[aprssig] 30M APRS and digipeaters...

[Stephen H. Smith]

On 7/11/2012 9:11 PM, Keith VE7GDH wrote:
> What are the current thoughts about digipeaters and APRS on HF,
> specifically on 30M? My understanding was that digipeaters would
> be a bad thing... i.e. if there is propagation, everyone would hear
> everything, and the digipeated beacons would just be QRM and
> take up bandwidth and time slots. I was basically saying iGates
> (properly configured) good, digipeaters evil.
>
> I ask because I said on another list that having a 30M APRS digi
> would not be a good thing and received one contrary (off list) reply.
>


1)    Propagation does not ensure everyone hears everyone.  On 30M, depending 
on solar activity and time of day, one can have a dead ("skip") zone of 
300-1000 miles around your station where you can't hear any nearby stations at 
all. Ironically, stations can be "too close" to hear each other.

 From my QTH in central Michigan, I frequently hear stations in the midwest or 
mid south (Wisconsin, Illinois, Kentucky, etc -- range of  200-500 miles) 
bouncing off digipeaters on the east coast twice as far away as they are, -- 
but I don't hear them direct at all. Other times, I won't hear the east coast 
stations at all, but do hear them digipeated by VE4GLS in Winnipeg, MB.

Still other times I have heard W6OTH in the California bay area digipeated by 
W7 stations in the Pacific Northwest, but not direct, even though they are just 
about the same distance from me, but on different headings.

Since this behavior is so unpredictable, digipeating has little value for 
consistently enabling two-way contact between specific stations.  On the other 
hand, for transmit-only mobile trackers on 30M, spraying packets around 
randomly every-which-way can maximize the  chance of hitting an igate station 
SOMEWHERE in North America regardless of propagation.

(Although I agree that the ultimate diversity receiving system on 30M would be 
lots more geographically-dispersed igates to accommodate changing propagation - 
currently they tend to be mostly bunched up on the two coasts.  We need more in 
places like Salt Lake City, Denver, Omaha, Calgary, Sault Ste Marie, etc where 
500-1000 mile skip distances would cover most of the North American interior 
when coastal stations aren't being heard. )



Many times I have been parked outside a motel room in western Nebraska, or in 
canyons in central Colorado, or the interior of Utah or Nevada where there is 
absolutely NOTHING on two meters. Monitoring findu.com or APRS.fi from the 
motel's (or McDonalds) wi-fi, I can pop off a posit on 30M from the mobile, and 
least 90-95 percent of the time, it will immediately show up on the Internet. 
Over a span of a couple of hours, I have seen the igate station shift from 
California to the Pacific northwest to the midwestern heartland in Chicago, Des 
Moines or St Louis, with the path showing digipeats sometimes and not others.

Two years ago, I made a vacation trip from my then-home in Pasadena, CA up I-5 
to Vancouver, BC, then over the Canadian Rockies to Banff, then straight south, 
re-entering the US in northern Idaho, and then down through eastern Washington 
and Oregon to Reno & Lake Tahoe.   In other words, thousands of mile of driving 
in some very empty places.    Two meters was non-existent on about half the 
trip.   The APRS.fi  track line on 30 meters was almost continuous, with igate 
points-of-entery literally in every corner of North America!

I will be repeating this kind of trip in about two weeks.   I'm heading from 
central MI to Everygreen, CO for the annual jazz festival the last weekend of 
July.  Then onward to Las Vegas & southern Cal, then up I-5 to Sacramento after 
an outing with a friend to the central CA coast.   After a week or so in Sac, I 
will head east over the Sierras to Lake Tahoe and then US-50 ("the loneliest 
road in America") across the Nevada & Utah interior back to Colorado before the 
final stretch back to MI. This trip will take most of August.

As I always do, I will be beaconing as:

o    WA8LMF (no SSID) on two meters.

o    WA8LMF-2 on 30 meter conventional AX.25 APRS.

o    WA8LFM-63 on the new 30-meter APRS-over-PSK/GMSK/MFSK16.

With the different SSIDs, you can compare the relative coverage of the two 
bands, and of the two modes on 30 meters.




2)   In the beginning (some 2 decades ago), 30M HF APRS activity started 
primarily to support mobile beacons in remote areas. Many 30-meter fixed 
stations used Kantronics KAM dual-port TNCs to gateway received 30M traffic 
over to two meters where normal 2M igates would pass the traffic to the 
Internet.   (The KAM's internal firmware can do this stand-alone with no PC 
required if the HF user places the keyword "GATE" in their APRS path.)   To 
distinguish between HF and VHF digipeating,  "ECHO" was adopted as the UNPROTO 
path component for HF digipeat hops, instead of the usual VHF "WIDE" or 
WIDEn-N".   Early adopters would use a path setting like
.     APRS via ECHO, GATE,WIDE2-2.

All the "old-timer" fixed stations respond to ECHO .   However many recent 
arrivals from two meters are now, through ignorance, configuring digis to 
respond to the VHF convention of WIDE instead.   It's now a complete mess, with 
about half the 30M fixed stations using ECHO and about half using some variant 
of WIDE.

I've seen HF mobiles using insane paths like "APRS via WIDE3-3,ECHO,GATE" which 
virtually GUARANTEES they won't get gated since the probability of making FOUR 
hops on HF is zero.

To add to this mess, the author of the new APRS-over-digimodes PSK/GMSK/MFSK16 
program "APRS Messenger" has hardwired a digipath setting of WIDE2-1 into the 
program. You can toggle the digipeater function on or off,   and you can toggle 
the WIDE2-1 hop on or off in your own transmissions (there's a check box 
"Digipeat Me") -- BUT -- you can't change the digipeat path setting!    This is 
further encouraging newcomers to try to use digipeating on HF.


-------------------------------------------------------------------------------

--

Stephen H. Smith    wa8lmf (at) aol.com
Skype:        WA8LMF
Home Page:          http://wa8lmf.net


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"APRS 101"  Explanation of APRS Path Selection & Digipeating
   http://wa8lmf.net/DigiPaths