I seem to recall this subject was written about in Communications
Quarterly. Now QEX.
Then debated in the next few issues in the Technical Conversations section,
and then
there may have even been a second article written as debate to the first.
I thought the key term was conjugative match, but with a quick look all I
found was
a winter 1999 article more about tuners and cable "VSWR, Reflections, and
the Conjugate
Impeadance Match.
Your subject "tube r-f amplifier using a tuned tank circuit (output
impedance)" was very
hotly debated over several issues. It was all above my ability, but I
gleaned a little from it.
Someone with an ARRL membership can search the index and get the issues
with the articles
and all the technical correspondence.
Mike








"Richard Fry" wrote in message
...
It has been theorized that a circuit consisting of a Class C vacuum-
tube r-f amplifier using a tuned tank circuit in its output network
provides an operational “non-dissipative source resistance” of 50 ohms
for energy present at the output connector of the transmitter.

However the information and measured data provided in the text
excerpts below is not very supportive of that theory.

These excerpts discuss and show the effects of the energy entering a
transmitter at its output connector by frequencies offset from the
transmitter frequency.

There is direct applicability of the conclusions of the paper showing
that the operational source impedance of the transmitter near/at the
carrier frequency is much different than 50 ohms.

If it WAS a functional 50 ohms, then the termination provided to the
transmission line for signals entering the transmitter by its output
connector (whether on or off frequency) would not be present at the
plate of the PA tube to react with the power being generated by the PA
tube.

Rather the data leads to a logical conclusion that the operational
source impedance of this configuration at the carrier frequency will
be very low (approaching zero), when it is optimally tuned/adjusted to
produce its rated output power.

Further discussion or comment is invited.

RF

From:

A STUDY OF RF INTERMODULATION BETWEEN FM BROADCAST TRANSMITTERS
SHARING FILTERPLEXED OR CO-LOCATED ANTENNA SYSTEMS, by Geoffrey N.
Mendenhall, P.E.*

II. INTERMODULATION AS A FUNCTION OF "TURN-AROUND-LOSS".
"Turn-Around-Loss" or "Mixing Loss" describes the phenomenon whereby
the interfering signal mixes with the fundamental and its harmonics
within the non-linear output device. This mixing occurs with a net
conversion loss, hence the term "Turn-Around-Loss" has become widely
used to quantify the ratio of the interfering level to the resulting
IM level. A "Turn-Around-Loss" of 10dB means that the IM product fed
back to the antenna system will be 10dB below the interfering signal
fed into the transmitter's output stage.

"Turn-Around-Loss" will increase if the interfering signal falls
outside the passband of the transmitter's output circuit, varying with
the frequency separation of the desired signal and the interfering
signal. This is because the interfering signal is first attenuated by
the selectivity going into the non-linear device and then the IM
product is further attenuated as it comes back out through the
frequency selective circuit.

"Turn-Around-Loss" can actually be broken down into the sum of three
individual parts:
(1) The basic in-band conversion loss of the non-linear device.
(2) The attenuation of the out-of-band interfering signal due to the
selectivity of the output stage.
(3) The attenuation of the resulting out-of-band IM products due to
the selectivity of the output stage.

Of course, as the "Turn-Around-Loss" increases, the level of
undesirable intermodulation products is reduced and the amount of
isolation required between transmitters is also reduced.

The small portion of the interfering signal that is not reflected is
what causes intermodulation products to be generated. Obviously the
lower the output source impedance, the more complete the reflection
(lower return loss), with the result being less production of
intermodulation products.

III. EQUIPMENT PARAMETERS THAT AFFECT INTERMODULATION LEVELS.
The interfering signal must be coupled into the transmitter's output
stage before the IM products are produced and the output level of the
intermodulation products will be related to the interfering signal
level.

The two parameters (outside of the filterplexing equipment) that most
affect the interfering signal level into the transmitter's output
circuit are the output loading and the circuit's frequency selectivity
(loaded "Q"). These two parameters are interrelated because the degree
of output loading will change the loaded "Q" of the output circuit
while also affecting the return loss of the interfering signal looking
into the output circuit.

"Output Return Loss" is a measure of the amount of interfering signal
that is coupled into the output circuit versus the amount that is
reflected back from the output circuit without interacting with the
non linear device.

To understand this concept more clearly, we must remember
that although the output circuit of the transmitter is designed to
work into a fifty ohm load, the output source impedance of the
transmitter is not fifty ohms. If the source impedance were equal to
the fifty ohm transmission line impedance, half of the transmitter's
output power would be dissipated in its internal output source
impedance. The transmitter's output source impedance must be low
compared to the load impedance in order to achieve good efficiency.

The transmitter therefore looks like a voltage source driving a fifty
ohm resistive load. While the transmission line is correctly
terminated looking toward the antenna (high return loss), THE
TRANSMISSION LINE IS GREATLY MISMATCHED LOOKING TOWARD THE OUTPUT
CIRCUIT OF THE TRANSMITTER (LOW RETURN LOSS). THIS MEANS THAT POWER
COMING OUT OF THE TRANSMITTER IS COMPLETELY ABSORBED BY THE LOAD WHILE
INTERFERING SIGNALS FED INTO THE TRANSMITTER ARE ALMOST COMPLETELY
REFLECTED BY THE OUTPUT CIRCUIT.

VI. CONCLUSIONS
1. "Turn-Around-Loss" is a function of the particular non-linear
device and the amount of loading on its output circuit.
2. "Turn-Around-Loss" increases as the interfering signal and the
resulting IM products are moved away from the carrier and out of the
output circuit passband.
3. "Turn-Around-Loss" will be least when the interfering signal is
within the transmitter's passband.

The figure posted at the link below shows the measured data supplied
with this paper.

http://i62.photobucket.com/albums/h8.../TAL_Chart.gif


* Geoffrey Mendenhall presently is Vice President, RF Engineering at
Harris Corporation Broadcast Division, and a recognized authority on
transmitter system design. Harris Broadcast is one of the largest
manufacturers in the world of AM/FM/TV broadcast transmitters, rated
for power outputs up to 2,000 kW.

On Jun 12, 7:30*pm, Richard Clark wrote:

Deja Vu all over again. *I suppose you posted this to inspire me to,
once again, remind you from Mendenhall's own notes about Class C
amplifier construction - and so I will: ...

If you believe by your understanding of the clips you quoted from
Mendenhall that a conventional, single vacuum tube Class C r-f
amplifier provides a functional termination of 50+j0 ohms for energy
applied to the output connector of the transmitter, then please
explain why that termination allows such signals to reach the plate of
the PA tube -- whose non-linear characteristics created the
_measured_ r-f intermodulation products and other performance data
given in the Mendenhall paper I quoted.

RF

On Sun, 13 Jun 2010 05:17:54 -0700 (PDT), Richard Fry
wrote:

the PA tube -- whose non-linear characteristics

From Mendenhall:

"VHF amplifiers often exhibit a somewhat unusual characteristic when
tuning for maximum efficiency. ... If the amplifier is tuned exactly
to resonance, the plate load impedance will be purely resistive and
teh load line will be linear."

73's
Richard Clark, KB7QHC

On Jun 13, 12:12*pm, Richard Clark wrote:
On Sun, 13 Jun 2010 05:17:54 -0700 (PDT), Richard Fry
wrote:

the PA tube -- whose non-linear characteristics

From Mendenhall:

"VHF amplifiers often exhibit a somewhat unusual characteristic when
tuning for maximum efficiency. ... If the amplifier is tuned exactly
to resonance, the plate load impedance will be purely resistive and
teh load line will be linear."

'Unusual'? Certainly not unexpected once one thinks about it. For a
given controlled power, the minimum dissipation in the controlling
device will occur when the minimum voltage occurs at the time of
maximum current, and vice versa, i.e. the voltage and current in the
load are in phase, or equivalently, the load impedance is resistive.

I would expect this to be a fundamental characteristic, and not just
for VHF amplifiers.

The 60Hz folk are trying to achieve the same result as they strive
for a power factor of 1.

....Keith

On 12 jun, 23:42, Owen Duffy wrote:
walt wrote :

...

What you have said above is the key to the concern over the output
resistance of a Clsss C amplifier being non-dissipative. What seems to
be universally misunderstood is that there are really two separate
resistances in the operation of these amps; one, the cathode-to-plate
resistance, which is the dissipative resistance Rpd that accounts for
all the heat, due to the electrons striking the plate; and two, the

It is my understanding that the average power (heat) generated at the anode *
of a triode can be found by averaging the product of the instantaneous
anode current and anode-cathode voltage over time. In a Class C amplifier,
the voltage and current are not linearly related to each other, ie there is
no constant of proportionality, no constant or fixed resistance.

I don't understand why then, that people try to explain the anode
dissipation in terms of some value of resistance.

Owen


Hello Owen,

I fully agree with you. When I am doing power electronics, I show
myself the instantaneous current*voltage plot, and the averaged
integral. The last one shows the dissipated power, the first one
tells me where I have my losses (and also what I have to change to
reduce the overall loss or reduce component stress).

Regarding the class C output impedance issue, I updated my simulations
with a 3.6 MHz output stage with a 6146 tube and real class C
operation.

You can almost get every impedance you want, also the conjugated match
condition, but you need to do many simulations to find that point.
With hours of time, I couldn’t get closer to an output VSWR of 1.58
with 79% efficiency. I know you can get closer, but at his moment I
cannot provide you a spice file as I don’t have it anymore and I also
dropped orcad long time ago. .

The smallest change in output loading or drive level results in
significant change of output impedance. So take an arbitrary class C
amplifier, measure its output impedance and it will very likely be way
off the intended load (in my example 3500 Ohm).

If you want to use load change method with some different resistive
loads, you should make phase measurements to get valid results, as you
don't know whether you amplifier has real or complex output
impedance.

For the class C amplifier I changed the drive by +0.36 dB, resulting
in an increase of plate efficiency from 79% to 80%, but the output
VSWR changed from 1.58 to 3.6. Also reduction of input drive gives
significant change in VSWR (magnitude and phase).

I hope that people will reproduce some of the simulation themselves to
develop a solid opinion on their own.

Best regards,


Wim
PA3DJS
www.tetech.nl
without abc, PM will reach me.

Hello,

I forgot the link: www.tetech.nl/divers/PA_impedance.pdf

Best regards,

Wim
PA3DJS

On Jun 13, 11:12*am, Richard Clark wrote:

From Mendenhall:
...

Would you please respond in your own words directly to my
previous statement (repeated below)?

If you believe by your understanding of the clips you quoted from
Mendenhall that a conventional, single vacuum tube Class C r-f
amplifier provides a functional termination of 50+j0 ohms for energy
applied to the output connector of the transmitter, THEN PLEASE
EXPLAIN WHY THAT TERMINATION ALLOWS SUCH SIGNALS
TO REACH THE PLATE OF THE PA TUBE -- whose non-linear
characteristics created the _measured_ r-f intermodulation products
and other performance data given in the Mendenhall paper I quoted.

RF

On Sun, 13 Jun 2010 10:03:40 -0700 (PDT), Keith Dysart
wrote:

'Unusual'? Certainly not unexpected once one thinks about it.

Hi Keith,

The "unusual" was lost in the ellipsis that I will reveal:
"... The highest efficiency operating point does not exactly coincide
with the lowest plate current because the power output continues to
rise for a while on the inductive side of resonance coming out of the
dip in the plate current. ..."

Continuing, Mendenhall presents the problems of trade-offs between
what would seem to be maximum power for worsening characteristics in
performance - the goal is what he calls minimizing synchronous AM
versus Efficiency.

Synchronous AM is a problem that can be introduced by measurement
equipment, as Mendenhall relates: "The input impedance of the
envelope detector must provide a nearly perfect match ... 30dB return
loss ... to the sampling line."

This sidebar relates to what Owen characterized as "Usability" where I
have recited the objective technical specification to reduce the
subjectivity of the term.

There's more that could be said, but I am waiting to see if Richard is
willing to subscribe to his own reference's writings. If not, and
Mendenhall doesn't mince words on the topic, then as Perry Mason would
observe "The D.A. is impeaching his own witness!"

I've always loved Perry Mason.

73's
Richard Clark, KB7QHC

On Jun 13, 12:35*pm, Richard Clark wrote:

I am waiting to see if Richard is willing to subscribe to his own reference's writings

I responded to you some 20 minutes before you posted, and now await
your response.

Also please comment on whether or not a Class C amplifier operating on
a linear portion of its transfer curve will function as a linear
amplifier.

RF

On Sun, 13 Jun 2010 10:14:21 -0700 (PDT), Richard Fry
wrote:

Would you please respond in your own words

Hi Richard,

I conform to Mendenhall to the specific statement I responded to.

If you believe

What I believe has been succinctly laid out in my subscribing to
Mendenhall's explicit statement.

THEN PLEASE EXPLAIN WHY

I don't see that elaboration is going to improve what has been
presented.

Yes, it is a difficult concept that many struggle with and few have
had experience in making a sufficiently accurate determination of.
Consult Walt's 333 line posting and examine how experience comes to
bear and through my recitations reveal the dovetail fit to theory.

The Only Explanation Possible:

Here's a modest proposal, Mendenhall constructed a power amplifier
that is within the technical grasp of many here to achieve at a modest
workbench. The design is quite spartan. The design is quite
understressed (there is nothing "forcing" a conclusion). The design
conforms to all engineering standards.

Build your own. [It feels strange to have to offer that option to a
group of Hams.]

Having this amplifier before you, observe all the variables, play with
them. Measure their impact on NOhms. Account for the heat with
direct measurement and note what does not conform to convention.

When that is finished the real work begins. Calibrate your tools and
repeat this for accuracy.

73's
Richard Clark, KB7QHC