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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 14th Jan 2018, 11:53 pm   #61
Phil G4SPZ
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Default Re: Simple analogue pantry transmitter ?

This is really very interesting. My home-made ferrite-rod-cored coil shown in post #55 has 66 turns in a single layer and measures 276uH in total, but the two individual 33-turn halves measure just 74uH and 88uH respectively. Using Lawrence's theory, this implies a coupling coefficient of 0.85.

The formula for the inductance of a single-layer air-cored solenoid suggests that L is proportional to N squared but inversely proportional to length. Doubling the number of turns in a single layer coil results in a winding of twice the length, hence the total inductance is doubled, not quadrupled. Obviously, if the additional turns are wound on in a second layer and the length remains the same, the inductance will increase accordingly.

However, when wound on a ferrite rod, the situation becomes more complex, and factors like the length of the rod and the position of the coil relative to the centre of the rod become important. There's a very good paper here which illustrates graphically how the inductance of a ferrite-cored coil varies with the number of turns. The relationship is a linear one, but displaced from zero. The published data shows good agreement between my 66-turn main winding at roughly 275uH. Extrapolating from the graph in fig 4 also agrees with my finding that each individual 33-turn half-winding should measure around 80uH.

I may be missing something, as it's been decades since I studied ferrite rod design (and I think I was off sick and missed that particular lecture ) so I'm finding this combination of theory and practical construction most enlightening!
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Old 15th Jan 2018, 7:07 am   #62
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Default Re: Simple analogue pantry transmitter ?

It's always nice to see the maths coming out right
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Old 15th Jan 2018, 7:42 am   #63
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Default Re: Simple analogue pantry transmitter ?

Don't forget that the ferrite rod in itself has no useful role in the transmission of EM waves, its cross sectional area is far too small. You can use one as a "transformer core" (but its no better than a ferrite pot core) to form an oscillator circuit or as the load choke for an RF output stage, but you still have to attach a long wire to it for any reasonable transmission range for a pantry transmitter. This is quite different to a reasonable sized loop antenna.
While it might seem a good & noble quest for a minimal component pantry transmitter, the price to pay is a combination of horribly non linear amplitude modulation and poor transmission range.
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Old 16th Jan 2018, 10:49 pm   #64
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Default Re: Simple analogue pantry transmitter ?

It is interesting yes, thank you one and all.
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Old 16th Jan 2018, 11:12 pm   #65
Phil G4SPZ
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Default Re: Simple analogue pantry transmitter ?

I've made a start on constructing a compact one-transistor pantry transmitter based on the circuit in post #50. One of the ideas I had was to try to improve the linearity and reduce the amount of FM by applying the modulating signal via a small audio transformer with its secondary in place of the earphone.

If and when I get it finished and working, I'll start a new thread in the "Homebrew Equipment" section.
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