> Hi Peter, Most of the ham amps now use a 4to1 step down
> transformer in
> place of the switched L coil. Does this provide the same
> reduction of
> harmonics?
> Regards, Jim
> K2ZO
Jim,
The reason manufacturers get away without a pi-L is simple.
Pi-L's aren't generally needed nor do they always reliably
improve harmonic suppression.
If the source was a pure constant resistance on all
frequencies and the load looked like 50 ohms on all
frequencies including the harmonics, and if the source was
always -6dB on the second harmonic, and if the tank
components were ideal perfect components and there was no
mutual or coupling, we could use numbers like Peter did.
The fact is the real world isn't even close to any of that.
I just looked at the harmonic test on a 3CX800A7 amp, and it
was -52dBc or better on the bands where it used a pi. It was
no better at all on bands where it used a pi-L. Some
harmonics got worse, some got better with the pi-L. That was
with a 50 ohm load. If you stick a real antenna on the amp
that isn't 50 ohms on the harmonic suppression can get much
worse with the pi-L than without it.
The reason the 4:1 transformer is used is to reduce the
required size of the loading cap. That's the very same
reason I used pi-L's in amps only on 160 and 80. The pi-L,
in an actual working PA, didn't really help anything except
matching range. The only bands with limited matching range
were 160 and 80, so why add L sections that aren't needed???
73 Tom
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