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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