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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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20th May 2017, 12:42 pm | #1 |
Octode
Join Date: Jan 2004
Location: Southport, Merseyside, UK.
Posts: 1,156
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Tank Coil question
I have a question regarding a small transmitter I have constructed, it's a 6AG7 crystal oscillator driving a 807 final, the 807 tank coil is puzzling me, the design of this is taken from formulae in the RSGB 'Radio Communications Handbook' and Pat Hawkers book 'A guide to Amateur Radio' and they specify that the coupling coil should be wound around the cold end of the tank coil, now why is this? Isn't the only coupling between the two coils the alternating magnetic field and wouldn't this be exactly the same at either end of the tank coil? I've attached a scan showing the relevant article in Pat Hawkers book.
Could someone please enlighten me? Thanks John |
20th May 2017, 7:32 pm | #2 |
Pentode
Join Date: Apr 2011
Location: Swansea, Wales, UK.
Posts: 143
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Re: Tank Coil question
Hello John
There will be capacitive coupling between the two coils as well, which will transfer RF to the link coil, particularly the higher harmonics. The lowest RF voltage to ground is at the cold HT end of the coil. Well done on your home brew. I always prefer the pi tank output as it naturally minimises the harmonics but the usual 2.5mH choke is hard to find. I have used a toroid in that position without problem in spite of what theorists might say. 73, Andrew |
22nd May 2017, 6:56 am | #3 |
Octode
Join Date: Jan 2004
Location: Southport, Merseyside, UK.
Posts: 1,156
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Re: Tank Coil question
Hi Andrew,
Thank you for your explanation, so considering the inductive coupling only the link coil could be at either end of the tank coil, but because there is some capacitive coupling as well it should always be at the cold end to reduce harmonics, this link coil position is always mentioned but the reason for doing it isn't stated. 73's John |
22nd May 2017, 10:29 am | #4 |
Rest in Peace
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
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Re: Tank Coil question
Putting it on the cold end also reduces the strain on the insulation as it sees mostly DC with only a smaller amount of AC.
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25th May 2017, 10:35 am | #5 |
Pentode
Join Date: Apr 2011
Location: Swansea, Wales, UK.
Posts: 143
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Re: Tank Coil question
I wondered why it is called a TANK coil. A suggestion is it stores the RF energy pulses then releases them via a tap. A condenser was required to capture the RF, like a still.
73, andrew |
25th May 2017, 4:20 pm | #6 |
Dekatron
Join Date: Jul 2007
Location: Penrith, Cumbria, UK.
Posts: 3,684
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Re: Tank Coil question
It's an Americanism, as I understand it. They started calling it that! It's (the parallel-resonant circuit, that is, of which the coil is part) a 'tank of RF energy' that is topped up with a trickle of make-up current of Ic/Q.
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Regds, Russell W. B. G4YLI. |