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PA2AGA > HDDIG 23.09.99 04:51l 185 Lines 7946 Bytes #-9770 (0) @ EU
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Date: Wed, 22 Sep 99 21:15:27 MET
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From: pa2aga
To: hd_broadcast@pa2aga
Subject: HamDigitalDigest 99/237H
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would be any different.
> >1)
> >Some folks live out in the country.
> >Sometimes WAY out in the country.
> >5-10 watts won't cover the 50-150 mile paths required.
> >Multipath can be handled by the use of appropriate antennas.
>
> Our longest single hop is 90 miles. We do that with 4 watts.
> Of course it is a mountain top to mountain top path. On an
> obstructed path, all you have is multipath. Usually, you can't
> resolve that with antenna orientation or gain because too many
> of the multiple paths are in the beamwidth of any reasonable
> gain antenna. So you have to engineer paths that clear the
> first Fresnel zone.
I was talking line of sight paths. Lots of mountains here. TOO many.
Get reflections from all directions due to those big white volcano things.
So we use yagis.
> Paths that don't qualify simply aren't used. That's just good
> network engineering practice. That often means that the most
> direct path isn't usable, and we have to organize the network
> topology to use an alternative routing. Sometimes it means
> rural hams are left out of the network unless they want to fund
> multiple hops just to serve one station. That's the way it goes.
Works where there are people or available sites. We have a real
lot of empty space, without people or roads. Often it is not feasible
to put relays in place. Cost of obtaining a site, creating road access
to it, etc. is way out of line. So we have long haul links Sometimes
those have to cross mountain ranges. Knife edge works. Not at
high data rates though, due to the problems you mention.
> >2)
> >Some of us want to build long haul networks.
> >100 mile or longer paths are needed.
>
> But they usually aren't feasible. Over the horizon isn't
> tenable for fast amateur data networks. You need to
> arrange your system around the idea that the average
> hop must be less than 30 miles, or settle for a much
> lower data rate where multipath delay doesn't cause
> symbols to overlap excessively. In exceptional cases,
> like the one noted above, topography may allow a
> longer hop, but it will be the exception, not the rule.
Hmmm ... not talking "Over the horizon". My horizon out the
shack window is over 150 miles to the south, 70 to the east,
and about 100 to the north east. I'm not even in a REAL good
location, just "good". My horizon to the north, however, is about
one mile. But I still have a 9600 baud link in that direction, of
about 25 miles. 25W, 10 el yagis on both ends. Works well,
even though we are way out of the first Fresnel zone.
> You need a BER of under 1 in 10E6 or your network
> performance becomes dismal out past 3 or 4 hops.
> That means you have to engineer things *right* on
> every link.
Not possible. We have these hills and mountains.
The 100 mile link to the south works very very well.
However that link of 25 miles to the north would take
three relay points to make it line of sight. Oops.
> >Got to get the power spread.
>
> Alligator thinking. Just being loud doesn't cut it. The
> key factor is the ability to receive a clean signal.
Not always an option.
> That requires a clean path, or your BER goes through
> the roof due to multipath induced intersymbol distortion.
> (SS has the potential of helping significantly with that,
> but we don't have any off the shelf SS equipment for
> amateur use right now, and there are other problems
> with SS systems co-located on high RF sites that I
> expect will be difficult to resolve.)
Nice idea, but not always an option.
> We tried 50 and 100 watt amplifiers early on. Didn't
> help. We could *hear* the alligators, but the BER was
> so bad that they degraded the performance of the
> entire network.
Our experience is quite different. Without the high ERP,
we had NO LINK AT ALL.
> >Takes aluminium as well as silicon.
> >The only other choice is to use the internet.
> >That ain't ham radio. No challenge involved.
>
> The other choice is to not succumb to alligator thinking,
> and instead do the hard RF path engineering to make
> the system work. It is hard work, it does require a lot of
> resources, but that's how it is done. AT&Tdid it many
> years ago with their RF relay network. If we want the
> same RF network capabilities, we have to repeat what
> they did. Physics hasn't changed.
I simply do not agree. There are not resources enough to
bridge the long hauls using short hops. The money ain't
there. So it is better to do SOMETHING rather than
throw up one's hands and claim it cannot be done because
it cannot be done "perfectly".
> Grandiose fast amateur RF networks spanning thousands
> of miles are just a pipedream for any group with less resources
> than AT&T. We can't fund it, we can't build it, we can't maintain it,
> and we can't manage it. We don't have the resources or the cooperative
> spirit to do it purely on amateur radio. There are too many square miles
> and too few hams standing on them in the US to allow that to happen.
Nonesense. It COULD be done, but not with the "I need a relay
point every few miles" or "The BER must be perfect" thinking.
> Perhaps in the densely populated little nations of Europe national
> networks can be done, but not across our vast rural spaces. That's
> sad, but it is real. We need to face up to reality and do what we are
> best suited to do, and that is the smaller scope metropolitan and
> relatively small regional networks that we can practically fund,
> build, and maintain.
>
> The GRAPES network covers an area of roughly 45,000 square
> miles, and that's about as big as is practical for a fast amateur
> network in our part of the US. We don't have the people, money,
> or sites to grow it much bigger. California might be different. They
> have 10 times our population, and a bunch of natural high sites. I
> haven't seen an organization with the energy and dedication of
> GRAPES building a fast network out there, however.
California is mostly open space with no people, just like the rest
of the West. It's a BIG place. The big cities are, north to south,
with approximate distances:
Vancouver, BC - Seattle, WA - 50 miles.
Seattle, WA - Olympia/Tacoma, WA - 50 miles.
Olympia/Tacoma, WA - Portland, OR - 100 miles.
Portland, OR - San Francisco, CA - 600 miles.
etc.
Yes, there are some small cities, and a lot of small (very small)
towns in between, but few of them have enough hams (or
any hams for that matter) to support intermediate link sites.
So we have fair network from Vancouver, BC south to Portland.
Then it gets pretty thin for a long way. The reason for the long
haul links is to bring places like Eugene, OR into the network.
That took a 100 mile hop. The next hop south from Eugene
is very tough; the transverse ranges north of the Siskyous chop
the paths up into 10-20 mile chunks. For a couple hundred miles.
Then you hit the CA Central valley, and all becomes easy again.
Um ... you did have to cross Siskayou Summit and Mt. Shasta
to get there though. If one could accomplish all the above, then
one has a network roughly following I-5 down the coast.
But what about inland? More big mountains in the way.
So we use a lot of long haul to very long haul 2M 1200 baud links.
It's that or nothing.
> >So you are saying these modules do not need any tune up?
> >Just plug everything together, hook to an antenna, it works?
> >Could you be more specific about the test gear required
> >beyond your basic solder gun, screwdriver and pliers?
To be continued in digest: hd_99_237I
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