On 6/27/2015 7:48 AM, amdx wrote:
On 6/26/2015 9:46 PM, rickman wrote:
On 6/26/2015 12:38 PM, amdx wrote:
On 6/26/2015 7:24 AM, rickman wrote:
I read this post in an antenna group and I don't get how this guy is
coming up with a negative noise figure. Looks to me like he is
calculating the noise figure of a resistor, not the amplifier. Anyone
care to explain this to me?

The part that seems bogus is this...

The negative NF is defined as the amplifier noise being less than
the
increase in noise due to the amplifier gain.

I thought noise figure was NF = SNRin / SNRout

Rick

I once ask Ratzlaff about the FSL antenna, in his comments he said,
"high Q or low Q has little relation to how the FSL will actually
perform for receiving weak signals.", but then went on to say, "Of
course using Litz is the only type of wire to use."
When I ask him about why Litz is important if Q isn't, he got ****ed
off and wrote back, "When you make sarcastic replies to the info I
passed along, and question what I say, then I write you off as just a
tire-kicker, not interested in possibly learning something, and I have
no interest in saying anything more."
Seems to me he said something incorrect and didn't like having someone
ask him to clarify it. I'll leave it to you to figure out which part was
incorrect.
I thought Q would be important, an FSL is a ferrite loaded tuned
loop.
With magic mixed in! ;-)
If Q is not important, why use Litz?
I suspect you might have already tried to ask him, and now he's mad at
you too!
Mikek

I suspect he said he didn't like your sarcastic replies because you made
sarcastic replies much more than he didn't want to discuss anything he
said.

I was not being sarcastic in my response when I ask about the Litz.

Look at the larger picture.


--

Rick

On Sat, 27 Jun 2015 06:48:59 -0500, amdx Gave us:

On 6/26/2015 9:46 PM, rickman wrote:
On 6/26/2015 12:38 PM, amdx wrote:
On 6/26/2015 7:24 AM, rickman wrote:
I read this post in an antenna group and I don't get how this guy is
coming up with a negative noise figure. Looks to me like he is
calculating the noise figure of a resistor, not the amplifier. Anyone
care to explain this to me?

The part that seems bogus is this...

The negative NF is defined as the amplifier noise being less than the
increase in noise due to the amplifier gain.

I thought noise figure was NF = SNRin / SNRout

Rick

I once ask Ratzlaff about the FSL antenna, in his comments he said,
"high Q or low Q has little relation to how the FSL will actually
perform for receiving weak signals.", but then went on to say, "Of
course using Litz is the only type of wire to use."
When I ask him about why Litz is important if Q isn't, he got ****ed
off and wrote back, "When you make sarcastic replies to the info I
passed along, and question what I say, then I write you off as just a
tire-kicker, not interested in possibly learning something, and I have
no interest in saying anything more."
Seems to me he said something incorrect and didn't like having someone
ask him to clarify it. I'll leave it to you to figure out which part was
incorrect.
I thought Q would be important, an FSL is a ferrite loaded tuned loop.
With magic mixed in! ;-)
If Q is not important, why use Litz?
I suspect you might have already tried to ask him, and now he's mad at
you too!
Mikek

I suspect he said he didn't like your sarcastic replies because you made
sarcastic replies much more than he didn't want to discuss anything he
said.

I was not being sarcastic in my response when I ask about the Litz.
My understanding is Litz reduces R losses thus Q increases, if Q is not
important, why is Litz the only type of wire to use?
He probably realized that what he wrote made no sense and being ask
about it offended his ego and I'm that's why got angry.
On the other hand, if both of his statements are correct, it would be
interesting to learn why. I would learn something, because as it stands,
I can't reconcile the two statements.
In the end, the ferrite loaded antenna I was working on had a low Q,
I was using some surplus ferrite material I had, and it was lossy,
especially in the upper AMBCB. It had Q's under 100 down to 40. To
support Ratzlaff's theory, it did bring in stations that the radio
didn't hear if not near the Ferrite loaded antenna. But that's
subjective and I don't know what it would be like if it had a Q of 800.
Mikek

The µ of the ferrite makes a difference.

Especially in cost..

There is also an "ideal" (or nearly so) diameter (and length) to best
use.

One can buy one inch specimens and glue them together when tuning to
find a "best use" scenario for the desired fo under which to continue
tuning practices.

On 6/27/2015 4:07 AM, Jeff wrote:
On 26/06/2015 13:24, rickman wrote:
I read this post in an antenna group and I don't get how this guy is
coming up with a negative noise figure. Looks to me like he is
calculating the noise figure of a resistor, not the amplifier. Anyone
care to explain this to me?

The part that seems bogus is this...

The negative NF is defined as the amplifier noise being less than the
increase in noise due to the amplifier gain.

I thought noise figure was NF = SNRin / SNRout

Rick


Both definitions are correct and mean the same thing; a negative NF,
when expressed in dB, would be when the SNRout is less than the SNRin.
However, the big but is that an negative NF is not possible.

I don't think both definitions mean the same thing. If the amplifier
adds *any* noise it increases the NF above zero by the conventional
definition. The only way the NF can be negative is if the amplifier
removes noise from the input, or in other words, increases the SNR.

What he seems to be suggesting is that NF is the ratio of the signal
noise to the amplifier noise.


It only appears to be the case due to the fact that the OP is not
comparing like with like, the test method used is only valid if the
system impedance remains the same. You cannot compare oranges with lemons.



--

Rick

On Sat, 27 Jun 2015 13:43:16 +0100, Jeff Gave us:

On 27/06/2015 13:26, rickman wrote:
On 6/27/2015 4:07 AM, Jeff wrote:
On 26/06/2015 13:24, rickman wrote:
I read this post in an antenna group and I don't get how this guy is
coming up with a negative noise figure. Looks to me like he is
calculating the noise figure of a resistor, not the amplifier. Anyone
care to explain this to me?

The part that seems bogus is this...

The negative NF is defined as the amplifier noise being less than the
increase in noise due to the amplifier gain.

I thought noise figure was NF = SNRin / SNRout

Rick


Both definitions are correct and mean the same thing; a negative NF,
when expressed in dB, would be when the SNRout is less than the SNRin.
However, the big but is that an negative NF is not possible.

I don't think both definitions mean the same thing. If the amplifier
adds *any* noise it increases the NF above zero by the conventional
definition. The only way the NF can be negative is if the amplifier
removes noise from the input, or in other words, increases the SNR.


Yes that is correct, but the definitions are also correct. The flaw in
the negative noise figure argument is that it is not possible to have a
better SNRout than SNRin *for the same system conditions*.

The apparent negative noise figure only come about by comparing the NF
of the amp in a 50ohm system with the output from a system with
something different on the input.

The test method used is also very prone to measurement errors for low
noise figures.

Jeff

To me, NF refers to "noise floor".

Lets see him go below that.

GPS received signals are among the lowest "power" signals we currently
grab. They sit just above the noise floor.

On Sat, 27 Jun 2015 14:02:49 +0100, Jeff Gave us:


Is that formula correct? If the input SNR is poor, an amplifier with a
high NF has very impact on the output SNR.

Also, are the units ratios, or are they in dB?



For a particular NF the effect on the output s/n ratio is always the
same regardless of the actual input s/n, until you get to the point
where the signal vanishes in the noise, but even then it still holds
true but you just can't see it.

The signal will go up by the gain of the amplifier, and the noise will
go up by the sum of *power* of the input noise times the gain and the
noise power of the calculated from the NF times the gain.

The noise powers being in watts calculated from the NF; in a 1Hz
Bandwidth by convention. So its dB above kTB converted to watts if you
are working with NF in dB.

So for a particular NF the added noise is always the same, therefore the
SNRin/SNRout holds, and is a standard definition of NF (not in dB).

Jeff

https://en.wikipedia.org/wiki/DBm

Look at the last four entries in the table.

On 6/27/2015 7:11 AM, DecadentLinuxUserNumeroUno wrote:
On Sat, 27 Jun 2015 06:48:59 -0500, amdx Gave us:

On 6/26/2015 9:46 PM, rickman wrote:
On 6/26/2015 12:38 PM, amdx wrote:
On 6/26/2015 7:24 AM, rickman wrote:
I read this post in an antenna group and I don't get how this guy is
coming up with a negative noise figure. Looks to me like he is
calculating the noise figure of a resistor, not the amplifier. Anyone
care to explain this to me?

The part that seems bogus is this...

The negative NF is defined as the amplifier noise being less than the
increase in noise due to the amplifier gain.

I thought noise figure was NF = SNRin / SNRout

Rick

I once ask Ratzlaff about the FSL antenna, in his comments he said,
"high Q or low Q has little relation to how the FSL will actually
perform for receiving weak signals.", but then went on to say, "Of
course using Litz is the only type of wire to use."
When I ask him about why Litz is important if Q isn't, he got ****ed
off and wrote back, "When you make sarcastic replies to the info I
passed along, and question what I say, then I write you off as just a
tire-kicker, not interested in possibly learning something, and I have
no interest in saying anything more."
Seems to me he said something incorrect and didn't like having someone
ask him to clarify it. I'll leave it to you to figure out which part was
incorrect.
I thought Q would be important, an FSL is a ferrite loaded tuned loop.
With magic mixed in! ;-)
If Q is not important, why use Litz?
I suspect you might have already tried to ask him, and now he's mad at
you too!
Mikek

I suspect he said he didn't like your sarcastic replies because you made
sarcastic replies much more than he didn't want to discuss anything he
said.

I was not being sarcastic in my response when I ask about the Litz.
My understanding is Litz reduces R losses thus Q increases, if Q is not
important, why is Litz the only type of wire to use?
He probably realized that what he wrote made no sense and being ask
about it offended his ego and I'm that's why got angry.
On the other hand, if both of his statements are correct, it would be
interesting to learn why. I would learn something, because as it stands,
I can't reconcile the two statements.
In the end, the ferrite loaded antenna I was working on had a low Q,
I was using some surplus ferrite material I had, and it was lossy,
especially in the upper AMBCB. It had Q's under 100 down to 40. To
support Ratzlaff's theory, it did bring in stations that the radio
didn't hear if not near the Ferrite loaded antenna. But that's
subjective and I don't know what it would be like if it had a Q of 800.
Mikek

The µ of the ferrite makes a difference.

Especially in cost..

There is also an "ideal" (or nearly so) diameter (and length) to best
use.

One can buy one inch specimens and glue them together when tuning to
find a "best use" scenario for the desired fo under which to continue
tuning practices.

These FSL antennas are a different breed, they use 10's of rods or
bars in a 4" to 10"+ diameter cylinder. Then a coil is wrapped around
and tuned with an air capacitor.
Here's a good link.
http://www.am-dx.com/antennas/FSL%20...timization.htm
Mikek


---
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On Sat, 27 Jun 2015 15:49:12 +0100, Jeff Gave us:


To me, NF refers to "noise floor".

Lets see him go below that.

GPS received signals are among the lowest "power" signals we currently
grab. They sit just above the noise floor.


It might to you, but in this context it means either Noise Factor or
Noise Figure.

Of course you can go below the Noise Floor, and in some circumstances
and modes the signal is receivable and decodable.

30dB below the noise floor....

http://www.bentongue.com/xtalset/1nlxtlsd/1nlxtlsd.html

The answer to all your needs. Less is more. That Chef's Hat
conglomeration is overkill.

In message , Jeff writes

To me, NF refers to "noise floor".

Lets see him go below that.

GPS received signals are among the lowest "power" signals we currently
grab. They sit just above the noise floor.


It might to you, but in this context it means either Noise Factor or
Noise Figure.

But you have to be careful, as "noise factor" is a numerical ratio, and
"noise figure" is in dB.

Of course you can go below the Noise Floor, and in some circumstances
and modes the signal is receivable and decodable.

In the analogue cable TV world, the noise figure (in dB) can be looked
at as the amount of noise power that (say) a real-world amplifier
notionally has at its input in excess of that which would be generated
from a perfect resistor as its source impedance.

As a rule-of-thumb, in a 4MHz vision bandwidth, a perfect 75 ohm
resistor generates -59dBmV. [Subtract around 48dB if you want dBmW.]

The output of a noiseless amplifier would be -59dBmV + G, where G is the
gain in DB.

The output of a real-world amplifier would be -59dBmV + NF + G, where N
is the noise figure.

One method of measuring the noise figure is first to feed the amplifier
first from a resistive source, and measure the output noise level. Next,
feed the amplifier from a source containing a known amount of noise, and
note the increase of output noise. The noise figure can then be
calculated.

In practice, the noisy source is usually a calibrated noise meter*. The
first reading is taken with the noise meter set at zero additional noise
output, and then the noise output is increased until the amplifier
output level rises by 3dB. This means that the noise meter is now
contributing the same amount of noise as the amplifier, and the noise
figure can be read directly from its output display. [This conveniently
saves having to do any further calculations.]

*Usually, a noise meter has a calibrated output meter or other display,
and this indicates the level of its noise output in a stated bandwidth -
both as an absolute level, and as the equivalent in dB with respect to
the basic minimum absolute level. In the cable TV world, the minimum
would be -59dBmV (probably shown in microvolts) in a 4MHz bandwidth, or
0dB. If, to increase the amplifier output level by 3dB, the noise meter
output had to be turned up to -49dBmV / 10dB, its noise figure would, of
course, be 10dB.


--
Ian

On Sat, 27 Jun 2015 15:49:50 +0100, Jeff wrote:


For a particular NF the effect on the output s/n ratio is always the
same regardless of the actual input s/n, until you get to the point
where the signal vanishes in the noise, but even then it still holds
true but you just can't see it.

The signal will go up by the gain of the amplifier, and the noise will
go up by the sum of *power* of the input noise times the gain and the
noise power of the calculated from the NF times the gain.

The noise powers being in watts calculated from the NF; in a 1Hz
Bandwidth by convention. So its dB above kTB converted to watts if you
are working with NF in dB.

So for a particular NF the added noise is always the same, therefore the
SNRin/SNRout holds, and is a standard definition of NF (not in dB).

Jeff

https://en.wikipedia.org/wiki/DBm

Look at the last four entries in the table.


..and your point is???

....between its shoulders.

On Sat, 27 Jun 2015 12:50:39 -0400, krw Gave us:

On Sat, 27 Jun 2015 15:49:50 +0100, Jeff wrote:


For a particular NF the effect on the output s/n ratio is always the
same regardless of the actual input s/n, until you get to the point
where the signal vanishes in the noise, but even then it still holds
true but you just can't see it.

The signal will go up by the gain of the amplifier, and the noise will
go up by the sum of *power* of the input noise times the gain and the
noise power of the calculated from the NF times the gain.

The noise powers being in watts calculated from the NF; in a 1Hz
Bandwidth by convention. So its dB above kTB converted to watts if you
are working with NF in dB.

So for a particular NF the added noise is always the same, therefore the
SNRin/SNRout holds, and is a standard definition of NF (not in dB).

Jeff

https://en.wikipedia.org/wiki/DBm

Look at the last four entries in the table.


..and your point is???

...between its shoulders.

krw is a pointless jackass, despite what some have said.

http://www.imdb.com/title/tt0067595/