On 7/7/18 12:41 PM, Richard (Rick) Karlquist wrote:
Interesting paper, lots of stuff to absorb.
The discussion of receiver noise figure brings
up a topic that has bothered me for a long time:
the use of excessively high gain external antenna preamps
(as high as 40 dB). Somehow, this is supposed to
make up for a really low gain receiving antenna,
say -40 dBi. Most practical external preamps have
an NF of around 3 dB. With a gain of 10 dB, they
should be able to overcome receiver noise in most
cases, especially of the receiver's internal preamp
is selected. If high gain is used, you are simply
listening to amplifier noise, instead of band
noise.
with beaucoup gain at the antenna, you can use really lossy cable back
to the receiving point. Most "active GPS" antennas (which is virtually
all of them) have about 30-40dB gain, so there's little point in
running, say, LMR-400 (other than for shielding effectiveness).
The issue with excessive gain is on the top end - is environmental noise
going to saturate the final stage. 40dB seems high, unless you've got
some selectivity in the front end.
I've got a HF receiver in LEO feeding a 1V ADC and it has about 30 dB
gain from a 6m long dipole. We went back and forth on the gain, but
settled on "enough gain to get the LNA noise just above the ADC noise",
and have a switchable 20dB pad in the front end.
The noise figure of the amplifier should be low enough that
environmental noise is above the amplifier noise. For HF, if the
ionosphere is transparent, your dominant noise is galactic background
which Hilary Cane measured quite accurately Plus local thunderstorm
noise. If the ionosphere is reflective it's manmade + thunderstorm noise.
The ITU-R P.372-8 report (which you can download from itu.org) gives
the curves, but for general purposes, the noise figure at 10 MHz is
about 30dB. That is, an isotropic antenna will have a noise that is 30dB
above kTB (-174 dBm/Hz). Atmospheric noise will raise that to 40dB, and
"median business area man made" might be around 50dB.
At this kind of level, NF of the amplifer is probably irrelevant.
With a "short antenna", the situation changes a bit.. at 10MHz a full
size dipole is 5m long, so with a 1m whip, you're down about 14 dB, and
it gets worse with lower frequencies.. At 1 MHz, even though the
environment might be 50-100 dB above the thermal noise floor (into an
isotrope), with a short antenna, you might have 30-40 dB of "mismatch loss"
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