Сообщение от K2AV
T300A-2 is a bare minimum toroid volume for #2 material in this application
T300A-2 has the minimum inside diameter size to support a transformer (not inductor) with 20 or 21 bifilar turns of Teflon sleeved #14 wire laid flat on the toroid.
Teflon sleeved double polyamide coated #14 and the T300A-2 was the first combination in our seven years of experience with these devices that had no arcing, shorting, burning, nicking, and never ever broke down again, no matter what the winding use was.
That combination of components, wound as an isolation transformer in an L over FCP, was stone cold after 15 minutes continuous key down 1500 watts. So far after three years I have not a single report of one going bad. Several were accidentally miswired as a balun. Accidentally wired as a balun performs very poorly between L and FCP.
I have many reports of various failures when these specifications were not followed.
The actual best size for performance on 160 is T400A-2. But since we had so many complaints from people wanting to use whatever toroid they had on hand, we only specify T300A-2 as the bare minimum and I constantly get requests to not even use the T300A-2.
A pair of T300-2 taped together or better three T300-2 will work fine, while keeping a sufficient inside diameter.
Some smaller diameter cores have a high enough AI number but the inside diameter is not large enough to support enough turns of Teflon sleeved wire in a flat layer. Smaller wire sizes without sleeving, that will form a flat layer in smaller cores, are subject to insulation nicks or perforation by wind static or lightning induced voltage and those breaks will deteriorate into arc paths and destroyed cores, even at low powers. This is not speculative, we've burned up more stuff than I want to admit.
QRO needs less loss of larger wire to keep from breaking down. QRP needs less loss to be heard. Both need minimum loss.
There is a physically smaller version that is OK but involves Teflon insulated #18 wire that is hard to find.
