The current in the shorted turns is such that it produces a magnetic field
opposing the magnet field in the rest of the
coil. Or you may say that the shorted turns resist a change in the magnetic
fields thru them. Either way it forces the
magnetic lines to spread out at the beginning of the shorted turns and loop
back around to the other end of the used
portion of the coil. If the shorted turns were super conducting there
would be no change in the magnetic field passing
thru that portion. In fact that is why chunks of super conducting material
will levitate over a strong magnetic.
The magnetic field passing thru the super conductor can not change.
Any change the field results in an infinite amount of eddy current due to
its zero resistance. There is a critical limit to the magnitude of the
field however where the super conductor will become a normal material and
at that instant everything changes.
73
Bill wa4lav
At 07:58 AM 8/6/2003 +0100, Ian White, G3SEK wrote:
>Stan Wigh wrote:
>>A year or two ago there was a discussion on the wildly accepted practice of
>>shorting out unused sections of a PI or PI-L output tank when not in use.
>>One commentator gave an exception that when toroids were used, shorting out
>>part of the inductor could be problematic.
>
>You can short out whole chunks of a normal air-wound solenoid coil because
>the magnetic coupling between the turns is not overly tight. You don't see
>high circulating currents (the "shorted turn effect") in the shorted part.
>The reason for shorting the coil rather than leaving unused parts
>open-circuit is that it prevents high voltages appearing at the open end
>of the coil, and also helps avoid stray resonances.
>
>The turns of a toroid are very tightly-coupled magnetically, so any
>shorted turns will almost short the whole coil.
>
>
>>I do notice that several of the
>>latest ARRL Handbooks do exactly that in their "Saint Petersburg" amplifier
>>as well as Command Technologies and QRO amplifiers, judging from their ad
>>photos. So what determines whether shorting out toroidal turns in the PI-Net
>>will work or not?
>
>What works is to short the whole toroid. That's why you usually see
>multiple toroids in these amps (I believe Command and QRO also use a
>toroid transformer at the output, but that's a separate thing).
>
>It can also OK to make a non-shorting tap on the winding, but only if that
>doesn't cause high voltages at the open end.
>
>
>--
>73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
> Editor, 'The VHF/UHF DX Book'
>http://www.ifwtech.co.uk/g3sek
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