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