> 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
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