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


Hello Group,

Approximately 8 weeks ago fellow group member Steve Ratzlaff and I
had an discussion about the noise performance of amplifiers for Non
Directional Beacon work. Steve suggested that it may be a good idea
to look at the Antenna Amplifier noise floor with a simulated antenna
inductance, rather than using a resistance equal to the amplifier
input z.

Steve's suggestion makes perfect sense when one considers that the
real and radiation resistance of loop antennas are very small in
comparison to the loops inductance.

First I retested two active antennas; Wellbrook's ALA100 and
ALA100LN in the paddock using 20m circumference loops. Loop area 21
sq.m. Both antenna use noiseless feedback. The ALA100 uses Bipolar
transistors, the ALA100LN uses JFETS.

The gain comparison on MW/LW was an increase of 2-4dB in favour of
the ALA100LN.

Unfortunately the ambient noise level was too high to see any noise
floor difference.

Next; Bench Tests ( gain and Noise Figure NF of both ALA100 amps.)
were conducted using a Marconi 2019A Sig. Gen. and WinRadio
Excalibur. A 9dB Norton amplifier in the Antenna Interface was used
to ensure that the total gain was approx. 10dB higher than the
Excalibur NF. Thus to ensure that the higher amplifier gain would
mask the receiver NF affecting the measurements. A 20uH inductor was
used to simulate the loop’s reactance.

The NF was measured using the gain method i.e. the excess noise above
the amplifier gain when the input is terminated with a resistor. The
negative NF is measured with a inductance connected to the amplifier
input.

Both ALA100s have an approx. 50 Ohm resistive input z as determined
using an Array Solutions AIM 4170C Antenna Analyser.

WinRadio Excalibur set 1kHz BW and the S Meter to RMS AVG.

-144dBm is used as reference for the 1kHz BW

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

Test results.

Old ALA100 1MHz gain = 27dB (18dB with Passive Interface )

Noise with 50 Ohm input = –116dBm

NF = 1.0dB

Noise with 20uH input = –118dBm ( -127dBm with Passive
Interface )

NF = –1dB.


New ALA100LN 1MHz gain = 28dB (19dB with Passive Interface )

Noise with 50 Ohm input = –116dBm

NF = 0.0dB

Noise with 20uH input = –124dBm ( -133dBm with Passive
Interface )

NF = –8dB.


3 other ALA100LNs were tested, the worst case was a -6dB NF.

Looking at the above one can see that there is only a 1dB difference
in the gain and NF of the two Head amps with a 50 Ohm input.

The apparent -1dB NF of the ALA100 with the 20uH input is probably
due to the fact that Inductors are noiseless compared to a 50 Ohm
resistor.

However, what is striking, is the apparent high negative NF of the
ALA100LN. My first thoughts were when this showed up, that the gain
had dropped by 8dB because of the loop inductance and this would
show up as low antenna gain. However, the on air comparison suggested
that this wasn't the case. Also Dave Aichelman ran some comparative
tests confirming my observations.

I do not have an explanation yet as to why the ALA100LN has an
apparent negative NF or why the noise floor drops by 8dB, but it
certainly accounts as to why some users are seeing improved
performance. The theoretical NF of the ALA100LN is close to 0dB.

One could speculate that there is a significant mismatch of the
antenna noise from its real and radiation resistance being a small
fraction of the amplifier input z. However, I have only been able to
achieve a significant negative NF using JFETS.

The ALA100LN uses 8 x J309 Fets. with noiseless feedback.

The negative NF can be increased to approx. -10dB by altering the
feedback and increasing the amplifier gain. Also the negative NF
increases by a couple of dB where the loop inductance is higher e.g.
a 20m circumference thin wire loop ( 30uH ). Also the negative NF is
maintained as the frequency rises.

The ALA100LN has a sister, “Florence� the FLG100LN with an input
matching/isolation transformer of 1152 Ohms for Terminated loop
antennas e.g. Flag and KAZ. However, the antenna termination
resistance makes the negative NF feature unrealisable.

One may ask what is the significance of the amplifier having a
negative noise figure of -8dB?

It basically means, that there is the potential of an 11dB s/n
improvement when compared to a conventional amplifier of the same
gain and NF of say 3dB with a similar loop size. This feature could
be used to compensate for any mismatch loss over wide bandwidths.

Additionally the negative noise figure could compensate for usual NF
degradation when loops are run as close spaced Phase Arrays.

I am currently exploring to see if the apparent negative NF can be
exploited with smaller loops. To this end I have extended the
bandwidth of the ALA100LN from 10MHz to 30MHz by configuring the FETs
to run in cascode with two Bipolar devices. However, there may be a
bandwidth limitation due to the fixed input impedance of the
amplifier compared to the ALA1530 with its loop/amp. impedance
tracking verses frequency.


--

Rick

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








Hello Group,

Approximately 8 weeks ago fellow group member Steve Ratzlaff and I
had an discussion about the noise performance of amplifiers for Non
Directional Beacon work. Steve suggested that it may be a good idea
to look at the Antenna Amplifier noise floor with a simulated antenna
inductance, rather than using a resistance equal to the amplifier
input z.

Steve's suggestion makes perfect sense when one considers that the
real and radiation resistance of loop antennas are very small in
comparison to the loops inductance.

First I retested two active antennas; Wellbrook's ALA100 and
ALA100LN in the paddock using 20m circumference loops. Loop area 21
sq.m. Both antenna use noiseless feedback. The ALA100 uses Bipolar
transistors, the ALA100LN uses JFETS.

The gain comparison on MW/LW was an increase of 2-4dB in favour of
the ALA100LN.

Unfortunately the ambient noise level was too high to see any noise
floor difference.

Next; Bench Tests ( gain and Noise Figure NF of both ALA100 amps.)
were conducted using a Marconi 2019A Sig. Gen. and WinRadio
Excalibur. A 9dB Norton amplifier in the Antenna Interface was used
to ensure that the total gain was approx. 10dB higher than the
Excalibur NF. Thus to ensure that the higher amplifier gain would
mask the receiver NF affecting the measurements. A 20uH inductor was
used to simulate the loop’s reactance.

The NF was measured using the gain method i.e. the excess noise above
the amplifier gain when the input is terminated with a resistor. The
negative NF is measured with a inductance connected to the amplifier
input.

Both ALA100s have an approx. 50 Ohm resistive input z as determined
using an Array Solutions AIM 4170C Antenna Analyser.

WinRadio Excalibur set 1kHz BW and the S Meter to RMS AVG.

-144dBm is used as reference for the 1kHz BW

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

Test results.

Old ALA100 1MHz gain = 27dB (18dB with Passive Interface )

Noise with 50 Ohm input = –116dBm

NF = 1.0dB

Noise with 20uH input = –118dBm ( -127dBm with Passive
Interface )

NF = –1dB.


New ALA100LN 1MHz gain = 28dB (19dB with Passive Interface )

Noise with 50 Ohm input = –116dBm

NF = 0.0dB

Noise with 20uH input = –124dBm ( -133dBm with Passive
Interface )

NF = –8dB.


3 other ALA100LNs were tested, the worst case was a -6dB NF.

Looking at the above one can see that there is only a 1dB difference
in the gain and NF of the two Head amps with a 50 Ohm input.

The apparent -1dB NF of the ALA100 with the 20uH input is probably
due to the fact that Inductors are noiseless compared to a 50 Ohm
resistor.

However, what is striking, is the apparent high negative NF of the
ALA100LN. My first thoughts were when this showed up, that the gain
had dropped by 8dB because of the loop inductance and this would
show up as low antenna gain. However, the on air comparison suggested
that this wasn't the case. Also Dave Aichelman ran some comparative
tests confirming my observations.

I do not have an explanation yet as to why the ALA100LN has an
apparent negative NF or why the noise floor drops by 8dB, but it
certainly accounts as to why some users are seeing improved
performance. The theoretical NF of the ALA100LN is close to 0dB.

One could speculate that there is a significant mismatch of the
antenna noise from its real and radiation resistance being a small
fraction of the amplifier input z. However, I have only been able to
achieve a significant negative NF using JFETS.

The ALA100LN uses 8 x J309 Fets. with noiseless feedback.

The negative NF can be increased to approx. -10dB by altering the
feedback and increasing the amplifier gain. Also the negative NF
increases by a couple of dB where the loop inductance is higher e.g.
a 20m circumference thin wire loop ( 30uH ). Also the negative NF is
maintained as the frequency rises.

The ALA100LN has a sister, “Florence� the FLG100LN with an input
matching/isolation transformer of 1152 Ohms for Terminated loop
antennas e.g. Flag and KAZ. However, the antenna termination
resistance makes the negative NF feature unrealisable.

One may ask what is the significance of the amplifier having a
negative noise figure of -8dB?

It basically means, that there is the potential of an 11dB s/n
improvement when compared to a conventional amplifier of the same
gain and NF of say 3dB with a similar loop size. This feature could
be used to compensate for any mismatch loss over wide bandwidths.

Additionally the negative noise figure could compensate for usual NF
degradation when loops are run as close spaced Phase Arrays.

I am currently exploring to see if the apparent negative NF can be
exploited with smaller loops. To this end I have extended the
bandwidth of the ALA100LN from 10MHz to 30MHz by configuring the FETs
to run in cascode with two Bipolar devices. However, there may be a
bandwidth limitation due to the fixed input impedance of the
amplifier compared to the ALA1530 with its loop/amp. impedance
tracking verses frequency.




---
This email has been checked for viruses by Avast antivirus software.
http://www.avast.com

On Fri, 26 Jun 2015 08:24:25 -0400, 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


Hello Group,

Approximately 8 weeks ago fellow group member Steve Ratzlaff and I
had an discussion about the noise performance of amplifiers for Non
Directional Beacon work. Steve suggested that it may be a good idea
to look at the Antenna Amplifier noise floor with a simulated antenna
inductance, rather than using a resistance equal to the amplifier
input z.

Steve's suggestion makes perfect sense when one considers that the
real and radiation resistance of loop antennas are very small in
comparison to the loops inductance.

First I retested two active antennas; Wellbrook's ALA100 and
ALA100LN in the paddock using 20m circumference loops. Loop area 21
sq.m. Both antenna use noiseless feedback. The ALA100 uses Bipolar
transistors, the ALA100LN uses JFETS.

The gain comparison on MW/LW was an increase of 2-4dB in favour of
the ALA100LN.

Unfortunately the ambient noise level was too high to see any noise
floor difference.

Next; Bench Tests ( gain and Noise Figure NF of both ALA100 amps.)
were conducted using a Marconi 2019A Sig. Gen. and WinRadio
Excalibur. A 9dB Norton amplifier in the Antenna Interface was used
to ensure that the total gain was approx. 10dB higher than the
Excalibur NF. Thus to ensure that the higher amplifier gain would
mask the receiver NF affecting the measurements. A 20uH inductor was
used to simulate the loop’s reactance.

The NF was measured using the gain method i.e. the excess noise above
the amplifier gain when the input is terminated with a resistor. The
negative NF is measured with a inductance connected to the amplifier
input.

Both ALA100s have an approx. 50 Ohm resistive input z as determined
using an Array Solutions AIM 4170C Antenna Analyser.

WinRadio Excalibur set 1kHz BW and the S Meter to RMS AVG.

-144dBm is used as reference for the 1kHz BW

That is 174 dBm/Hz, indicating about 300 K noise temperature (room
temperature). UHF people would call that 3 dB reference level and it
is not so hard to go down to 100 K (1 dB NF) in a good preamplifier.

If the noise level is below 3 dB, someone would call it a negative
NF:-).

Anyway, at VLF/LF the band noise is so huge, that I do not understand
what a low NF would help, unless the antenna is extremely small and
lossy, such a ferrite bar with -60 to -80 dB antenna efficiency and
hence similar gain.

A several meter in diameter loop will have a half decent efficiency,
so extremely low amplifier NF is seldom justified. Of course, putting
the amplifier at the antenna will help keeping interference from
entering the downlead.

On Fri, 26 Jun 2015 08:24:25 -0400, 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

Yes, it is. NF = 0dB is about as low as you can go without magic.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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.

--

Rick

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



** SNIPPED for brevity **
Even at DC frequencies (eg: broadcast FM or even AM) Litz wire gives
an edge for higher Q; that is a proven fact.
BUT....
If you do not give a rat about Q or gain losses or noise figure
(again, at DC frequencies), then bag Litz and pay the price on all of
them....

In message , Jeff writes
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.

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.

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?


--
Ian

On Fri, 26 Jun 2015 23:04:43 -0700, Robert Baer
wrote:

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



** SNIPPED for brevity **
Even at DC frequencies (eg: broadcast FM or even AM) Litz wire gives
an edge for higher Q; that is a proven fact.
BUT....
If you do not give a rat about Q or gain losses or noise figure
(again, at DC frequencies), then bag Litz and pay the price on all of
them....

Litz is useful at VLF/LF/MF due to the high inductance and hence large
number of turns required. On a small coil former, the requirement is
to use the available copper cross section as effectively as possible.

At high HF and VHF, fitting a few turns into a coil former is
required, so using 0.5 mm solid thick wire to contain the RF within
the skin dept is not a problem.

For a single turn loop antenna, there are no such size restrictions. A
1-2 m loop antenna made of 10-20 mm copper tubing works quite well as
a transmitting antenna at 3.5 MHz with perhaps 1 % efficiency (-20 dB
gain).

I once made a loop antenna into a hoola hoop using a flat cable using
"off by one" connection to create a multiturn loop. The problem was
that the self capacitance between turns made it tunable only at VLF
frequencies.

On 6/27/2015 1:04 AM, Robert Baer wrote:
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



** SNIPPED for brevity **
Even at DC frequencies (eg: broadcast FM or even AM) Litz wire gives
an edge for higher Q; that is a proven fact.
BUT....
If you do not give a rat about Q or gain losses or noise figure
(again, at DC frequencies), then bag Litz and pay the price on all of
them....

Yes, his comment made no sense to me. So I ask about it, that was a
mistake!
Mikek

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