[Amps] Pi-L Network

[Tom W8JI]

> Tom said:
>>It's pretty tough and very expensive to obtain the 
>>perceived
> harmonic advantages of a pi-L in the real world of
> multi-band amps. It's much cheaper to do it other ways.<
> So why did so many professional continuous coverage QRO 
> amps (7.5kW and up) go to the trouble of a pi-L then?
> I know of at least 4 different manufacturers who did 
> so....
> 73
> Peter W6/G3RZP this week

I wouldn't have any idea why someone else does something. I 
only know how it actually works.

I use pi-L's to increase matching range and reduce the size 
of loading caps.  Even my 1950's Globe Scout has a pi-L that 
was installed just specifically for that reason, and it even 
says so in the manual.

It's easily proven the harmonic suppression change, unless 
we use a very good layout and a center impedance that is 
very high, is insignificantly better than a pi of the same 
overall Q. With real loads might actually be worse than a pi 
because it terminates in a low reactance series reactance, 
not a low reactance shunt reactance. It's easily proven it 
is cheaper and more reliable to get suppression by other 
methods.

When I need large amounts of harmonic suppression I add a 
trap or correct the problem some other way. It's generally 
much less expensive and works better than using a high 
center impedance with a L network.

Of course you COULD be confusing a double L with a pi L. 
Circuit wise they look exactly the same, and double L's are 
VERY common in wideband transmitters. A double L is a two 
stage L network that uses very low Q to match impedances. 
These were very common in marine transmitters, where the 
operator had to move over wide frequency excursions without 
retuning. I suppose at a quick glance without looking at 
component values people would mistake a double L, used to 
increase bandwidth, with a pi L since physically they have 
the same component layout....just different reactance 
values.

You aren't confusing a double L with a pi-L are you? You 
aren't assuming marine rigs used pi-Ls are you....when they 
actually commonly used double L's?

73 Tom