[amsat-bb] XW-2D Returns to Life!

[Hasan al-Basri]

Hi Zach,
Since we were speaking of SSB SNR, the bandwidth would be similar. In our
cases we use 2.2 kHz. I forgot to even mention it.

The ...and the point of the whole process was to keep it simple. The more
precise, the more complex and the less likely anyone is going to bother.

The perfect is the enemy of  the good.

Suffice it to say, with hours and hours of measurements using the approach
indicated, it is clear that if the antenna gain is increased by x dB or the
uplink power is changed by x dB, the reference calibration between two
setups holds nicely. There are a ton of other factors at any given moment
that one can get lost in, but wash out with repeated measurement.

Using the approach I described, can a minimally equipped (test) station
determine whether they have adequate sensitivity for their sat ops, or if
they have improved their setup? Of course they can and they won't be stuck
doing math they don't understand and has little bearing on the real world
performance of their satellite station.

Many, many years ago Amsat Journal published an article on Link Budget
Analysis that did all the math and painstaking analytics you describe. How
many people made use of it....next to none. I know, because I wrote it.

What I was aiming at was a simple, quick and dirty "rough estimate" of :
1. Is my system sufficiently sensitive
2. Am I running too much power.

Boltzman is not needed for that. We aren't doing EME. Sat ops are a
relatively strong signal mode. It says a lot that many receive setups are
performing so poorly that "rough and dirty"  (and simple) approaches like I
outlined can make a big difference in overall efficiency. I just hope that
it helps people hear better.
(and subsequently reduce their uplink power)

73, N0AN
Hasan


On Thu, Mar 5, 2020 at 11:16 AM Zach Metzinger via AMSAT-BB <
amsat-bb at amsat.org> wrote:

> On 2020-03-05 04:06, Hasan al-Basri via AMSAT-BB wrote:
> > 1. Connect a 50 ohm resistor to the antenna or preamp input.
> > 2. Set the rx for maximum sensitivity (RF Gain)
> > 3. Adjust IF Gain so that the 50 Ohm resistor produces a signal level of
> > -95 dBM
> > 4. This creates a common noise floor level calibration point so that we
> can
> > compare local noise (environmental) as well as satellite signal levels.
>
> Hello Hasan,
>
> Ah, noticed one more thing. This is only true if the measured thermal
> noise power bandwidth is the same on both setups.
>
> Thermal noise power is k*T*B, where k is Boltzmann's constant (1.38E-23
> J/K), K is "temperature" of your source in Kelvins (290 is about room
> temp), and B is the bandwidth of the noise in Hertz (1/seconds).
>
> We assume that there is no current flowing through the resistor for this
> example.
>
> (1.38E-23 J/K)(290 K)(1 Hz) = 4.002E-21 Watts
>
> [Kelvins cancel out, leaving Joules/seconds, which is Watts -- math is
> cool!]
>
> However, this is an annoying figure to remember, so we convert it to dBm:
>
> (4.002E-21 W)(1000 mW/W) = 4.002E-18 mW
>
> 10 * log10(4.002E-18 mW / 1 mW) ~= -174 dBm
>
> (~= is "approximately equal", rounded)
>
> Now that's an easy number to remember, and all radio amateurs should!
>
> Notice that we initially computed this noise floor with Bandwidth of 1
> Hz. Not even the slowest CW would fit in that, so let's make it
> something more reasonable like 3kHz for voice.
>
> Instead of doing all that math again, we can use logarithms to fix this
> up. Multiplication becomes addition when we're in a log scale (mmm,
> slide rules..), so:
>
> -174 dBm + log(3000 Hz / 1 Hz) ~= -170 dBm
>
> This is the thermal noise floor in a 3 kHz bandwidth from a noise source
> at 290 Kelvin (62 deg F).
>
> Anyway, all of that was to show that bandwidth is very important when
> comparing noise power of two receivers.
>
> If anyone is interested in learning more about noise floor, noise
> figure, or required SNR, here's a great article:
>
> https://tinyurl.com/t7o4n34
>
> --- Zach
> N0ZGO
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