1/4 vs 1/2 wavelength antenna

[John Woodgate]

Not if we were there the moment the later wave turned on. I heard that
amateur operators hate splatter. RC appears to be an exception,
however.

See 'at any later time' in my text.

 

[gwhite]

See 'at any later time' in my text.

Oh yeah.

 

[Richard Clark]

Ah, at last a relevent question/statement. See my first post in this thread.

Mmm-Hmm


RF transmitters are not impedance matched to antennae in the sense of maximum
transfer of power.

Hi OM,

As I've noted in the past, you can fill a library with negative
assertions without ever offering an answer, eg.:

RF transmitters are not Nuclear resonated to antennae in the sense of maximum
transfer of power.

RF transmitters are not impedance matched to antennae in the sense of maximum
balance of payments.

RF transmitters are not cosmically matched to antennae in the sense of maximum
psychrotropic power.

The list could go on, be completely accurate, and yet never actually
mean anything in the end much as the nonsense you offered from the
start.

You sighed with content at being offered a "relevent
question/statement" Your re-iterative response contains the same (how
could it be otherwise?) slack of precision that started this. Want to
try again?

You could have as easily expressed what sense they ARE matched, but
instead this time offer what Basis of Matching you are attempting to
describe. This is the more rigorous approach that eliminates vague
descriptions and uses standard terms. If you have to query about what
"Basis" means (used by professionals - namely metrologists who can
quantify Output Z of all sources) - then we can skip it as a topic out
of the reach of amateur discussion.

Note:

Again, RF PA's should be load-line matched.

Does not qualify as a Basis. It is suggestive of one, but because you
indiscriminately mix several Basis within your discussions, it is your
responsibility to be precise. If you can accomplish this, then we can
proceed to review how little it all matters.

Barring resolving any of these issues of precise language, I notice
that you rather enjoy fruitless jousting with them than challenging my
support of Ken's (supposed) statement that you say is your focus:

That's because you don't understand the difference between impedance matching
and ac load line matching.

We will leave that as another dead-end.

73's
Richard Clark, KB7QHC

 

[gwhite]

You entirely missed the point. You don't know the output impedance because you
don't have a way of determining it by swinging the output full-scale.

You don't have to swing the output full-scale to measure the impedance.[/QUOTE]

Playing along with the idea that there is some meaningful fixed Z of the device
for large swings, yes you would have to do so to prove the concept. You would
need to prove that output Z was the same for driving 1 W into the output as for
driving 100 W into the output. I also predict that even the small signal output
Z of the power amp will not be that conjugate impedance you think it is for a
properly designed PA. (I am not making a claim that it would *never* be so for
any PA.)

Any change in the load, no matter how small, will cause a change in the
output voltage and the output current.

Likewise, a change in the output Z would do the same thing. Since you're
presuming linearity, we can include gain linearity. I.e., the gain with "-10
dB" of drive is the same as the gain with "0 dB" drive. I'll define the 0 dB
gain as associated with the 1 db compression point. Since the gain is defined
as linear (really fixed regardless of drive), and the load is fixed, something
must have "caused" the compression. A way to *model* the compression is a
changed output Z as a function of drive. While I realize this is an
unconventional view of output compression modeling, I believe it is fair, since
you are making the linear presumption. I think this is fair also because the
impedance concept is a linear/sinusoid one. Under that presumption, you've given
me license to disregard distortion.

From these you can calculate the
output impedance at the current operating point.

When a transistor is operating under large signal conditions into a tuned
load, there is still an output impedance and this impedance still
discribes what will happen for small changes in the load.

Let's do another example.

Say the device we've selected has an Imax rating of 1 amp and a generator
resistance of 100 ohms. Per standard linear theory, we do our norton model of
Igen in parallel with the 100 ohms. Under standard conjugate matching theory,
we should load it with 100 ohms.
Now with the 100 ohm load, we get a 50 V peak for Imax = 1 amp. But what if
both our DC supply and device breakdown won't allow this? We have a practical
limiting Vmax not at all included in linear theory. Due to breakdown or supply
rail concerns, we'll see our Imax quite short of the 1 amp we expect when the
device is loaded with 100 ohms. We won't be getting all the power out of it we
"expect" because of practical limitations not built into linear conjugate
matching theory.

How do we select the best load, since conjugate loading clearly does not use the
device to its full potential? We seek Ropt, or what is commonly referred to as
the load line match.

Ropt = Vmax/Imax

where Ropt << Rgen, if not

(Rgen + Ropt)/(Rgen*Ropt) = Vmax/Imax


So even looking into the PA output in the small signal sense (or tweaking the
impedance as you suggest), we won't likely see Ropt = Rgen, because we are
dealing with some practical design limitations not accounted for in linear
theory.

Perhaps a couple of quotes from Cripps would be nice:

http://www.amazon.com/exec/obidos/tg/detail/-/0890069891/

"The load-line match is a real-world compromise that is necessary to extract the
maximum power from RF transistors and at the same time keep the RF voltage swing
within specified limits and/or the available DC supply." p13

"A final note here concerns the nebulous and highly questionable concept of
large signal impedance. The reason for the load-line match is to accommodate the
maximum allowable current and voltage swings at the transistor output. That says
nothing about the impedance of the device, which remains the same throughout the
linear range. Once a device starts to operate in a significantly nonlinear
fashion, the apparent value of the impedances will change, but the whole concept
of impedance starts to break down as well, because the wave forms no longer are
sinusoidal." p14

 

[Ken Smith]

LOL. Sure, the purpose of a power amp is to actually extract power. This is a
good start.

No, the purpose of the power amp is to deliver power, not extract it.

Perhaps a simplistic (and of course idealized) class A example would help. And
I want to remind that this is a simplification of the first order design cut.

Don't bother with the over simplified Class A case. RF power
amplification is rarely done class and and it is a digression from the
actual topic.

[...]

Our circuit loaded with 10 ohms delivers twice as much power as with the lesser
5 ohms or greater 20 ohms. That is, extracted output power is peaking at some
finite non-zero value. This is also easily seen to be most efficient point for
this simplistic example.

At some point as you decrease the resistance, the output will drop to zero
as the amplifier fails or it will start to decrease in some more
controlled manner as the protection circuits take control. If we assume
the latter case, it is easy to see that the power reaches a maximum value
and then decreases as the resistance is lowered. The point at which the
power is at the maximum is the point at which the load is matched. If you
make a small change in the load and observe the voltage and current when
that small change is made, you will see that that is indeed the output
impedance of the amplifier. I think this is the part you are not
grasping.

 

[Ken Smith]

Here's the original quote [Ken]:

"When the correct matching is done, the antenna works as an impedance mathcing
network that matches the output stages impedance to the radiation resistance."[/QUOTE]

Yes, I stand by and have just in another part of the thread once again
explained that indeed the impedance is matched. ie: If you make a small
change in the impedance in any direction the power decreases. Increasing
the resistance is the obvious one. The other three are because the
protection circuits act. The OP had a completed transmitter he was
connecting to a length of wire.

 

[Ken Smith]

The point is that if you want to talk/write about one of these
impedances, you need, to prevent misunderstanding, use a precise term,
such as 'incremental output source impedance' and define it.

You are right. I really needed to be more clear in the first posting I
did. That bridge has now been crossed and this is getting tiresome. If
the OP doesn't come in with more questions, I'm out of here.

 

[gwhite]

No, the purpose of the power amp is to deliver power, not extract it.

Well really, once the device and supply have been determined, we can indeed view
it as extraction. We'll load it to extract the most. If you want to mince
words and call it "deliver," that is fine.

Don't bother with the over simplified Class A case. RF power
amplification is rarely done class and and it is a digression from the
actual topic.

Well, class A is certainly done. Two cases are where the extra little bit of
linearity is desired and at high frequencies, were PAE starts to take a bite as
the gain drops below 10 dB.

At some point as you decrease the resistance, the output will drop to zero
as the amplifier fails or it will start to decrease in some more
controlled manner as the protection circuits take control. If we assume
the latter case, it is easy to see that the power reaches a maximum value
and then decreases as the resistance is lowered. The point at which the
power is at the maximum is the point at which the load is matched. If you
make a small change in the load and observe the voltage and current when
that small change is made, you will see that that is indeed the output
impedance of the amplifier. I think this is the part you are not
grasping.

No, this is exactly where I'm saying you are incorrect. You are not getting the
practical limitations and are mistakenly applying linear concepts. It doesn't
work if you want to extract maximum power from the DC supply through a real
device, converting the DC power into RF power.

 

[gwhite]

Here's the original quote [Ken]:

"When the correct matching is done, the antenna works as an impedance mathcing
network that matches the output stages impedance to the radiation resistance."

Yes, I stand by and have just in another part of the thread once again
explained that indeed the impedance is matched. ie: If you make a small
change in the impedance in any direction the power decreases.[/QUOTE]

Driven to max swing, this is true. But it is because of asymmetrical clipping,
not because of conjugate mismatch. For lower drives, what you say won't
necessarily be true *unless* you've mis-designed according to conjugate match
ideals. Your argument is circular.

If you design for conjugate match, you're right. I'm saying: don't do that. If
I design for load line match and you design for conjugate max (both pf us using
the same device and supply), I will get a higher peak power than you will.
However, you'll get to be right about how your amp acts regarding diverging from
conjugate load. But it is irrelevent: you made a fundamental mistake.

 

[gwhite]

As I've noted in the past, you can fill a library with negative
assertions...

The troublesome assertion is not the negative one. It is that RF PA's are
conjugate matched. Neither you nor Ken has provided a single example of such a
design that also extracts the maximum amount of "linear" power from a device and
essentially its power supply (after all, that is what it is: a _power_ amp).
Your example said nothing about output-Z, which suggests you have no clue, since
you didn't even remotely address the issue.

For Ken's part, he recently obfuscated by dismissing an example that was
primarily intended to be illustrative, but yet holding the salient points. He
completely ignored (or didn't understand) the clipping issue. Further
obfuscation was provided by talking about "protection circuitry," which may or
may not exist in a circuit, but adds zero to a discussion regarding how the PA
is to be loaded. "Protection" is a non-stater because the PA is either off or
impaired.

Ken's argument is circular. He say's that if a design is done for conjugate
match,
then it will behave as if it is conjugately matched. Well of course (or at
least sort of under specific test conditions and circuits)! It is
self-fullfilling prophecy but it unfortunately makes no statement regarding
obtaining the maximum power out of the circuit in the sense of turning DC power
into RF power (yes, *extracting* power from the DC supply and transformed to
RF). This is paramount to PA design. To use the device to maximum efficacy, as
Cripps puts it, a load-line match is needed. Ken's "conjugate match" design
won't do that, and that's why PA's aren't designed that way.

The bottom line is that if I design an amp via load line techniques using the
same device and power supply as Ken (him using conj-match), my amp will deliver
higher unclipped PEP than his. That is the factual result you resist. Now if
you want to pay for extra power and big devices, that's your business--go ahead
and attempt to conj-match your amp--but engineers who design PA's don't do that.

Another idealized and hypothetical example to elucidate the load-line principle
is offered.

Let's say we have a 10 W FET we'll build into a class A circuit. An RF choke is
used to supply drain current. We DC bias it to Vd = 10 V and Id = 1 A. Just
for argument sake, let's say it has a constant internal resistance of 110 ohms
and the device will break down at 25 V. According to the most idealized and
standard load-line theory, we should load it to rL = Vd/Id = 10 Ohms. This
idealization includes the definition of positive and negative clipping --
whichever comes "first" -- of being the operational limit for output voltage
swing. Clipping is associated with severe distortion.

Since we need rL to be 10 ohms, and Ri = 110 ohms, we need to make the actual
load resistor equal to: RL = 11 Ohms. Let's check that result and see if it
meets the clipping constraint for maximum available power.

positive swing = Id*rL = 1*10 = 10 V
negative swing = Vd = 10 V
Power delivered to RL: Pload = 10^2/(2*11) = 4.55 W
The efficiency is a little under 50% because of the internal resistance. Note
the Load resistance is decidely not the conjugate of the internal resistance.

Let's spot check the load to see if it at least appears to be the peak available
power, by testing two loads "immediately" on either side of our optimum 11 ohms.

Let RL = 10 ohms
positive swing = Id*rL = 1*9.17 = 9.17 V
negative swing = Vd = 10 V
Since we positive clip at 9.17 V, we are limited by our design clipping
constraint to only driving the PA such that 2*9.17 V is the maximum available
voltage swing.
Power delivered to RL: Pload = 9.17^2/(2*10) = 4.20 W

Let RL = 12 ohms
positive swing = Id*rL = 1*10.82 = 10.82 V
negative swing = Vd = 10 V
Since we negative clip at 10 V, we are limited by our design clipping constraint
to driving the PA such that 2*10 V is the maximum available voltage swing.
Power delivered to RL: Pload = 10^2/(2*12) = 4.17 W

Sure enough, the power peaked at a load of 11 ohms, just like load-line theory
says it will. Now let's see what the available power hit of conjugate matching
is.

By definition, conj-match insists RL = Ri = 110 ohms. Again we are limited in
our clipping constraint by static drain current, and supply voltage,
specifically 10 V.

Our negative swing limit is, as ever, 10 V (the drain voltage).

positive swing = Id*rL = 1*55 = 55 V

This would breakdown the device, but the lower negative swing will force us to
back down the drive to meet the design defined clipping constraint.

Pload = 10^2/(2*110) = 0.455 W

Conjugate matching resulted in a 10*log(0.455/4.55) = 10 dB available power
hit. Power amplifiers are not designed with conjugate matching in mind. You
don't need to re-invent the wheel. Just follow well established principles when
doing cookie cutter PA design.

The list could go on,...

LOL. Given your pattern, I am sure it will.

You sighed with content at being offered a "relevent
question/statement" Your re-iterative response contains the same (how
could it be otherwise?) slack of precision that started this. Want to
try again?

Not really. The problem isn't precision, it is you can't, or refuse, to
comprehend what is being said, which I presume is why you instead write with the
most bizarre terms and phrasology that has nothing of import to the topic at
hand.

You could have as easily expressed what sense they ARE matched,

For what seems like the billionth time now: they are load-line matched.

...but instead this time offer what Basis
of Matching you are attempting to
describe.

I've given a didactic example (actually a couple), you just don't--or more
likely won't--get it. If you don't like my example, you can refer to Cripps,
who is considered one of the preeminant RF PA experts in the world.

Even more simplistic is Malvino's discussion on pp177-185 of the first edition
((c) 1968) of "Transistor Circuit Approximations." It is basically a technician
level description, so perhaps it is well-suited to you. In academics, load-line
theory is presented down to tech level courses and up across to engineering.
That some engineers and techs aren't clear on the load-line concept for PA's (or
*any* circuit needing a wide symmetrical swing) is notwithstanding.

This is the more rigorous approach that eliminates vague
descriptions and uses standard terms. If you have to query about what
"Basis" means (used by professionals - namely metrologists who can
quantify Output Z of all sources) - then we can skip it as a topic out
of the reach of amateur discussion.

I see you still don't know what impedance is. In any case, it doesn't mean that
looking into a properly designed PA output with a network analyzer confirms the
conj-match precept, it doesn't.

Impedance is a *linear* conception, a portion of linear theory, and again by
definition:

Z = V/I

V and I are sinusoids (phasors). But with power amps, substantial non-linearity
exists (destroying the linearity assumption of impedance), thus applying a
linearly defined concept to a non-linear milieu is a misapplication. You are
attempting, as is Ken, to stuff a square peg down a round hole. Why?

The concept is even questionable for the most linear of the power amps: class
A. In any case, given real devices with real supplies, the conj-match ideal is
next to worthless. While I could agree that the borderline may be fuzzy
regarding where and when to drop the impedance notion, it still stands that the
concept is not useful in determining how to optimally load an RF PA.

At this point you own the conj-match assertion as much as Ken. Prove it! You
can't because it is fundamentally incorrect.

Note:
Does not qualify as a Basis.

Load-line matching is such a basic electronic concept it is unbelievable how
oblivious you are to the concept. Read a basic book. Don't rely on me: look it
up and do your own design!

It is suggestive of one, but because you indiscriminately
mix several Basis within your discussions, it is your
responsibility to be precise.

You just like to hear yourself talk. I've been explicit and precise. You just
don't know anything about the elementary electronics principle of load line
matching. I presume this is why your comments have zero substantive
responsiveness.

If you can accomplish this, then we can
proceed to review how little it all matters.

If you keep ignoring what I've written, and that which is written in elementary
electronics texts, you can remain happily ignorant of understanding the
simple-basic-fundamental concept presented. Your choice.

Barring resolving any of these issues of precise language,...]

The guy ignorant of the definition of impedance and that s-domain theory *is*
linear circuit theory (and more goodies) is talking about "precise language."
Amusing.

I notice
that you rather enjoy...

No, I don't enjoy it at all. Your lack of electronic understanding is dismal,
especially given your tone. It would have been a lot easier for me if Ken
hadn't made the erroneous
statement in the first place and made a correct one instead. That would have
been my preferance.

..fruitless jousting with them than challenging my
support of Ken's (supposed) statement that you say is your focus:
We will leave that as another dead-end.

I suspect you will. I already understand it -- you're the one who doesn't.


"One of the principal differences between linear RF amplifier design and PA
design is that, for optimum power, the output of the device is not presented
with the impedance required for a linear conjugate match. That causes much
consternation and has been the subject of extensive controversy about the
meaning and nature of conjugate matching. It is necessary, therefore, to swallow
that apparently unpalatable result as early as possible (Section 1.5), before
going on to give it more extended interpretation and analysis (Chapter 2)." --
Cripps, p1


The quote is on Page 1. Swallow it now. Learn something for a change.

 

[John Woodgate]

By definition, conj-match insists RL = Ri = 110 ohms. Again we are
limited in our clipping constraint by static drain current, and supply
voltage, specifically 10 V.

Our negative swing limit is, as ever, 10 V (the drain voltage).

positive swing = Id*rL = 1*55 = 55 V

This would breakdown the device, but the lower negative swing will force
us to back down the drive to meet the design defined clipping
constraint.

Pload = 10^2/(2*110) = 0.455 W

And the power dissipated in the device is also 0.445 W. Matching
according to the 'maximum power theorem' or conjugate matching, results
in equal power in the PA and load. That's why it isn't useful for power
amplifiers.

Doesn't everyone know that an audio amplifier that id designed to feed
an 8 ohm load MUST have an output source impedance of 0.0000001 ohms or
less. An output source impedance of 8 ohms would dramatically decrease
the electromagnetic damping on the loudspeaker voice-coil - by the huge
factor of .... two!(;-)

 

[Richard Clark]

Not really. ....

I suspect you will.

Hi OM,

224 line postings to produce this little qualitative information?

73's
Richard Clark, KB7QHC

 

[Richard Clark]

Doesn't everyone know that an audio amplifier that id designed to feed
an 8 ohm load MUST have an output source impedance of 0.0000001 ohms or
less.

Hi John,

I hope that was a joke.

73's
Richard Clark, KB7QHC

 

[gwhite]

224 line postings to produce this little qualitative information?

I see one line here with no content. You don't have any argument because you
have zero understanding. No one can cure that but you. But can you?

"Stupid is as stupid does." -- Forrest Gump

 

[gwhite]

And the power dissipated in the device is also 0.445 W.

I think it is 1A*10V - 0.455 W = 9.545 W
^^^^^^ ^^^^^^^
DC input Power
Power delivered
to RL


The resistance dissipated in the "internal AC resistance" is equal to RL in the
conj-match condition. Of course, we're ignoring input power here, which is
"small" when the gain is > +20 dB.

Matching
according to the 'maximum power theorem' or conjugate matching, results
in equal power in the PA and load. That's why it isn't useful for power
amplifiers.

Amusingly for my hypothetical class A conj-match example, the "equal power
dissipation" isn't such a big deal, since it is class A and the fractional power
dissipated in either the internal AC resistance or the external load resistance
is rather small compared to DC dissipation (less than 10%).

Doesn't everyone know that an audio amplifier that id designed to feed
an 8 ohm load MUST have an output source impedance of 0.0000001 ohms or
less. An output source impedance of 8 ohms would dramatically decrease
the electromagnetic damping on the loudspeaker voice-coil - by the huge
factor of .... two!(;-)

Nice one.

 

[Rich Grise]

Doesn't everyone know that an audio amplifier that id designed to feed
an 8 ohm load MUST have an output source impedance of 0.0000001 ohms or
less. An output source impedance of 8 ohms would dramatically decrease
the electromagnetic damping on the loudspeaker voice-coil - by the huge
factor of .... two!(;-)

Yeah - isn't that why the TOOB amps had those taps? So you could get that
rich, full-bodied TOOB sound? ;-)

Cheers!
Rich

 

[Rich Grise]

Hi John,

I hope that was a joke.

Please! You know Mr. Woodgate _hates_ explaining his jokes:

"Doesn't everyone know that an audio amplifier that [is] designed to feed
an 8 ohm load MUST have an output source impedance of 0.0000001 ohms or
less[?] An output source impedance of 8 ohms would dramatically decrease
the electromagnetic damping on the loudspeaker voice-coil - by the huge
factor of .... two! (;-)
[^^^^]

Please notice the last sentence in that paragraph. ;-)

73's

Best regardses? ;-)

Cheers!
Rich

 

[Richard Clark]

I see one line here with no content.

Hi Forrest,

Great! Now try with the other eye.

73's
Richard Clark, KB7QHC

 

[gwhite]

Please! You know Mr. Woodgate _hates_ explaining his jokes:

Mr. Clark _hates_ reading and comprehending. I forsee a clash royal.

 

[Richard Clark]

Please! You know Mr. Woodgate _hates_ explaining his jokes:

Hi Rich,

Some love explaining their jokes. I've gotten quite a bit of
correspondence to that matter already.

73's
Richard Clark, KB7QHC