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PA2AGA > HDDIG 25.09.99 02:23l 179 Lines 7882 Bytes #-9767 (0) @ EU
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From: pa2aga
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Subject: HamDigitalDigest 99/239C
X-BBS-Msg-Type: B
>> >news:tinlN9QLyMKyqvyn62g+MzZSipIu@4ax.com...
>> >
>> >> Note, an external amplifier is not a good idea. You don't
>> >> need the extra TR delay, and any path that can't be made
>> >> with the 5-10 watts available from the transverter probably
>> >> has too much multipath to be useful anyway (though with
>> >> the FEC now built into the modems, multipath is less of a
>> >> concern).
>> >
>> >Gary,
>
>Hmmm ... "external" amplifier. Didn't think I said that ...
You said,
"Some assembly required i.e. the transverter and amplifier
you will need to make a useful system".
The only way to read that is if you're talking about an amplifier
external to the transverter. Otherwise, no assembly would be
required between the transverter and the amplifier.
>Why any extra delay? Whether the final transistor is 10W
>or 100W, the delay should be (about) the same: a few ms. max.
>Certainly true for my HF radios. No reason to think vhf/uhf/shf
>would be any different.
It is *possible* to design an amp like that, but off the shelf
amateur UHF external amps generally don't have fast TR.
100 to 200 milliseconds is typical, and far too slow for use
on a fast network unless you're doing permanently nailed
up duplex point to point links. The thing is, permanent network
links need to be properly engineered anyway, so the whole
issue of alligators becomes moot at that point.
>> >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.
Sure, we use beams too, when we're establishing a dedicated link. If you
have line of sight, then you certainly don't need much power. A few hundred
milliwatts does fine out to LEO distances when you can employ gain antennas
and nothing is in the way of your signals.
Terrestrially, the less power you run, the less multipath you have to deal
with on the other end, because the attenuation of the reflected signals puts
them below your noise floor. More power always makes multipath problems
*worse*. We only need a CNR of about 10 dB to meet our BER target
against gaussian noise. Running more power than that on a link will just
exacerbate any multipath problems you may have.
(Actually, you do want to build in some fade margin on a path that may have
to traverse regions where inversions may form, but not the 60 dB or greater
that most alligator stations use. Use of circular polarization on both ends of
the link is also highly recommended because reflections reverse polarization
sense, so you can get another 20 dB or so of multipath suppression this way.)
I'm going to break the message here because it is getting very long.
I'll touch on other points in another message.
Gary
Gary Coffman KE4ZV | You make it |mail to ke4zv@bellsouth.net
534 Shannon Way | We break it |
Lawrenceville, GA | Guaranteed |
>.
------------------------------
Date: Wed, 22 Sep 1999 12:36:17 -0400
From: Gary Coffman <ke4zv@bellsouth.net>
Subject: Let's look at real numbers for TNC software sales
On Mon, 20 Sep 1999 20:02:33 -0700, "Hank Oredson" <horedson@att.net> wrote:
>KE4ZV wrote:
>> 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.
Yep, exactly so. If you're satisfied with low rate and non-realtime usages,
then you can do moderately long haul dedicated links. You can't sustain
an extensive packet network that way, however, because latency becomes
so great that very few people will tolerate it, even for non-realtime uses. In
other words, if a Pony Express rider could beat your packets across the
country, your network wouldn't be sustainable.
That's what happened to the BBS "network" (which wasn't really ever a
network in the sense of a packet switched network anyway). When it was
the only game in town, it got a bit of use. But as soon as something faster
became widely available, it fell into disrepair. In other words, most of your
users abandoned you for the internet. You and Charles can rail about that,
but it is fact, and ultimately one has to face facts or become permanently
disassociated from reality.
There are a number of historical parallels to that. The telegraph supplanted
the Pony Express, steamships supplanted windjammers, the airplane
supplanted the ocean liner for intercontinental travel, etc. If you can't
deliver
speed and convenience, your users will abandon you for systems which can.
You have to define your niche, and fill it the best way that you can. The
bicycle is a wonderful and efficient tool for getting around a relatively
small area, but few people would choose it for a cross country journey.
Amateur packet radio networks can be wonderful at the MAN level, but
when you try to stretch them across the vast empty spaces, you run into
the sparse network topology problem, ie there's a whole lot of nothing
between the centers of activity where a viable network can be supported.
>> >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".
That is not just an exceptional location, it is an *extremely*
exceptional location compared to what is available to most
of the country. When you start talking about continental
scale networks, you have to look at more of the continent than
that part of it immediately outside your window.
>My horizon to the north, however, is about
>one mile. But I still have a 9600 baud link in that direction, of
To be continued in digest: hd_99_239D
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