[AMPS] Re: Parasitics

[Ian White, G3SEK]

Jon Ogden wrote:
>
>How can a series resistance of 100 Ohms or so be greater than the K-Ohm 
>impedance of the anode circuit?
>
A 100-ohm resistor can LOOK like 1K to the tube, because of the
impedance-transforming properties of the inductances in the circuit. 

Pulling some example figures out of the air, consider a suppressor that
consists of 100 ohm resistor paralleled by 100nH. In series with that is
say 200nH of connecting lead inductance between the tube and the pi-
tank. At 100MHz, this network transforms into 1058 ohms in parallel with
279nH.

That's where your K-ohm comes from...  and yes, you ARE supposed to ask
that question!


>The conventional concept of a supressor is that the coil chokes the VHF 
>signal and the resistor then absorbs it.  However, if you think about 
>impedances and such, that doesn't make sense.
>
There are certainly better ways to look at it - especially if you want
to play the numbers game.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.demon.co.uk/g3sek

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