Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #21   Report Post  
Old June 13th 10, 07:20 PM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Jul 2006
Posts: 2,951
Default "Non-dissipative Source Resistance"

On Sun, 13 Jun 2010 11:04:50 -0700 (PDT), Richard Fry
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.


So your subscription to your own authority is contingent upon me?

I'm flattered.

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.


"If the amplifier is tuned exactly to resonance, the plate load
impedance will be purely resistive and the load line will be linear."

I would note that the bulk of his paper concerns frequency products -
something the bears quite intimately upon linear operation.

Your question is rather opaque in that the appearance of subtext is
undoubtedly bound up in the term "linear" which Mendenhall has already
employed within the norms of amplifier design.

How linear is linear? Give me a technical criteria (subjectivity
abounds and is not very informative except for statements of fashion).
Express the least departure from a line response in percent that you
accept as being linear. If it exceeds Mendenhall's design or
commercial product, then I am afraid you won't be satisfied with any
answer.

73's
Richard Clark, KB7QHC
  #22   Report Post  
Old June 13th 10, 08:16 PM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Jun 2006
Posts: 440
Default "Non-dissipative Source Resistance"

On Jun 13, 1:20*pm, Richard Clark wrote:

How linear is linear? *Give me a technical criteria...


A truly linear device does not produce r-f intermodulation products
when signals of different frequencies are applied to it at the same
time.

For many practical uses a linear device does need to be perfectly
linear. Let's say that the IM products need to be only - 20 dB w.r.t
the lowest amplitude value of the various input frequencies. The
Mendenhall paper I quoted conclusively shows that even such nominally
linear performance is not an attribute of Class C vacuum tube r-f
amplifiers -- especially at, and near the center frequency of the PA
output tuning/matching network.

That Mendenhall paper also shows that such r-f amplifiers do not
present a functional, 50+j0 termination to r-f energy entering via the
tx output connector. That is the reason why such energy is present in
the PA plate circuit, giving rise to r-f intermodulation and/or
changing the power dissipation there.

RF
  #23   Report Post  
Old June 13th 10, 08:31 PM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Jul 2006
Posts: 2,951
Default "Non-dissipative Source Resistance"

On Sun, 13 Jun 2010 12:16:40 -0700 (PDT), Richard Fry
wrote:

On Jun 13, 1:20*pm, Richard Clark wrote:

How linear is linear? *Give me a technical criteria...


Let's say that the IM products need to be only - 20 dB w.r.t
the lowest amplitude value of the various input frequencies.


Medenhall's paper shows "RF spectrum showing distortion products" that
are 30dB down.

His statement: "If the amplifier is tuned exactly to resonance, the
plate load impedance will be purely resistive and the load line will
be linear." is attached to that linearity which is greater than you
expect.

73's
Richard Clark, KB7QHC
  #24   Report Post  
Old June 13th 10, 08:58 PM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Dec 2006
Posts: 102
Default "Non-dissipative Source Resistance"

On Jun 13, 3:31*pm, Richard Clark wrote:
On Sun, 13 Jun 2010 12:16:40 -0700 (PDT), Richard Fry
wrote:

On Jun 13, 1:20*pm, Richard Clark wrote:


How linear is linear? *Give me a technical criteria...


Let's say that the IM products need to be only - 20 dB w.r.t
the lowest amplitude value of the various input frequencies. *


Medenhall's paper shows "RF spectrum showing distortion products" that
are 30dB down.

His statement: "If the amplifier is tuned exactly to resonance, the
plate load impedance will be purely resistive and the load line will
be linear." is attached to that linearity which is greater than you
expect.

73's
Richard Clark, KB7QHC


To Richard Fry,

Richard, it appears that you have ignored my post where I reported
data that proves the output resistance of Class B and C RF amps is non-
dissipative. Have you reviewed or ignored my reference to Chapter 19
in Reflections 3, or the combined sections of that chapter that appear
in my web page at www.w2du.com that uses an example from Terman's
Radio Engineers Handbook? If you haven't yet reviewed it I urge you to
do it, and then if you disagree with my measured data I'd like for you
to explain the reason for your disagreement.

Walt, W2DU
  #25   Report Post  
Old June 13th 10, 09:36 PM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Jun 2006
Posts: 440
Default "Non-dissipative Source Resistance"

On Jun 13, 2:31*pm, Richard Clark wrote:


Medenhall's paper shows "RF spectrum showing distortion products"
that are 30dB down.


Not near the resonant frequency of the output tuning/matching network.

He shows as little as 5 dB at 0.8 MHz offset of the external signal
from the carrier of that tx. It would be less than 5 dB for lesser
separations.

The reason that the IM performance improves for greater frequency
separations is related to the r-f bandwidth of the output network --
not the "linear" operating characteristics of the PA, or that a 50+j0
ohm load is seen by the external energy reaching the tx output
connector.

RF


  #26   Report Post  
Old June 13th 10, 10:04 PM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Jun 2006
Posts: 440
Default "Non-dissipative Source Resistance"

On Jun 13, 2:58*pm, walt wrote:

To Richard Fry,

Richard, it appears that you have ignored my post where I reported
data that proves the output resistance of Class B and C RF amps
is non-dissipative.


I know your conclusion, thanks Walt.

The impedance of a perfect transmission line also is non-dissipative,
but that does not change the power dissipated in a termination of that
transmission line equaling its Zo.

Do you agree that the Mendenhall paper I have quoted shows that the
source impedance of an operating Class C vacuum tube PA and its output
tuning/matching network designed for a 50 ohm load is not, by itself,
a functional 50+j0 ohms?

RF
  #27   Report Post  
Old June 13th 10, 10:33 PM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Dec 2006
Posts: 102
Default "Non-dissipative Source Resistance"

On Jun 13, 5:04*pm, Richard Fry wrote:
On Jun 13, 2:58*pm, walt wrote:



To Richard Fry,


Richard, it appears that you have ignored my post where I reported
data that proves the output resistance of Class B and C RF amps
is non-dissipative.


I know your conclusion, thanks Walt.

The impedance of a perfect transmission line also is non-dissipative,
but that does not change the power dissipated in a termination of that
transmission line equaling its Zo.

Do you agree that the Mendenhall paper I have quoted shows that the
source impedance of an operating Class C vacuum tube PA and its output
tuning/matching network designed for a 50 ohm load is not, by itself,
a functional 50+j0 ohms?

RF


No Richard, I've not made that inference. The point I'm emphasizing
is that because the output resistance of these amps is non-
dissipative, no reflected power from a mismatched load enters the amp,
but is totally re-reflected in the forward direction. I make no
comment on whether the output resistance is 50 +j0, because when all
the available power is being delivered at given grid drive the output
resistance equals the load resistance, which doesn't have to be 50 +
j0. Proof of that is that when the load resistance is either increased
or decreased the output power decreases.

Walt, W2DU
  #28   Report Post  
Old June 13th 10, 11:09 PM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Jun 2006
Posts: 440
Default "Non-dissipative Source Resistance"

On Jun 13, 4:33*pm, walt wrote:
The point I'm emphasizing
is that because the output resistance of these amps is non-
dissipative, no reflected power from a mismatched load enters the amp,
but is totally re-reflected in the forward direction.


Walt, wouldn't that non-dissipative output resistance also reflect
reverse power on other close frequencies, such as those coupled from a
co-located transmission system within the bandwidth of the tx output
network?

If so, such external signals would never enter the transmitter, and
could not cause r-f intermodulation by mixing with the main signal in
the (non-linear) PA plate circuit of that transmitter to produce a 3rd
order product at 2F1 - F2, and other combinations.

But such r-f intermods are a fact of life, as shown in the Mendenhall
paper, and in the existence of elaborate and expensive r-f filtering
hardware needed to enable several stations to operate from one
location while meeting a radiated r-f intermod spec of -80 dBc -- even
from one antenna!

RF
  #29   Report Post  
Old June 13th 10, 11:25 PM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Dec 2006
Posts: 102
Default "Non-dissipative Source Resistance"

On Jun 13, 6:09*pm, Richard Fry wrote:
On Jun 13, 4:33*pm, walt wrote:

The point I'm emphasizing
is that because the output resistance of these amps is non-
dissipative, no reflected power from a mismatched load enters the amp,
but is totally re-reflected in the forward direction.


Walt, wouldn't that non-dissipative output resistance also reflect
reverse power on other close frequencies, such as those coupled from a
co-located transmission system within the bandwidth of the tx output
network?

If so, such external signals would never enter the transmitter, and
could not cause r-f intermodulation by mixing with the main signal in
the (non-linear) PA plate circuit of that transmitter to produce a 3rd
order product *at *2F1 *- F2, and other combinations.

But such r-f intermods are a fact of life, as shown in the Mendenhall
paper, and in the existence of elaborate and expensive r-f filtering
hardware needed to enable several stations to operate from one
location while meeting a radiated r-f intermod spec of -80 dBc -- even
from one antenna!

RF


Richard, I'm not qualified to answer your question about intermods, so
I won't even try. However, interference to the amp from radiation from
an antenna driven by another amp will not be phase or frequency
coherent with the tx receiving the interference. Perhaps that's the
reason the interfering signal gets through while the waves reflected
from a mismatch don't? In other words, could the coherent relationship
between the forward and reflected waves result in the total re-
reflection at the non-dissipative output resistance of waves returning
from the mismatched load?

Walt
  #30   Report Post  
Old June 13th 10, 11:42 PM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Jun 2006
Posts: 440
Default "Non-dissipative Source Resistance"

On Jun 13, 5:25*pm, walt wrote:
However, interference to the amp from radiation from
an antenna driven by another amp will not be phase or frequency
coherent with the tx receiving the interference. Perhaps that's the
reason the interfering signal gets through while the waves reflected
from a mismatch don't?


Measurements made of broadcast transmitters in the engineering test
lab at Harris Broadcast Division show that power reflected from a
mismatched load does indeed enter the PA plate output circuitry, and
change the amount of power dissipation and cooling requirement for the
transmitter.

That is the reason why virtually every modern manufacturer of
broadcast transmitters specifies a maximum value of load VSWR for
which their maximum rated powers are permissible.

For Harris FM transmitters that value is 1.7:1 (any phase angle). As
the load VSWR exceeds that value the transmitter progressively "folds
back" its output power to protect itself from component damage.

RF
Reply
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
"meltdown in progress"..."is amy fireproof"...The Actions Of A "Man" With Three College Degrees? K4YZ Policy 6 August 28th 06 11:11 PM
any source for a plug and play phase arry "turning" system for 6m an old freind General 12 June 9th 06 01:04 PM
any source for a plug and play phase arry "turning" system for 6m Not Cocksucker Lloyd Policy 0 June 9th 06 01:04 PM
any source for a plug and play phase arry "turning" system for 6m an old friend CB 0 June 9th 06 02:03 AM
source for plug and play phased array "turner" for 6m [email protected] Antenna 3 June 5th 06 06:12 AM


All times are GMT +1. The time now is 02:15 PM.

Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 RadioBanter.
The comments are property of their posters.
 

About Us

"It's about Radio"

 

Copyright © 2017