[Amps] Pi-L Network

[Tom W8JI]

> 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