Geurts, Gerard wrote:
>Some people use resistive inductors to improve their surpressors, so why
>not use inductive resistors? From a circuit analysis point of view a
>non-inductive (I know, they don't exist) resistor in parallel with an
>inductor is the same as an inductive resistor in parallel with a
>slightly bigger inductor.
>
That's true, but the effective values of R and L will vary with
frequency, so you need to check the behavior of the network at both VHF
and HF. Above all you need to either make measurements or plug in some
numbers - or best of all, do both.
>The same goes for the series resistor we put in the high voltage line to
>limit the current. I see references to non-inductive resistors for this
>application all the time, but this resistor is switched in series with
>the plate RFC. Again, from a circuit analysis point of view a
>non-inductive resistor in series with an inductor (the plate RFC) is the
>same as an inductive resistor in series with a slightly smaller RFC. The
>only thing we need to make sure is that the resistor can handle the
>energy dumped in it during a major 'event'.
>
There isn't any particular reason for this resistor to be non-inductive,
because it isn't part of the RF circuit. The only reason why globar-type
resistors are better is that they are least likely to break down when
the full B+ voltage appears along their length. Spiral-wound resistors
are more likely to arc from turn to turn - especially if the turns
spacing isn't quite uniform, or if the resistor is physically too short.
If it were not for those problems, some series inductance in the surge
limiting resistor would actually help to reduce the current surge. G4GCM
has had good results with a home-made resistor wound from resistance
wire with a wide turns spacing (about 1 wire dia).
>The extreme case of this would be if you wind the RFC using resistance
>wire. This would integrate the RFC and limiting resistor in one
>component, reducing component count, and would also lower the Q of the
>RFC making it easier to produce one without resonances.
That doesn't seem like a good idea, because it would introduce losses at
a critical high-impedance point in the RF circuit. There are good
reasons to keep the RF functions in one component and the DC functions
in another.
>Does anybody
>know a source of suitably insulated resistance wire so I can give this a
>try?
In the UK, try the Scientific Wire Co (01181 505 0002).
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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