On Sun, 6 Feb 2005 08:03:39 -0600, "Richard Fry"
wrote:

It can transmit a constant (DC) video value of any amplitude

Hi OM,

This is absurd. Video is a modulation being carried on RF, no one
broadcasts baseband on anything but copper.

The equally absurd notion offered here originally of DC offset being
RECEIVED is another fanciful illusion. Any such measurement was
clearly an error of measurement or understanding.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message...
On Sun, 6 Feb 2005 08:03:39 -0600,
"Richard Fry" wrote:

It can transmit a constant (DC) video value of any amplitude

Hi OM,

This is absurd. Video is a modulation being carried on RF, no one
broadcasts baseband on anything but copper.
__________________

Before calling this reality absurd, consider that a television station
transmits a video signal in/on an RF channel. The demodulated video
waveform in the TV receiver will be identical to the baseband video signal
applied to the TV tx -- including its DC components (subject to any
distortions along the transmission path).

If it wasn't ~ identical, a TV set could never "fade to black" when the
original image did, and low-luminance colors such as blue, red, brown etc
would be impossible to reproduce with their original chromaticity.

RF (ex-RCA Field Engineer, and installer of
hundreds of TV color studio and film cameras)

"Roy Lewallen" wrote in message
...
The average value of a signal is its DC value. To receive and reproduce
a signal with a non-zero DC component means that your antenna, as well
as your receiver, has to have response to DC. To generate such a signal
would require a static electric and/or magnetic field, which can't
propagate. So it's not possible for a signal you're receiving to have a
non-zero average value.

sure it is. and while the 'static' field itself doesn't propagate the
leading edge of a step from 0 to some 'static' value can propagate and if
you measure as it passes you will see the received signal go from 0 to the
static value and then stay there.

On Sun, 6 Feb 2005 15:59:36 -0000, "Dave" wrote:

leading edge of a step from 0 to some 'static' value

Hi Dave,

....negates the usage of "static" - clearly.

A step pulse is not "static" and in fact contains an infinite range of
frequencies all of which are NOT DC. This is called the genii out of
the bottle and no one here can (but no doubt will try to) put it back.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
On Sun, 6 Feb 2005 15:59:36 -0000, "Dave" wrote:

leading edge of a step from 0 to some 'static' value

...negates the usage of "static" - clearly.

A step pulse is not "static" and in fact contains an infinite range of
frequencies all of which are NOT DC. This is called the genii out of
the bottle and no one here can (but no doubt will try to) put it back.
______________

You mis-read. There is a DC component _required_ to convey the steady
voltage values preceding and following the step pulse transition. He's not
saying that the step pulse transition itself is comprised of "DC."

RF

"Richard Clark" wrote in message
...
On Sun, 6 Feb 2005 15:59:36 -0000, "Dave" wrote:

leading edge of a step from 0 to some 'static' value

Hi Dave,

...negates the usage of "static" - clearly.

A step pulse is not "static" and in fact contains an infinite range of
frequencies all of which are NOT DC. This is called the genii out of
the bottle and no one here can (but no doubt will try to) put it back.

73's
Richard Clark, KB7QHC

you will of course note that i quoted the term 'static' to denote that it
was indeed not static in the infinite sense of mathematics, but in the real
sense of it being a constant value over some measured time period.

the other obvious thing you are missing is that DC is a frequency... and it
does come out in the Fourier transform as a frequency of zero with some
finite magnitude for any waveform who's average value is not zero. it does
not even require that the waveform stay at some value forever, a short pulse
going from 0v to 1v and back to 0v will have a DC component in its spectrum
both mathematically and on any meter that can properly respond to it.

First of all, my apologies. As you can gather it is the first time I post in
this newsgroup. In the process, the computer kept on telling me that I
required authentification (I had forgotten to click on one of the boxes) and
said that it could not send the message. I therefore ended up writting it
several times. As it turned out, two of the times where it told me it had
not sent it, it had. I greatly apologise for all the inconvenience.

Secondly, yes, I mean to say the signal has a DC component. I would have
thought that impulsive noise, being cause by electron movement would produce
an AC signal, otherwise where does the DC come from? The importance of the
time is because I would have thought that in the long term the antenna would
resonate so as to produce a zero mean signal (AC). Sorry, the answer might
be common sense but I just fail to see it.

Thank you for your time and once again, sorry.


"Galilea" wrote in message
...
Hello, thank you for reading this post.

When analysing wideband impulse signals from a wideband antenna I have
realised that the average signal magnitude is not zero. I have thought
this
is because the reactance of the antenna at different frequencies varies
and
since it is a wideband antenna there can be energy measured since it is
only
for an extremely small period of time of 2-4 us. However, I am not sure
and
would greatly appreciate the views of this newsgroup.

yes, a signal can have a dc component, even from a small antenna. (just
watch the arguments over this one) it gets even worse if you sample over a
short period like a few microseconds depending on the characteristics of the
signal you are measuring. and it can get much worse over a long period
because of numeric integration errors accumulating, or small measurement
bias in instrumentation accumulating.

"Galilea" wrote in message
...
First of all, my apologies. As you can gather it is the first time I post
in
this newsgroup. In the process, the computer kept on telling me that I
required authentification (I had forgotten to click on one of the boxes)
and
said that it could not send the message. I therefore ended up writting it
several times. As it turned out, two of the times where it told me it had
not sent it, it had. I greatly apologise for all the inconvenience.

Secondly, yes, I mean to say the signal has a DC component. I would have
thought that impulsive noise, being cause by electron movement would
produce
an AC signal, otherwise where does the DC come from? The importance of the
time is because I would have thought that in the long term the antenna
would
resonate so as to produce a zero mean signal (AC). Sorry, the answer might
be common sense but I just fail to see it.

Thank you for your time and once again, sorry.


"Galilea" wrote in message
...
Hello, thank you for reading this post.

When analysing wideband impulse signals from a wideband antenna I have
realised that the average signal magnitude is not zero. I have thought
this
is because the reactance of the antenna at different frequencies varies
and
since it is a wideband antenna there can be energy measured since it is
only
for an extremely small period of time of 2-4 us. However, I am not sure
and
would greatly appreciate the views of this newsgroup.

Galilea wrote:
Secondly, yes, I mean to say the signal has a DC component. I would have
thought that impulsive noise, being cause by electron movement would produce
an AC signal, otherwise where does the DC come from?.

The DC is the result of your 2-4 uS sampling window. Make
your sampling window one second long and see what you get.

For instance, if you sample a 250 kHz signal for 2 uS and
if you happen to hit it at a zero-crossing going positive,
you will read the DC RMS value of the wave, having sampled
only 1/2 cycle.
--
73, Cecil http://www.qsl.net/w5dxp


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I can understand all you are saying but in this case it is am impulsive
wideband signal (500 - 2.5 GHz), with a defined start and finish, caused by,
say things like car ignition systems and faulty thermostats. Surely a signal
like that would have all frequency components starting and ending at a
zero-crossing and then have full cycles, wouldn't it? Or maybe not because
of the multipath effects? Do you believe this can be the case in my
situation?

Thank you.



"Galilea" wrote in message
...
Hello, thank you for reading this post.

When analysing wideband impulse signals from a wideband antenna I have
realised that the average signal magnitude is not zero. I have thought
this
is because the reactance of the antenna at different frequencies varies
and
since it is a wideband antenna there can be energy measured since it is
only
for an extremely small period of time of 2-4 us. However, I am not sure
and
would greatly appreciate the views of this newsgroup.