[aprssig] weak signal ISS packet

[Robert Bruninga]

Yes, again, "best packet"  all depends on the settings of the transmitting
station (and matching it on receive).

When the ISS used a kenwood, then another kenwood would always work the
best. Because both are built at the factory for optimum performance (to each
other) and they use direct modulation and demodulation to the same levels.
My kenwoods can decode another kenwood (direct) with only 3 bars.    BUT
similarly, I can see many signals that appear to have full scale signals and
then barely deocde because so many signals on the air were never adjusted
properly.  AND there are two standards.

Do the sending and receiving radios use pre- and de-emphasis (speaker and
Mic connections)? Or do they operate flat with direct modulation and
demodulation.  Is the receiver an FM demodulator or Phase demodulator?

BUT on ISS, I believe they are currently using another radio, not a kenwood.
SO now you need a receiving system best matched to this radios transmitted
waveform.

Maybe someone on the AMSAT-BB has captured the existing ISS waveform and we
can see what kind of receiver is best.  But then again, the captured
waveform MUST indicate exactly what kind of receiver (with or without
de-emphasis) was used in capturing the waveform!

Two manufactured APRS radios work BEST between them because they were
factory adjusted for BALANCED tone levels and NO preemphasis or de-emphasis.
But other stations may just use the Mic connections and  speaker connections
and get over 6 dB of performance loss if they are not accounted for.

The use of speaker/mic connections(with pre- and de-emphasis) and direct
modulation and discriminator demodulation can each have optimum performance.
But there are always problems when crossing between the two.  BUT, it IS
possible to adjust on the transmit side for a compromise between the two and
then receivers of each type can decode pretty good.

Though maybe 1% of hams go to this trouble.  Packet radio is NOT
Plug-N-play.  The transmitter must be adjusted for optimum waveform!

I should be able to tell everyone what the RIGHT way to do it is, but I
never trust my memory anymore, and don’t have time today to go do all the
research again...  But I'm going to recall from memory, that I think the
WORST situation of most TNC's is if the LOW tone is higher than the HIGH
tone, that decoding always suffers.  And suffers FAR WORSE than the other
way around.  So that is why the compromise is to make the high tone about 3
dB higher (30% on an OO scope)  so that it doesn’t get completely lost when
it goes through a speaker connected receiver and gets de-emphasized.

OR, If  I remember wrong and it is the other way around!

Good luck.

Bob

-----Original Message-----

I was hoping for some ideas to better my station and am very pleased you
have responded. I want to better utilize my station and increase my
throughput with the ISS in particular. I have available 10 element crossed
Yagi antennas and Yaesu az el rotor and several radios with TNC's built in
including Yaesu FTM-350 and FTM-400 and Kenwood
TM-D700 and TS-2000. MFJ and Kantronics TNC's as well as products from
Byonics and Argent Data are found on my bench. I am currently running a
Kenwood TM-D700 to a M2 Eggbeater as a 24/7 ISS station with UISS software
and modules to upload to the internet.
         I want to increase my capabilities with respect to the ISS and
APRS. I have found that the Kenwood TM-D700 requires that you get about
almost a half scale reading to effect good packet reception. I don't want to
use the fully automated  station (FT-847) that I have when I'm not home
because in the past a failure caused my rotor to jam and I had to lower my
tower to remove and repair the az-el rotor. I suspect that the best way
might be to use a sound card software and computer to give the best
decoding. Thanks for the response Steve KB1CHU