[AMPS] Re: Parasitics
Ian White, G3SEK
Fri, 15 May 1998 14:02:00 +0100
Jon Ogden wrote:
>>A 100-ohm resistor can LOOK like 1K to the tube, because of the
>>impedance-transforming properties of the inductances in the circuit.
>I well, so you say....
Network theory says so, and it's true.
>Transmission lines, etc can act as impedance transformers. This is the
>whole idea behind phasing harnesses on antennas, 1/4 wave transformers,
>However, a supressor is typically not any appreciable wavelength from the
>anode. So how can you say that it's impedance is transformed.
There's more than one way to transform an impedance. This is impedance
transformation using lumped reactances - essentially the same kind that
takes place in your pi-network (different circuit, but covered by the
same network theory).
>you do get say, 1 KOhm of impedance from all the inductances, that's NOT
>a REAL resistance. That is a reactance.
No it isn't. Think about your pi-network terminated in a 50 ohm load.
The load impedance is transformed to say 2K across the tube - take away
the dummy load and the 2K resisitive part disappears to infinity
(ignoring network losses). Also, the 50 ohm resistor receives exactly
the same amount of RF power as a 2K resistor wired across the tube
The same applies to a 100 ohm suppressor resistor, when it is
transformed to represent 1K in parallel with the tube. It will dissipate
exactly as much power as a 1K resistor connected across the tube. In
that sense the 1K is a real resistance.
Since 1986 I've been developing and using a "network calculator"
program, imaginatively called NETCALC, which makes all this
series<>parallel stuff very quick and easy.
There's nothing clever about the network math - it's all standard stuff
from the ARRL Handbook or any other textbook. NETCALC simply mechanizes
the math, so that you're free to understand about the network.
NETCALC.EXE and its HTML documentation will be on my web site sometime
over the weekend. Its 1980s-style DOS interface is a bit clunky but it
works, and the math routines have been validated by many users over the
last 12 years.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
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