Chuck wrote:
Doesn't the impedance transformation ratio have an effect on the
efficiency of a pi network?

The question is not: Can a particular pi-network plus
load be 30-50% efficient? Certainly, it can. The question
is: Are all pi-networks 30-50% efficient? The answer is "No!".
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Chuck wrote:
Doesn't the impedance transformation ratio have an effect on the
efficiency of a pi network?

The question is not: Can a particular pi-network plus
load be 30-50% efficient? Certainly, it can.


No disagreement, Cecil. My question
wasn't intended as a response to your
post. Sorry about that.

The question
is: Are all pi-networks 30-50% efficient? The answer is "No!".

If that is the question, then no
disagreement there either.

We don't know all of what W8QUR said,
and none of what he meant, but there are
contexts in which each of his three
assertions could be correct. I guess the
one thing he doesn't seem to have said
is that all pi-networks are 30-50%
efficient. ;-)

73,

Chuck





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Chuck wrote:
No disagreement, Cecil. My question wasn't intended as a response to
your post. Sorry about that.

Didn't mean to sound grouchy - it was before my first cup
of coffee.

We don't know all of what W8QUR said, and none of what he meant, but
there are contexts in which each of his three assertions could be
correct. I guess the one thing he doesn't seem to have said is that all
pi-networks are 30-50% efficient. ;-)

We do know what seemed to be inferred about the inference
of what W8QUR said. :-)
--
73, Cecil, w5dxp.com

"Jimmie D" wrote in message
...

"Owen Duffy" wrote in message
...
wrote in :

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?


Myron,

The temptation is to see that the second paragraph is about Pi networks,
though it doesn't actually use the term. It does refer to a "system" and
goes on to discuss efficiency in the context of "feeding the antenna".

There is no doubt that practical Pi networks in transmitters operate at
efficiencies much greater than 50%, and the design efficiency is a
trade-
off with harmonic suppression (for the low pass configuration in a
typical PA).

If the term "system" is to include more than just the Pi network, then
lower system efficiciency will prevail, but without a clear definition
of
the "system", it is not possible to comment on the reasonableness. For
example, if a Pi coupled transmitter feeds a full wave dipole via a
substantial length of coax, system efficiency might well be much less
than 10%. Does he include DC to RF conversion loss in his view of system
efficiency?

Owen

Giving W8QUR the benifit of the doubt
I thought he may be including feedline losses which could be from 1 to 2
db
for coax compared to balanced line used with a balanced output network.
I think something may be lost in the paraphrasing and this is probably a
comparison of balanced to unbalanced systems rather than a comparison of
Pi-net to other types of tuner networks.

Jimmie



In the 1960's it was common to refer to efficiency as relating to the entire
system converting AC or DC power into RF out of the antenna. Total system
efficiencies of 30-50% would have been normal taking into account the losses
involved in running valve heaters, HT valve supplies and final stage cooling
fans. The reference to Pi matching output circuits is a bit of a red
herring. These were probably the most commonly used system used to match
transmitters that were required to operate on a wide range of frequencies
because they were so effective at this task, and relatively cheap to
manufacture. Other more efficient matching methods could be used for fixed
frequency valve transmitters.

Even up to the 1980's, many shipboard emergency transmitters were valve
based. Radio Officers were required to make regular checks and efficiency
calculations to ensure that the emergency transmitters and receivers could
operate for a minimum specified period from a bank of emergency batteries.
Even with several hundred amp hours of battery capacity, only around 16
hours of full power (100watts) operation was the maximum that could be
expected.

Mike G0ULI

On May 23, 1:08 pm, Wes Stewart wrote:
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.


I haven't thought terribly deeply about this, but it occurs to me
you're caught between a rock and a hard place any time you are stuck
with a tube whose output capacitance represents a low reactance at the
operating frequency, and which wants to see a high load impedance.
However you resonate that capacitance, you end up with a high Q. It
is convenient that the Q of coils goes up as the frequency increases,
and for practical tubes at VHF/UHF, you can use transmission lines
that are physically large enough to have very high Qu.

In fact, it's not just the tube capacitance that gives you grief--it's
the ratio of the reactance and the desired load resistance. And for a
pure pi network, it's also the ratio between the resistance you're
matching: if you want to present a 5000 ohm load to a tube and
transform that to 50 ohms, the Q of the pi will be at least 10, at
which point the network has degenerated into a simple L with no output
capacitance. If you need to get from 10k ohms to 10 ohms, then the
loaded Q is 31.6 minimum.

But if you add just one more inductor forming a cascade of two L
networks each performing a 31.6:1 impedance transformation (for the
10k to 10 ohm example), the Ql of each will be about 5.6. The
capacitance at the plate end becomes much smaller, though, so this
method is only practical at lower frequencies. The comparison between
the "minimum Q" pi degenerated into a single L network and the cascade
of two L networks is interesting: the -3dB bandwidth of the single L
is about 6%, versus 26% for the cascade of two; but the harmonic
attenuation is better for the cascade: at the second harmonic, it's
42dB versus 33.5, and at the third, 59dB versus 42dB. Loss with Q=100
coils is also better for the cascade, about .48dB versus .72, although
if you use the same volume for the single coil case as you do for the
two coil network, the loss is pretty similar since the larger coil has
higher Qu. You can carry this even further and cascade more L
sections to get a flatter wide passband, better harmonic suppression,
and reasonably low loss.

Cheers,
Tom

On 24 May 2007 14:46:38 -0700, K7ITM wrote:

[all my good stuff snipped]


I haven't thought terribly deeply about this, but it occurs to me
you're caught between a rock and a hard place any time you are stuck
with a tube whose output capacitance represents a low reactance at the
operating frequency, and which wants to see a high load impedance.
However you resonate that capacitance, you end up with a high Q. It
is convenient that the Q of coils goes up as the frequency increases,
and for practical tubes at VHF/UHF, you can use transmission lines
that are physically large enough to have very high Qu.

In fact, it's not just the tube capacitance that gives you grief--it's
the ratio of the reactance and the desired load resistance. And for a
pure pi network, it's also the ratio between the resistance you're
matching: if you want to present a 5000 ohm load to a tube and
transform that to 50 ohms, the Q of the pi will be at least 10, at
which point the network has degenerated into a simple L with no output
capacitance. If you need to get from 10k ohms to 10 ohms, then the
loaded Q is 31.6 minimum.

But if you add just one more inductor forming a cascade of two L
networks each performing a 31.6:1 impedance transformation (for the
10k to 10 ohm example), the Ql of each will be about 5.6. The
capacitance at the plate end becomes much smaller, though, so this
method is only practical at lower frequencies. The comparison between
the "minimum Q" pi degenerated into a single L network and the cascade
of two L networks is interesting: the -3dB bandwidth of the single L
is about 6%, versus 26% for the cascade of two; but the harmonic
attenuation is better for the cascade: at the second harmonic, it's
42dB versus 33.5, and at the third, 59dB versus 42dB. Loss with Q=100
coils is also better for the cascade, about .48dB versus .72, although
if you use the same volume for the single coil case as you do for the
two coil network, the loss is pretty similar since the larger coil has
higher Qu. You can carry this even further and cascade more L
sections to get a flatter wide passband, better harmonic suppression,
and reasonably low loss.


Yep.

A number of years ago in this group our departed friend, Reg, made a
comment more or less saying that the fewer (non-ideal) reactances were
in the matching network, the lower the losses were.

I offered an example that proved this wrong. I'm extremely strapped
for time but I think the thread has something to do with L-networks if
anyone cares to search for it.

Wes

"Wes Stewart" wrote in message
...
On 24 May 2007 14:46:38 -0700, K7ITM wrote:
(snip)
A number of years ago in this group our departed friend, Reg, made a
comment more or less saying that the fewer (non-ideal) reactances were
in the matching network, the lower the losses were.

I offered an example that proved this wrong. I'm extremely strapped
for time but I think the thread has something to do with L-networks if
anyone cares to search for it.

Wes

http://tinyurl.com/2vn4sa

"Richard Clark" wrote in message
...
On Wed, 23 May 2007 23:33:45 -0400, "Jimmie D"
wrote:

Giving W8QUR the benifit of the doubt
is like saying a 4 cylinder Hummer's efficiency would be improved if
you removed the chassis.

73's
Richard Clark, KB7QHC

You either have to give the benifit of the doubt to W8QUR or the origonal
poster
W8QUR had benifit of editors that reviewed his work and decided it was
correct. Saying that a Pi-net is as inefficent as the OP interpretted W8QUR
as saying is a huge blunder that I doubt anyone who has the credentials to
author a magazine article would make. The OP only assumed he was talking
about Pi-networks and admitted
W8QUR did not directly refer to the network as this even though in 1965 it
was still called a Pi-net. It is also uncommon to call a pi-net a "system".
While you cant be sure unless you have the origonal article in its entirity
I will put my money on W8QUR in this case.

Jimmie

On Sat, 26 May 2007 11:15:06 -0400, "Jimmie D"
wrote:

You either have to give the benifit of the doubt to W8QUR or the origonal
poster

Hi Jimmie,

This is absurd on the face of it. Nearly every poster has performed
nothing more than "tea leaves reading" to massage inferences into
actualities. Look at the subject line heading every post and respond
to THAT. I've see nothing in three days that has diverged from my
initial response.

To mold supposed quotes into a new text that conforms to conventional
thinking does not confer nobility on the source. Using loose
references is, however, the staff of life in this forum.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Sat, 26 May 2007 11:15:06 -0400, "Jimmie D"
wrote:

You either have to give the benifit of the doubt to W8QUR or the origonal
poster

Hi Jimmie,

This is absurd on the face of it. Nearly every poster has performed
nothing more than "tea leaves reading" to massage inferences into
actualities. Look at the subject line heading every post and respond
to THAT. I've see nothing in three days that has diverged from my
initial response.

Nor Have I, As a matter of fact I hadnt read your post until just now.

I feel your explanation has credability but we will not know until the OP
further explains his paraphrase.



To mold supposed quotes into a new text that conforms to conventional
thinking does not confer nobility on the source. Using loose
references is, however, the staff of life in this forum.

I dont doubt the nobility of the source but I do doubt its accuracy.
Primarily because of the change of language from "Pi-network" paraphrased
part to "system" in the quoted part. I think it would be very unusual to
refer to a pi-net as a system While comparing a balanced system to a
single-ended sytem with 'system' meaning tuner plus transmission line would
be more logical. This assumption would also fit the W8QRU quote. While I
admit this is best guess I expect the OP to let me know if I am wrong, not
you, unless of course you have a copy of the related article. Either the OP
or W8QRU made a mistake. At this time I am slightly in favor of W8QRU.

This is why I was taught many years ago in Mrs. Mary Ruth Smiths English
class that when paraphrasing someone it is a good idea to retain certain key
words.

Sincerely
Jimmie