1/4 vs 1/2 wavelength antenna

[John Woodgate]

I read in sci.electronics.design that Tom Ring <news0001@taring.org>
wrote (in <4227cdce$0$92311$39cecf19@news.twtelecom.net&gt about '1/4 vs
1/2 wavelength antenna', on Thu, 3 Mar 2005:

He forgot to mention that for that output impedance to be relevant, you
need superconducting wire to the speakers as well as superconducting
voice coils.

See the last sentence, about the effect of an **8 ohm** source impedance
on damping.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[gwhite]

Ken Smith wrote:

The strongest argument for dropping the impedance matching concept is PA
efficiency, and therefore maximum signal swing. Obtaining maximum swing is a
load line issue.

What do you mean by "maximum signal swing" in this context. I can get a
bigger swing by leaving the output completely unloaded and hence causing
the actual efficiency to be zero.

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

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.

The first assumption/idealization for the class A example would be to demarcate
between strong and weak non-linearity. This demarcation is basically the
boundary of clipping, both positive and negative. That is, we want our class A
amp to swing to the rails but not go beyond. In our class A design we will
accept weak non-linearity but not strong non-linearity.

Lets say we have selected a class A device/amplifier for which we can statically
dissipate 10W. The drain DC circuit is simply an RF choke (see, it is a simple
example!). Let's say the quiescent values are a 10 V supply and 1 A of current
(10 W). The question is: how do we load the device to extract maximum power
given the "no clipping" (strong non-linearity) constraint?

Say we load it with 20 ohms, what happens? The max positive swing before
clipping is Id*rL = 1*20 = 20 V. The max negative swing is, of course, Vd = 10
V. Since the 10 V is the lesser of the two swings, our non-clipping design
constraint limits us to 20 Vp-p. So we can deliver (Vp)^2/(2*rL) =
(10)^2/(2*20) = 2.5 W.

Say we load it with 5 ohms, what happens? The max positive swing before
clipping is Id*rL = 1*5 = 5 V. The max negative swing is, of course, Vd = 10
V. Since the 5 V is the lesser of the two swings, our non-clipping design
constraint limits us to 10 Vp-p. So we can deliver (Vp)^2/(2*rL) = (5)^2/(2*5)
= 2.5 W.

Say we load it with 10 ohms, what happens? The max positive swing before
clipping is Id*rL = 1*10 = 10 V. The max negative swing is, of course, Vd = 10
V. Since they are equal, we get 20 V of p-p swing. So we can deliver
(Vp)^2/(2*rL) = (10)^2/(2*10) = 5 W.

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.

In no way was the ouput-Z of the amplifier considered in deciding how to load it
for the purpose of extracting maximum power from the circuit. The output-R is
completely irrelevent.

This example is intended to be illustrative rather than exact.

The reactive component issue is still there too. Reactive loads cause
increased currents in the output stage without delivering any power to the
load so they still need to be reduced as much as practical.

Yes, I already noted that for that portion of the impedance, it should be tuned
out *as best* possible.

"...(to be fair, the time-averaged reactive output component is tuned out as
best possible)."

 

[Thomas Magma]

Response is flattened through gain controlling the pre-amp from a look-up
table held in the micro's EEPROM. The alignment procedure is automated using
the HB-IP bus from the spectrum analyzer and a computer. The
computer/analyzer also looks for harmonic content and spurious emissions
during this procedure. Think it takes about ten seconds to do this.

"Richard Clark" <kb7qhc@comcast.net> wrote in message
news:luuf219fa7kg4up47ug631oc8fn6movce9@4ax.com...

On Fri, 04 Mar 2005 01:13:39 GMT, "Thomas Magma"
somewhere@overtherainbow.com> wrote:

Wow this is a long thread. Don't really know where I should put my two
bits
in, but here it goes.

I have designed several RF PA sections in the past. 500MHz at about 50W.
Pretty easy stuff if you have the right tools and know how to use them.
The
tools I like using for matching the power output FET is two triple stub
tuners. One on the input of the FET and one on the output. So it
goes...pre-amp (50 ohm output) -> stub tuner -> FET -> stub tuner -> 50
ohm
dummy pad -> spectrum analyzer. Then just tune the stubs for the
performance
you desire, these include: efficiency (thermal issues), harmonic content,
spurious emissions, load VSWR considerations, cold start, ect. Then
remove
the FET and look into the triple stub tuners with the network analyzer.
Model and duplicate the network out of discrete components that can
handle
the voltage/power, send the design off to the enviro test lab, and head
home
early for the day.

Cheers,
Thomas

Hi Thomas,

Thanx, your two bits were worth more than the academic plug nickel.
This is something that our original poster should hearken to as his
needs were obviously production oriented. Bench experience will trump
cut-and-paste theory in a heart-beat.

However, triple stub is pretty aggressive. How long did it take you
to flatten response?

73's
Richard Clark, KB7QHC

 

[Richard Clark]

On Fri, 04 Mar 2005 16:47:05 GMT, "Thomas Magma"
<somewhere@overtherainbow.com> wrote:

Response is flattened through gain controlling the pre-amp from a look-up
table held in the micro's EEPROM. The alignment procedure is automated using
the HB-IP bus from the spectrum analyzer and a computer. The
computer/analyzer also looks for harmonic content and spurious emissions
during this procedure. Think it takes about ten seconds to do this.

Hi Thomas,

10 seconds to adjust all 6 stubs?

73's
Richard Clark, KB7QHC

 

[Jim Kelley]

Thomas Magma wrote:

No the triple stub tuners are only for development. Production boards have
discrete components to form the match network. Power levelling or
"flattening the response" is computer adjusting the output power to
compensate for the reactive components to ensure a constant output power
over the entire band of the radio. We also put in a small temperature
compensation coefficient into the EEPROM because the PA tends to put out
more power when it is cold.

Richard was asking how long it took you to tune the triple stub filters
during devolpment.

I am curious about the exact nature of the impedance transmformation
these devices provided.

jk

"Richard Clark" <kb7qhc@comcast.net> wrote in message
news:f45h21p426bhp6gnqog44i2svdenb6qlhd@4ax.com...

On Fri, 04 Mar 2005 16:47:05 GMT, "Thomas Magma"
somewhere@overtherainbow.com> wrote:


Response is flattened through gain controlling the pre-amp from a look-up
table held in the micro's EEPROM. The alignment procedure is automated

using

the HB-IP bus from the spectrum analyzer and a computer. The
computer/analyzer also looks for harmonic content and spurious emissions
during this procedure. Think it takes about ten seconds to do this.


Hi Thomas,

10 seconds to adjust all 6 stubs?

73's
Richard Clark, KB7QHC

 

[Richard Clark]

On Fri, 04 Mar 2005 17:39:04 GMT, "Thomas Magma"
<somewhere@overtherainbow.com> wrote:

No the triple stub tuners are only for development.

Hi Thomas,

I thought 10 seconds was awful quick. How long would it take to
flatten the response when manually adjusting the triple stub tuners?

What merit did you find with triple that could not be found with
double stub tuners?

73's
Richard Clark, KB7QHC

 

[Thomas Magma]

Hi gwhite,

I would have to agree with you on most everything you have said through this
thread. I once saw my boss (with his "PHD") try to model and match a power
amp based on the small signal parameters off the datasheet. He insisted that
the stated input and output impedances were characteristic parasitics of
that amp and wouldn't change between a small or large signal. It was kind of
pathetic to watch him struggle for over a month on the matching network, and
I think he had resorted to guessing in the end.

I've often questioned why manufactures put small signal parameters on their
datasheets? Makes no sense to me. Even if they do publish some large signal
parameters it is unlikely to be the exact same mode of operation that you
need for your project.

Playing with triple stub tuners on PA's has shown me that there are many
combinations of input and output impedances that appear to give similar
results at any one frequency, but give different results at others
frequencies. So it's a matter of finding the input and output impedance that
give you adequate performance over the entire scope of your project.

Thomas

"gwhite" <gwhite@deadend.com> wrote in message
news:42289188.7FE6D8DB@deadend.com...

Richard Clark wrote:

On Fri, 04 Mar 2005 01:13:39 GMT, "Thomas Magma"
somewhere@overtherainbow.com> wrote:

Wow this is a long thread. Don't really know where I should put my two
bits
in, but here it goes.

I have designed several RF PA sections in the past. 500MHz at about
50W.
Pretty easy stuff if you have the right tools and know how to use them.
The
tools I like using for matching the power output FET is two triple stub
tuners. One on the input of the FET and one on the output. So it
goes...pre-amp (50 ohm output) -> stub tuner -> FET -> stub tuner -> 50
ohm
dummy pad -> spectrum analyzer. Then just tune the stubs for the
performance
you desire, these include: efficiency (thermal issues), harmonic
content,
spurious emissions, load VSWR considerations, cold start, ect. Then
remove
the FET and look into the triple stub tuners with the network analyzer.
Model and duplicate the network out of discrete components that can
handle
the voltage/power, send the design off to the enviro test lab, and head
home
early for the day.

Cheers,
Thomas

Hi Thomas,

Thanx, your two bits were worth more than the academic plug nickel.
This is something that our original poster should hearken to as his
needs were obviously production oriented.

I doubt you understand what he wrote. I can't fathom why you would be
concerned
with the OP when your own difficulties are so acute.

Bench experience will trump
cut-and-paste theory in a heart-beat.

How would you know?

However, triple stub is pretty aggressive. How long did it take you
to flatten response?

How long will it take you to figure out that he wrote not a wisp of a word
on
what the "output-Z" of the amplifier is? He did write that he determines
how
the amp was loaded to acheive power, something I've been saying is the
prime
concern.

 

[Thomas Magma]

If your amp has to operate over a wide frequency range it is not likely that
you can flatten the response just with stubs. Stubs should be looked at as
more single frequency devices than broadband networks. But you can use the
stubs to plot out the appropriate impedance curve on the Smith Chart to
ensure a flat response when you model in the discretes.

I usually just try to get the flatness of the response as close as possible
and rely on a software calibration routine to flatten it off. Saves a lot of
time.

It's my understanding that a triple stub tuner of the right length can reach
anywhere on the Smith Chart where as a double stub can not.

"Richard Clark" <kb7qhc@comcast.net> wrote in message
news:a39h215boqo45mq8k7pvbas78nhd1eiv28@4ax.com...

On Fri, 04 Mar 2005 17:39:04 GMT, "Thomas Magma"
somewhere@overtherainbow.com> wrote:

No the triple stub tuners are only for development.

Hi Thomas,

I thought 10 seconds was awful quick. How long would it take to
flatten the response when manually adjusting the triple stub tuners?

What merit did you find with triple that could not be found with
double stub tuners?

73's
Richard Clark, KB7QHC

 

[Richard Clark]

On Fri, 04 Mar 2005 19:09:20 GMT, "Thomas Magma"
<somewhere@overtherainbow.com> wrote:

If your amp has to operate over a wide frequency range it is not likely that
you can flatten the response just with stubs.

Hi Thomas,

Certainly not as conventional Triple Stubs. However, care to provide
some of the cogent details of that particular project? Any
interesting insights?

73's
Richard Clark, KB7QHC

 

[gwhite]

Thomas Magma wrote:

I've often questioned why manufactures put small signal parameters on their
datasheets? Makes no sense to me.

They might be of some use for specific cases. For example, if the PA is class
A, is used well backed off because of high PEP-to-avg ratios of the signal, and
you've managed to get the output load dialed in, s-params can be useful for a
first cut at the amplifier *input* match. I've always still had to do some
tweeking though. Also, with some work and considering the load-line match, they
can give you an idea of what gain can be accomplished. This might already be in
the data sheet though, as you mention.

Even if they do publish some large signal
parameters it is unlikely to be the exact same mode of operation that you
need for your project.

One of the large signal parameters I like best is how much power the device can
dissipate. ;-) Voltage breakdowns and Imax are nice too. ;-) ;-)

 

[Tom Ring]

John Woodgate wrote:

I read in sci.electronics.design that Tom Ring <news0001@taring.org
wrote (in <4227cdce$0$92311$39cecf19@news.twtelecom.net&gt about '1/4 vs
1/2 wavelength antenna', on Thu, 3 Mar 2005:


He forgot to mention that for that output impedance to be relevant, you
need superconducting wire to the speakers as well as superconducting
voice coils.


See the last sentence, about the effect of an **8 ohm** source impedance
on damping.

Get a sense of humor. Or maybe more ice and mixer.

tom
K0TAR

 

[Tom Ring]

John Woodgate wrote:

I read in sci.electronics.design that Tom Ring <news0001@taring.org
wrote (in <4227cdce$0$92311$39cecf19@news.twtelecom.net&gt about '1/4 vs
1/2 wavelength antenna', on Thu, 3 Mar 2005:


He forgot to mention that for that output impedance to be relevant, you
need superconducting wire to the speakers as well as superconducting
voice coils.


See the last sentence, about the effect of an **8 ohm** source impedance
on damping.

Oh, and going from 8 ohms output impedance to 10e-7 (unless I
miscounted) would take the damping factor from 1 to 8e7, which is a bit
more than 2. Ignoring the speaker wires of course.

tom
K0TAR

 

[John Woodgate]

I read in sci.electronics.design that Tom Ring <news0001@taring.org>
wrote (in <42290052$0$92310$39cecf19@news.twtelecom.net&gt about '1/4 vs
1/2 wavelength antenna', on Fri, 4 Mar 2005:

John Woodgate wrote:

I read in sci.electronics.design that Tom Ring <news0001@taring.org
wrote (in <4227cdce$0$92311$39cecf19@news.twtelecom.net&gt about '1/4 vs
1/2 wavelength antenna', on Thu, 3 Mar 2005:


He forgot to mention that for that output impedance to be relevant, you
need superconducting wire to the speakers as well as superconducting
voice coils.


See the last sentence, about the effect of an **8 ohm** source impedance
on damping.

Oh, and going from 8 ohms output impedance to 10e-7 (unless I
miscounted) would take the damping factor from 1 to 8e7, which is a bit
more than 2. Ignoring the speaker wires of course.

Also ignoring the ***voice-coil resistance***. If that is included, as

it must be for a correct analysis, you get 2.

F Langford-Smith 'invented' the concept of damping factor, and around
1949 accepted the point made by James Moir that, by not properly taking
into account the effect of the voice-coil resistance, it was a seriously
misleading concept. Yes, 60 years later, people are still being misled.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Tom Ring]

John Woodgate wrote:

I read in sci.electronics.design that Tom Ring <news0001@taring.org
wrote (in <42290052$0$92310$39cecf19@news.twtelecom.net&gt about '1/4 vs
1/2 wavelength antenna', on Fri, 4 Mar 2005:

Also ignoring the ***voice-coil resistance***. If that is included, as
it must be for a correct analysis, you get 2.

F Langford-Smith 'invented' the concept of damping factor, and around
1949 accepted the point made by James Moir that, by not properly taking
into account the effect of the voice-coil resistance, it was a seriously
misleading concept. Yes, 60 years later, people are still being misled.

Good point. I stand corrected.

The only nit I would pick would be that impedance be used, since you
need to measure it at the frequency(ies) in question, not DC. And then
there is that pesky crossover in most systems. Personally I like biamping.

tom
K0TAR

 

[Leon Heller]

"gwhite" <gwhite@deadend.com> wrote in message
news:421E98CC.6E3BDC78@deadend.com...

Richard Clark wrote:

On Wed, 23 Feb 2005 19:08:20 GMT, gwhite <gwhite@deadend.com> wrote:

RF transmitters are not ....

Sorry OM,

This was all nonsense.

Nice articulation. I don't know who OM is, but RF transmitter power amps
are
not "impedance matched." Neither are audio power amps for that matter.

'OM' is radio ham speak for 'Old man'.

Leon
--
Leon Heller, G1HSM
http://www.geocities.com/leon_heller

 

[John Woodgate]

I read in sci.electronics.design that Tom Ring <news0001@taring.org>
wrote (in <4229bd3a$0$92313$39cecf19@news.twtelecom.net&gt about '1/4 vs
1/2 wavelength antenna', on Sat, 5 Mar 2005:

Personally I like
biamping.

So do I.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk