[Amps] 200-ohm tank and "un-un" in HF amp design?

[Manfred Mornhinweg]

Jim,

> Assuming adequate core/wire sizing and airflow, can anyone think of a 
> solid reason *not* to design a legal-limit HF amp output tank for 200 
> ohms, then use a 4:1 "un-un" transformer to step it down to 50?

I think there is no big technical reason against this, but it may well 
end up significantly more expensive than using the Pi circuit alone to 
get 50 ohm.

> I'm curious because doing so would bring the loading cap requirement for 
> a 4000-ohm plate impedance down to under 1000pF at 1.8 mHz, which can be 
> done with a large air variable alone.

But that variable capacitor needs to be rated for twice the voltage than 
the one for a 50 ohm output. So it needs twice the spacing, and then a 
lot more plate area is required for a given capacitance. So there is no 
free lunch there.

The 200 to 50 ohm transformer is basically an added cost, without really 
saving on anything else.

Maybe in some particular case it does make sense to use that system.

Manfred

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