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

[Hasan al-Basri]

Hi George,

(and anyone else who is interested in an easy way to evaluate your
satellite rx performance)

These remarks are with respective to linear birds, not FM

Some of the satellite ops I work with run various SDRs (software defined
radios like the FunCube Pro+). The software we use with them is called SDR
Console v3
(Google SDR Radio or look at this group:

main at sdr-radio.groups.io)

What we do so we can compare apples to apples is the following:

The software is designed to be able to measure relative signal levels at
high accuracy (-dBM)

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.

5. Now disconnect 50 ohm resistor and change to "real" satellite antenna.

Observe the increase in noise floor. If there isn't one, you have poor rx
sensitivity. If there is an increase and it is 6 dB or more, you have local
noise problems (environmental). If there is a small increase in noise, you
likely have relatively low noise, and at least it exceeds the internally
generated noise of your radio/preamp combo.

Now, when we observe signals from the satellites, we can directly compare
peak signal strength in dBm between our satellite setups. This includes,
antenna polarity, antenna gain, feedline losses, differences in uplink
power, differences in receiver noise figure, and locally generated noise
like power line, WiFi hash, etc.  All...easily and accurately done by
having software that works correctly and a simple SDR Dongle using Simon's
(G3ELI) software (which is free).

For example:

If KB7IJ has a noise floor of -76 and I have a noise floor of -90 (dBm),
and he has a peak BEACON signal strength of -53 and I have a peak strength
of --60, we can see two things; (these are real and typical measurements,
btw)

1. His ambient noise is 14 dB higher than mine. He lives south of Dallas,
Tx, and has to shoot thru the city to see sats. Lots of environmental
noise! I live in rural Iowa. Much quieter.

2. His rx system has 7 dB greater gain than mine (antennas). That makes
sense as he has 2x7 EL yagis and I have a single 5 EL Yagi. So all else
being equal (and it almost never is), he should always see the birds 7 dB
louder than I do (actual signal level, not SNR)

Now look at SNR (Signal to Noise Ratio):...a much different story. How well
do we hear the birds?

His SNR (best): -76 dBm noise floor - (-53) signal = 23 dB SNR (signal is
23 above noise, loud!)

My SNR (best ): -90 dBm noise floor - (-60) signal = 30 dB SNR

So when we are both getting a satellite at our best signal level, I am
hearing the bird 7 dB better than he is, even though his rx antennas are
much better.

Look what happens when the ssb signal is much weaker:

If I am seeing the SSB signal at an SNR of 10 dB (the lowest signal
strength that an SSB signal can be understood at):

He is seeing it at only 3 dB SNR which is unreadable.

In general, I nearly always hear both him and myself way better than he
does, because my noise floor is so much better than his.

Keep in mind, these are all RELATIVE measurements not ABSOLUTE. They are
not meant to be lab grade, they are meant to be able to understand and
measure reasonably well,  how a satellite station is performing in the real
world. Common calibration between stations allows us to compare how our
systems are working. It's not perfect, but it is useful. (There are other
factors, like the practical limit of SNR from any bird based on passband
noise). If you can consistently see/hear passband noise, you are good. If
you only hear it now and then, your rx system needs work)

p.s. The software from Simon, also allows us to make MP4 recording (video
and audio) of our satellite qsos, which can demonstrate some of the very
dramatic ways to improve or ruin your ability to use satellites
efficiently. That is to say, receive properly and not overdrive the
satellite with excessive uplink power, because you can't hear worth a darn
and don't know it. (Just continue to increase transmit power so you can
hear yourself, taking more than your share of what is available, because it
is a shared resource).

I have hours or recordings of assorted stations doing exactly this (running
excessive uplink power and thus producing downlink signals 6 dB or more
louder than the CW beacon) and therefore stealing the available downlink
power of others, driving them down into the noise floor. It's embarrassing.

*The rule is simple: you should NEVER be louder than the CW beacon, period.*

73, N0AN
Hasan


On Wed, Mar 4, 2020 at 7:48 PM George Sakai <grsakai5120 at yahoo.com> wrote:

> Thanks for the info Hasan.  What do you use to measure signal levels in
> dBm?
>
> George N3GS
>
> On Wednesday, March 4, 2020, 03:43:43 PM CST, Hasan al-Basri via AMSAT-BB <
> amsat-bb at amsat.org> wrote:
>
>
> K7VNE reported to me this morning that he had been on XW-2D (long since
> non-operational) and sigs were very good, but no one else was on.
>
> Signals were of good quality, no FM'ing of my SSB signal.
>
> I just checked it and had a pass of Max EL 2 deg to my East. It is there,
> it is loud. *The CW beacon is raspy*.
>
> The passband was normal level and my signal was typical:
>
> Max Sig: -64 dBm
> Noise Floor: -81 dBM
>
> I made an mp4 recording of the pass, with me calling CQ and announcing
> Elevation and signal levels.
>
> https://drive.google.com/open?id=1lKYvARp4SyHUraAX9u6HQRbmnCiamJAM
>
> Download this mp4 file and play it back on any device (even a phone) and
> you will see and hear what I did.
>
> I have seen no announcement about not using the satellite, but will watch
> for same.
> 73, N0AN
> Hasan
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