on 11/3/99 16:45, measures at measures@vcnet.com wrote:
>> WHY? Why would a rheostat stop doing what a rheostat is supposed to do???
>>
> ? Hang onto your hat. The nichrome resistance wire that the rheostat
> was made from was subject to disintegration from atmospheric effects.
> The graphite contact in the movable arm would crumble. The carbon
> contact in the movable arm would wear away quickly.
So according to these "experts" a rheostat is not capable of varying the
magnitude of an AC voltage. Go figure. Hey, I have a rheostat on my
filament AND on my HV primary.
>>
>>> One should keep in mind that above its self-resonant frequency, a
>>> capacitor looks like an inductor. For example, your typical 0.01uF
>>> disc-ceramic universal "bypass" capacitor looks like an inductor on 20m -
>>> 10m.
>>
>> I agree with your comment. I am surprised though that the caps are that
>> poor.
>
> The caps were not poor.
Poor is a relative term. I mean poor RF performance. Looking like an
inductor at 14 MHz may be what is supposed to happen with that cap, but it
is still "poor" RF performance.
>
>> I know even most "quality RF" caps
>
> Even Quality cap leads have inductance.
Oh agreed, agreed. But then that can be minimized by shortening the leads
as much as possible. So is the poor 14 MHz performance due to longer leads
than should be used (sometimes you have no choice) or is the performance due
to the way the cap is internally constructed? I'm not disbelieving what you
measured, I am just trying to get a handle on the HF performance of a
typical disc capacitor.
>
>> (mica or otherwise) begin to look
>> really bad above 30 MHz, but I am surprised things look bad above 14 MHz.
>
> Perhaps you have not measured the resonance of many caps.
Not for a long time. The first time I did it was in college when I was
designing my first VHF amplifier. Using a vector impedance meter I got a
quick education into how even mica caps really looked at RF.
73,
Jon
KE9NA
--
-------------------------------------
Jon Ogden
KE9NA
http://www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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