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
>
> 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.
>
> 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.