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, W0PBV.
--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
NRA Life Member & Certified Instructor for Rifle, Pistol, & Home Firearm Safety
Also Certified Instructor for the Kansas Concealed-Carry Handgun (CCH) license

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

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

Hi Myron,

No. Efficiencies are found (or broken) elsewhere. As Terman offers,
the Loaded Q of the Tube output tank runs between 8 and 15 (the
efficiency you quote would have that plummet). The generalization
offered ("It is not possible to run more than 50% efficiency") is also
the conventional error of not being able to distinguish between a
Conjugate Match and a Z Match.

73's
Richard Clark, KB7QHC

Myron, W0PBV wrote:
"Is that low efficiency of 30-50% really true?"

No. The pi network would not be so popular were that true.

The efficiency of a Class B or Class C amplifier is higher than that and
the network by itself is very low loss. RF amplifiers typically have
efficiencies well above 50% because much of their source resistance is
of the lossless variety.

Search the internet for: "pi network antenna tuner". One entry near the
top of the list is from Collins for its 180S-1 Antenna Tuner. It is
basically a 1000 watt "pi" network for matching various antenna
impedances to a 50 ohm coaxial transmission line in the range of 3-30
MHz. In most cases it is used as an "L" network, but when the "L"
network cannot match the desired antenna, the complete "pi" circuit is
used. The vacuum variable capacitor employed in the output circuit can
be connected either in series or shunt with the antenna. The 180S-1 is
useful for tuning trailing wires on large aircraft.

Ralph Hanna, W8QUR had it wrong when saying "The big disadvantage of
this system is the low efficiency."

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote in message
...
Myron, W0PBV wrote:
"Is that low efficiency of 30-50% really true?"

No. The pi network would not be so popular were that true.

The efficiency of a Class B or Class C amplifier is higher than that and
the network by itself is very low loss. RF amplifiers typically have
efficiencies well above 50% because much of their source resistance is
of the lossless variety.

Best regards, Richard Harrison, KB5WZI


If the efficiency or loses were in 50% range, we would be "ungluing" the
components in the PI network at the 2 kW power levels. Can you picture 1 kW
being "lost" in the coil and capacitors?
Typical matching network or tuners in the transceivers have loss about 10%
when power output is measured with tuner in or out while maintaining same
input.
Amps with decent copper and quality caps should be less than 10% in loses.

Yuri, K3BU

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.

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

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

"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