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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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Old 20th May 2017, 12:42 pm   #1
John M0GLN
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Default 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
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Old 20th May 2017, 7:32 pm   #2
GW3OQK Andrew
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Default 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
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Old 22nd May 2017, 6:56 am   #3
John M0GLN
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Default 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
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Old 22nd May 2017, 10:29 am   #4
G8HQP Dave
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Default 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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Old 25th May 2017, 10:35 am   #5
GW3OQK Andrew
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Default 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
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Old 25th May 2017, 4:20 pm   #6
russell_w_b
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Default Re: Tank Coil question

Quote:
Originally Posted by GW3OQK Andrew View Post
I wondered why it is called a TANK coil.
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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