was coax (but a twisted pair can work too) and that the radio's chassis connection was connected to the sheath and the sheath connected to the V/C's body and the emitter circuit of the transistor. Otherwise that link wiring between this assembly and the radio will act as an antenna and receive the electric component of the EM wave, which must be significant currently, masking over the expected null when largely only the magnetic component is being received by the rod (I hope your valve radio doesn't have a hot chassis or you will need capacitive or RF transformer isolation).
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