[NLRS] Super tropo

[Dr. Gerald N. Johnson]

On 7/31/2012 1:57 PM, Doug Reed wrote:
>
>
> Hi Jerry.
>
> Yup, I can agree with everything you said about packet. But I doubt the
> SSB advantage is a full 30dB, most references say 7 to 10dB. I usually
> expect reasonable packet decodes at 20dB quieting, around .5uv or less.
> I expect pretty much guaranteed packet decodes near 1uv. In SSB I would
> expect easy Q's at under .25uv and I know we've managed to work 10GHz
> SSB well into the noise level. But those are contest Q's, not what I'd
> call ragchew quality.

I'll stick with 30 dB. MDS for a good receiver SSB bandwidth is less 
than -140 dBm. 10^-14 mw, or 10^-17 watts. 2.2 x 10^-8 volts. .02 
microvolts. 20 dB below .2 microvolts.

8 dB was the advantage 6 kHz double sideband AM lost to SSB with 3 kHz 
bandwidth.

FM receiver bandwidth is 15 to 20 kHz, plus the limiter and frequency 
discriminator set a hard threshold where the quieting effect is very 
significant improving S/N rapidly with little signal level change. Below 
that threshold there's no copy, but in SSB below the threshold there 
still is some copy.

There is quite a bit more noise power in the receiver IF in a 15 or 20 
kHz bandwidth than there is in a 2.3 kHz or so SSB bandwidth.

Modern narrower band FM works poorer for weak signals than wider 
deviations and receivers, but the FM industry wants channels, not range.
>
> To me, if the APRS beacons indicate a 500 mile path, that path, or
> something longer, has already been open for SSB operations for quite a
> while. We're back to "If a tree falls in the forest but nobody hears,
> did it make a sound..." If nobody is transmitting, is there really an
> opening? If everybody is listening on 144.200 but nobody transmits, how
> do I know if conditions are good? Use 144.205 for voice keyer beacons?

Definitely. And probably over a significantly longer path.

Activity, the lack thereof, I fear, misses many openings. I didn't make 
the point at this years CSVHF business meeting but I have at many 
previous ones, I say, "Our receivers are built wrong, they don't radiate 
to let others know we are listening." 50+ years ago I had use of a 2m 
superregen. It radiated so the hams in the neighborhood could tell I was 
listening. I have to admit, I'm not as good as some I know about getting 
on and calling CQ on a quiet band.
>
> The best thing about using APRS as a propagation indicator is that there
> are already thousands of beacon stations out there all across the
> country. It is not a perfect indicator because it could easily miss a
> weak opening. But it is active enough to catch most strong enhancements.
> In fact I'm inclined to argue it may be TOO active, too many beacons so
> they overlap!

Beacon collisions are guaranteed, because all the blatts are 
uncoordinated. That's why they are repeated at a few minute intervals, 
often ten.
>
> And there are enough APRS receiving stations out there collecting the
> beacons and sending them to a central server that the information is
> captured and available for further processing. If you are of the "I'd
> rather do it myself" type, you can setup an APRS receiver and search the
> received beacons to extract your own personalized propagation
> information. I think there may already be some filtering routines in
> APRSISCE that will do it for you if you just specify what local station
> to monitor.

The best of APRS antennas probably has 6 or 8 dBd gain, omni while a 
poor 2m horizontal does 10 or 12 dBd gain but its directional.
>
> Is it perfect, no. I could suggest several options that are technically
> better, starting with CW Skimmer, PropNET or WSPRnet. If you have a
> handy SDR receiver parked near 144.265MHz you could use SDR and CW
> Skimmer to monitor existing CW beacons and forward the captures to one
> of the propagation sites. That would get you maybe 50 beacons across the
> country. And how many people would dedicate a receiver to do this
> monitoring? There are nearly 50 APRS WIDE digis and Internet gateways in
> Minnesota alone with many more active stations.
>
> Yes, I would love to see a better system. But how do we get enough
> people interested to create the infrastructure. In broad numbers there
> are over 500 4-digit grid squares across the continental US. There is no
> way we'd get enough people interested in building CW beacons to have one
> per grid square. But there is probably near full penetration if using
> APRS. Or we could probably get by with the existing CW beacons if we had
> 50 CW Skimmer stations monitoring propagation.

I just wish for a 10 G beacon about Mason City that I could use to check 
out potential 10G+ operating points better than just checking that 
there's slope down to the north for a few miles and no trees, buildings, 
or tall corn or hills to have to work over. I still am trying to figure 
out how far one needs to be from an obstacle for the ordinary 
atmospheric refraction to bend over it. I came closer while driving 
looking for a wrench today.
>
> Until we get another, better, system for monitoring propagation, you can
> manually listen to CW beacons or you can watch for out-of-the-area FM or
> TV stations, or you can listen for distant NOAA weather radio stations.
> But all of these require your active attention, it isn't automated. Of
> all the systems out there, the APRS propagation monitoring map at
> Mountainlake seems to be the best option if you can apply a little
> mental enhancement to what the map shows you.

NOAA stations are difficult to chase, because the local ones are pretty 
strong, though sometimes they get wiped out. A common sheriff's 
frequency, like 155.340, mutual aid in Iowa can get interesting, though 
identification is a pain.
>
> What I was interested in was having people mention if they make 2M Q's
> based on what they see on the Mountainlake map. This morning when the
> map was showing propagation from Mpls to Duluth, could Bill near Grand
> Rapids talk to Iowa or Missouri? Bill did say that conditions during his
> Q with W0GHZ was well above normal, but how much further could he have
> gone? DX Sherlock attempts to collect QSO logging info to indicate
> activity and pull more people in. But since it relies on someone
> noticing activity, it makes a poor indicator.
>
> Anyway, my coax and antennas are so poor that I don't hear anything on
> the beacon frequencies even when Mountainlake is showing good
> conditions. :-)
>
> For reference, I use these numbers to indicate the relative sensitivity
> and "QSO Ready quality" of conditions.
> +10dB-	FM voice
> 0 dB -	SSB
> -6 dB-	CW
> -14dB-	PSK31
> -24dB-	WSPRnet
>
> I sometimes just lump CW and SSB together as 0dB, but that is a personal
> preference.... After considering FM vs SSB at 10dB, you also have to
> consider relative power levels. I'd guess the average APRS WIDE would be
> running around 40 watts. So on the order of 4 watts SSB should perform
> about as well. Or the other way around, a 100 watt SSB station has at
> least 14dB, over 2 S-units, advantage over an APRS station.

Think of the FM/SSB disadvantage another way. Here in the prairie, 60 
miles is practically DX on FM, while 60 miles is that "blank" local qrm 
who pegs my S-meter and hurts my ears. Without band openings, working 
200 or 300 miles on SSB is common though I will admit that takes decent 
antennas pointed at each other. And I've been known to cheat on FM by 
running a good vertical elevated 50 or 60 feet fed with 1/2" CATV cable. 
These days I'm running a Prime gain vertical, on a tripod on the house 
and I can work 50 miles most days to repeaters in Des Moines, but the 
Waterloo/Cedar Falls about 50 miles east is not reliable. One of my 
packet links for a while ran a 5 WL M2 yagi vertically polarized. 
Trouble was, any profile plots put the put 100 feet into the turf 15 
miles west of me. It worked some days anyway. And that was about 100 miles.
>
> Anyway, unless we can find some other fairly constant signal(s) to watch
> as automatic propagation indicators, I still think Mountainlake is as
> good as we can find. There is always the other problem too, that during
> the weekday there are far fewer stations available to work....

And even the retired aren't on the air all the time. I've found that I 
don't tolerate pure noise nearly as long as I once did.

73, Jerry, K0CQ
>
> 73, Doug Reed, N0NAS.
>
>