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PA2AGA > HDDIG 23.09.99 04:51l 173 Lines 7893 Bytes #-9770 (0) @ EU
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Subj: HamDigitalDigest 99/237G
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From: pa2aga
To: hd_broadcast@pa2aga
Subject: HamDigitalDigest 99/237G
X-BBS-Msg-Type: B
Alligator thinking. Just being loud doesn't cut it. The
key factor is the ability to receive a clean signal.
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.)
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.
>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.
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.
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.
>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?
Netting the various transverters together on a common frequency
is the only adjustment absolutely needed. That can't realistically be
done by ear. You can do it by monitoring the eye pattern (with a scope)
of another station known to be on frequency. If one of the group has an
IFR or the like, you can use that without needing a priori to have one
transverter spot on frequency.
Typically, we find that the transverters are a few kHz off frequency
when first crystalled up, so they have to be netted. You don't have to
get absolutely spot on, the modems use a tracking data detector
that can handle up to about 5 kHz frequency error without impairing
BER. But we'd like to reserve that as margin for thermal drift and
component aging.
It is also nice if you can monitor the eye pattern with a scope so you can
adjust the transverter PA drive of other systems for best linearity (and
monitor their received eye pattern to do the same for your PA). That'll
make you a better spectral neighbor, and lower BER a bit too. But we've
found that with the MMT transverters, turning the drive down to get about
4 watts out always gives us a clean spectrum, so you can do it with just
a Bird. (You can't get away with that with the Hamtronics units, however,
for them you *really* need access to a spectrum analyzer. Nasty things
unless they're tuned *just* right.)
No external test equipment at all is needed for the RF modems. There are
some switch settings to configure (if the defaults don't suit you). You can
select one of 10 frequencies (100 kHz spacing), select whether you want
to operate duplex or half duplex, select TTL or RS422 on the computer side,
select whether you want FEC or not, etc. But the modems come configured
to work right out of the box and shouldn't need any adjustments unless you
want something other than the default settings.
The stuff that had to be tweaked in the first generation modems has all
been put into ASIC for the current modem, so there is no tune up involved.
You can peak the IF filter if you like, but the modem has an internal method
of doing that which doesn't require any external test equipment. You just
set a jumper, then tweak for max on the signal bargraph display. It should
be fine right out of the box, however.
>Where does one get these various modules?
>What is their cost?
Paccomm (and GRAPES) sell the W&T RF modems. The cost is $349.
Transverters are available from their manufacturers, or at hamfests. I
usually find MMT transverters for around $150 at most of the hamfests
I attend. They're usually crystalled for the wrong segment of the band
(the satellite or weak signal segment), so you need to spend $15 for a
new set of crystals to move them into the data bandplan. New transverters
range in price from under $200 to over $600 depending on make, model,
and band. Virtually any linear transverter will work with the modems.
You need a HDLC card in the computer too. I've personally used both
the PI-2 card ($98) and the Gracilis PacketTwin (don't know the current
cost on that one, but it is more expensive since it is full duplex DMA).
There are others. A Mac can work directly from its serial port since it
comes stock with chips that can be programmed to handle RS422 and
HDLC directly (about the only case I can think of where having a Mac is
an advantage).
Basically, you're looking at $600 to set up for 56 kb. That's more than
it costs to dedicate a TNC and radio to 1.2 kb, but not much different
from what it costs to set up to do 9.6 kb correctly (that's because you
generally have to use an IF modulated multimode to get an acceptable
BER with G3RUH style 9.6 kb).
Gary
Gary Coffman KE4ZV | You make it |mail to ke4zv@bellsouth.net
534 Shannon Way | We break it |
Lawrenceville, GA | Guaranteed |
>.
------------------------------
Date: Mon, 20 Sep 1999 20:02:33 -0700
From: "Hank Oredson" <horedson@att.net>
Subject: Let's look at real numbers for TNC software sales
Gary Coffman <ke4zv@bellsouth.net> wrote in message
news:DHrmNx+s2SaofDUoViT9sz9jVUYr@4ax.com...
> On Sun, 19 Sep 1999 22:50:35 -0700, "Hank Oredson" <horedson@att.net>
wrote:
> >Gary Coffman <ke4zv@bellsouth.net> wrote in message
> >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 ...
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
To be continued in digest: hd_99_237H
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