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.