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Wes Stewart · May 23rd 07, 09:08 PM posted to rec.radio.amateur.antenna

On 23 May 2007 09:37:00 -0500, wrote:

Ralph Hanna, W8QUR, in a brief article "Pi Networks" on page 108 of
the December, 1965, issue of 73 MAGAZINE, after discussing power-
supply filters and high- and low-pass TV filters, wrote:

(Paraphrasing) "The most popular of all pi networks is the output
circuit of a transmitter ... with which the output of almost any
transmitter can be matched to almost any antenna ... another
advantage is the reduction of harmonics....

(Actual quote) "The big disadvantage of this system is the low
efficiency. It is not possible to run more than 50% efficiency
and it tends to be more like 30%. Other methods of feeding the
antenna will result in efficiencies of as high as 65% to 70%."

Is that "low efficiency" of 30-50% really true?

As others have stated, No.

Clearly at that time the author was talking about a vacuum tube
transmitter where the pi-network was used to transform the load
impedance (usually 50 ohm) up to the load that the tube(s) want to
see.

The usual implementation was the low-pass form of shunt C(s), series
L, although this isn't the only option. The network can be thought of
as two L-networks back-to-back with a "virtual" impedance common to
the midpoint. The usual design sets a overall network Q (the sum of
the two L-network Q's) at something between 10 and 12 for harmonic
suppression reasons.

The loss in the network is usually considered to be only in the
inductor, (although this isn't totally correct) because inductors
generally have lower unload Qs than the air or vacuum variable
capacitors that are typically used.

The network efficiency using this assumption is then:

eff = 1 - (Ql/Qu)

So for example if the inductor Q = 200 (a reasonable value) and the
network Q is set to 12 then the efficiency is 94%, a long way from
what the author claims.

At higher frequencies with tubes with high output capacitance it may
be necessary to design for a higher loaded Q than we would like. In
this case, the efficiency will reduce as is often the case with
amplifiers on 10-meters for example.

All of this stuff in any ARRL Handbook and can be worked out by the
reader.