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On Wed, 24 May 2006 16:51:16 -0500, (Richard
Harrison) wrote:

Not once do I recall our antenna system
causing distortion anywhere except in the edges of pattern nulls.

Hi Richard,

So in this exception, I would make one compelling correlation to your
own observation. That distortion of the pattern was a product of
non-linear behavior in the characteristic Z of the antenna. That
alone seems to be evident. It happens so frequently that we need
complex tools (NEC) to juggle the outcome.

We have non-linear behavior, distortion, and it didn't demand
harmonics, did it? Such non-linearity AND distortion were in all
likelihood trivial, and yet it was noticeable. Now, if no one
complained about pattern nulls being lost, would the non-linearity go
away? ;-)

Cecil would shrug off 59% worth of distortion to define it linear.

73's
Richard Clark, KB7QHC

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Richard Clark wrote:
Cecil would shrug off 59% worth of distortion to define it linear.

Richard, seems you suffer from the same affliction as Howard
Hughes, repeating the same psychotic nonsense over and over.
--
73, Cecil http://www.qsl.net/w5dxp

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"Richard Harrison" wrote in message
...

snip

Antennas are in general distortion free.

Yes, but "bad times" can make them non-linear. Consider the bolted joints
that get
corroded and semi-conductive over time, with rain and temperature changes to
help.

Nearby RF sources can generate distortion products in my antennas if I
haven't
kept up my maintenance.

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wrote in message
oups.com...
... By the way, generating new frequencies is not necessarily a violation
of superposition (though it usually is). Consider a system undergoing
a constant Doppler shift.

73,
Glenn AC7ZN


Glenn,
boy! I can't read all these posts, so I was trying to see where you were
going and asking by just skimming, primarily your posts.

The above caught my eye. Doppler is not "generating new frequencies" as a
non linearity does. A non-linear system will produce harmonics with one
exitation frequency and produce the common mixing / IM with multiple
exitation (superimposed) frequencies. I think trying to mix relativistic
effects in with the stationary world is an unnecessary complication of a
linearity discussion.
If I have time I'll try to follow the thread to see what you're really
after...but. If it takes mega watts to see some non linearity in an
antenna, who cares? and more importantly how will you know whree it is
occuring since things like the junction of two connectorc can produce enough
IM to mask other, smaller sources. If you tried an experiment looking
for IM / Mixing you might try to use a receiver becaue a receiver could be a
very sensitive detector...but you'd have to have a pretty good receiver.
Something like a kW LO and mixer to have a really good intercept.
I'm not sure of the point here... Do antennas cause IM?
Sounds like a deadend arena to me.

73, Steve, K9DCI

FIGHT! FIGHT! FIGHT!
 

Thanks, Steve,

Good post. You do need to read a little mo by mentioning Doppler
shifts, I was illustrating that a linear system can generate new
frequencies without violating the law of superposition. No
relativistic effects needed he a two-tone train whistle will not
distort in the superposition sense under Doppler shift, but it can
generate new (or different) frequencies. My point was that generation
of new frequencies is not necessarily a valid test for nonlinearity.


My position on antennas is the same as yours. Earlier posts were
confusing as the term 'linearity' was being applied to antennas in two
ways. One in the electrical superposition sense (by me) and the other
in current distribution along the antenna element. Electrically,
antennas are very linear (I believe) even when their current
distribution along the element length is not.

I stopped posting when I realized we were discussing two different
things.

73,
Glenn AC7ZN



Steve N. wrote:
wrote in message
oups.com...
... By the way, generating new frequencies is not necessarily a violation
of superposition (though it usually is). Consider a system undergoing
a constant Doppler shift.

73,
Glenn AC7ZN


Glenn,
boy! I can't read all these posts, so I was trying to see where you were
going and asking by just skimming, primarily your posts.

The above caught my eye. Doppler is not "generating new frequencies" as a
non linearity does. A non-linear system will produce harmonics with one
exitation frequency and produce the common mixing / IM with multiple
exitation (superimposed) frequencies. I think trying to mix relativistic
effects in with the stationary world is an unnecessary complication of a
linearity discussion.
If I have time I'll try to follow the thread to see what you're really
after...but. If it takes mega watts to see some non linearity in an
antenna, who cares? and more importantly how will you know whree it is
occuring since things like the junction of two connectorc can produce enough
IM to mask other, smaller sources. If you tried an experiment looking
for IM / Mixing you might try to use a receiver becaue a receiver could be a
very sensitive detector...but you'd have to have a pretty good receiver.
Something like a kW LO and mixer to have a really good intercept.
I'm not sure of the point here... Do antennas cause IM?
Sounds like a deadend arena to me.

73, Steve, K9DCI

FIGHT! FIGHT! FIGHT!
 

wrote
Electrically,
antennas are very linear (I believe) even when their current
distribution along the element length is not.

I stopped posting when I realized we were discussing two different
things.

In a linear system, there only needs to be a straight line function
between the input and output. The actual signals on the input wouldn't
be very useful if only straight line functions were allowed on that
input. The current distribution along an antenna element length only
ever approximates a straight line. The only requirement for that
current to obey the rules for a linear system is that it be a
linear function of the source current and it is at every point.
--
73, Cecil http://www.qsl.net/w5dxp

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Cecil Moore wrote:

In a linear system, there only needs to be a straight line function
between the input and output. The actual signals on the input wouldn't
be very useful if only straight line functions were allowed on that
input. The current distribution along an antenna element length only
ever approximates a straight line. The only requirement for that
current to obey the rules for a linear system is that it be a
linear function of the source current and it is at every point.

I mostly agree with your definition of linearity. Roy's point that an
offset in the straight line violates superposition is an example of a
straight line violating superposition. Also consider the throwing of
two dice. If the dice act independently they can be considered a
linear system with two outputs (the numbers that show on the dice)
which obeys the law of superposition. If, however, the dice collide
when thrown, they now influence each other, the system becomes
nonlinear, and the law of superposition is violated. It's pretty hard
for me to attach a straight line function to dice.

Mixers are especially interesting beasts when viewed in the light of
superposition. They are obviously highly nonlinear, yet we regularly
speak of mixer linearity. Here is the trick: when the local oscillator
is included in the input signal set, the mixer is highly nonlinear as
the LO influences every signal that comes in drastically, in the sense
of generating new frequencies.

But it is convenient to think of the local oscillator as just an
internal parameter of the mixer, and to not include it in the input
signal set. Under this assumption all the RF input signals
substantially obey the law of superposition (that is, they do not
influence each other) and discussing mixer linearity has enough meaning
that we can characterize it through standard tests. Note also that
under this assumption the output frequencies do not match the input
frequencies in general, yet the law of superposition holds over a wide
dynamic range.

Cecil's point that doubling antenna current will double the current at
each point in the antenna (I'm rephrasing) has a dual in mixers:
doubling any signal will double all the mixer's ouputs due to that
signal alone, and not any other outputs.


73,
Glenn Dixon AC7ZN

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wrote:
Cecil's point that doubling antenna current will double the current at
each point in the antenna (I'm rephrasing) has a dual in mixers:
doubling any signal will double all the mixer's ouputs due to that
signal alone, and not any other outputs.

Yep, as long as system linearity is maintained.
--
73, Cecil http://www.qsl.net/w5dxp

FIGHT! FIGHT! FIGHT!
 

wrote in message
oups.com...
Thanks, Steve,

Good post.

Thanks.

You do need to read a little mo by mentioning Doppler
shifts, I was illustrating that a linear system can generate new
frequencies without violating the law of superposition.

Hi Glenn,
I got that. Perhaps my use of "relativitistic effects" was inappropriate.
I still maintain that Doppler shift is not an example of (at least what I
would call) "creating new frequencies" because an electronis system did not
cause said shift. A system is linear or non-linear, but a system can't
"make Doppler happen". It is a frequency change, yes, but an "electronic
system" can't cause it. It occurs for reasons other than "system
characteristics". Yes, a space craft can be considered part of a system in
teh general sense, but not as I believe an "electronic system" should be
thought of when discussing such things. Perhaps symmantics in your view,
but not mine. I think it is a fundamental, but, perhaps can't explain
adequately why.
What a thread. usually when a thread gets this long it digresses far
outside the original intent.


...Earlier posts were
confusing as the term 'linearity' ...electrical superposition sense
... and ... current distribution along the antenna element.

So you *were* questioning the linearity of antennas as we understand the
term?

73, Steve


73,
Glenn AC7ZN



Steve N. wrote:
wrote in message
oups.com...
... By the way, generating new frequencies is not necessarily a
violation
of superposition (though it usually is). Consider a system undergoing
a constant Doppler shift.

73,
Glenn AC7ZN


Glenn,
boy! I can't read all these posts, so I was trying to see where you
were
going and asking by just skimming, primarily your posts.

The above caught my eye. Doppler is not "generating new frequencies" as
a
non linearity does. A non-linear system will produce harmonics with one
exitation frequency and produce the common mixing / IM with multiple
exitation (superimposed) frequencies. I think trying to mix
relativistic
effects in with the stationary world is an unnecessary complication of a
linearity discussion.
If I have time I'll try to follow the thread to see what you're really
after...but. If it takes mega watts to see some non linearity in an
antenna, who cares? and more importantly how will you know whree it is
occuring since things like the junction of two connectorc can produce
enough
IM to mask other, smaller sources. If you tried an experiment
looking
for IM / Mixing you might try to use a receiver becaue a receiver could
be a
very sensitive detector...but you'd have to have a pretty good receiver.
Something like a kW LO and mixer to have a really good intercept.
I'm not sure of the point here... Do antennas cause IM?
Sounds like a deadend arena to me.

73, Steve, K9DCI

FIGHT! FIGHT! FIGHT!
 

I haven't been following this thread lately, but happened on this
particular posting...

Seems to me you want to say "Linear TIME INVARIANT system" to get to
not generating new frequencies. Certainly I can build a system that is
linear but not time-invariant and generate new frequencies with that
system. A simple one is a signal going into a potentiometer, coming
out the wiper, in which the wiper is rotated continuously. It's
linear, but not time invariant, and obviously any input will be
amplitued modulated at the rate of the time variation.

Is a double-balanced mixer with LO a linear system (for input signals
in the intended amplitued range)? Increasing the input amplitude by
1dB causes the output amplitude to increase by 1dB, though the output
is not at the same frequency as the input. If the response of the
DBM/LO system to input x1 is y1, and to x2 is y2, then is (y1+y2) the
response to input (x1+x2)? Is a DBM/LO system time-invariant: if I
apply stimulus x1 at time t1 do I get the same response as if I apply
it at time t2 (where the response is also shifted by t2-t1)?

Perhaps this will be useful food for thought...

Cheers,
Tom

Steve N. wrote:
wrote in message
oups.com...
Thanks, Steve,

Good post.

Thanks.

You do need to read a little mo by mentioning Doppler
shifts, I was illustrating that a linear system can generate new
frequencies without violating the law of superposition.

Hi Glenn,
I got that. Perhaps my use of "relativitistic effects" was inappropriate.
I still maintain that Doppler shift is not an example of (at least what I
would call) "creating new frequencies" because an electronis system did not
cause said shift. A system is linear or non-linear, but a system can't
"make Doppler happen". It is a frequency change, yes, but an "electronic
system" can't cause it. It occurs for reasons other than "system
characteristics". Yes, a space craft can be considered part of a system in
teh general sense, but not as I believe an "electronic system" should be
thought of when discussing such things. Perhaps symmantics in your view,
but not mine. I think it is a fundamental, but, perhaps can't explain
adequately why.
What a thread. usually when a thread gets this long it digresses far
outside the original intent.


...Earlier posts were
confusing as the term 'linearity' ...electrical superposition sense
... and ... current distribution along the antenna element.

So you *were* questioning the linearity of antennas as we understand the
term?

73, Steve


73,
Glenn AC7ZN



Steve N. wrote:
wrote in message
oups.com...
... By the way, generating new frequencies is not necessarily a
violation
of superposition (though it usually is). Consider a system undergoing
a constant Doppler shift.

73,
Glenn AC7ZN


Glenn,
boy! I can't read all these posts, so I was trying to see where you
were
going and asking by just skimming, primarily your posts.

The above caught my eye. Doppler is not "generating new frequencies" as
a
non linearity does. A non-linear system will produce harmonics with one
exitation frequency and produce the common mixing / IM with multiple
exitation (superimposed) frequencies. I think trying to mix
relativistic
effects in with the stationary world is an unnecessary complication of a
linearity discussion.
If I have time I'll try to follow the thread to see what you're really
after...but. If it takes mega watts to see some non linearity in an
antenna, who cares? and more importantly how will you know whree it is
occuring since things like the junction of two connectorc can produce
enough
IM to mask other, smaller sources. If you tried an experiment
looking
for IM / Mixing you might try to use a receiver becaue a receiver could
be a
very sensitive detector...but you'd have to have a pretty good receiver.
Something like a kW LO and mixer to have a really good intercept.
I'm not sure of the point here... Do antennas cause IM?
Sounds like a deadend arena to me.

73, Steve, K9DCI