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Subject: HamDigitalDigest 99/250A
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Ham-Digital Digest Mon, 4 Oct 99 Volume 99 : Issue 250
Today's Topics:
56k UHF-100W / 19.2K VHF -50W (8 msgs)
Checking.
FA: Ku 14 Ghz Satellite Transceiver
Let's look at real numbers for TNC software sales (6 msgs)
Packet modems and Windows CE portables
Tandy 200 Laptop at eBay - Great for Hamming
Send Replies or notes for publication to: <Ham-Digital@UCSD.Edu>
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Archives of past issues of the Ham-Digital Digest are available
(by FTP only) from ftp.UCSD.Edu in directory "mailarchives/ham-digital".
We trust that readers are intelligent enough to realize that all text
herein consists of personal comments and does not represent the official
policies or positions of any party. Your mileage may vary. So there.
Loop-Detect: Ham-Digital:99/250
----------------------------------------------------------------------
Date: Sun, 3 Oct 1999 18:50:29 -0700
From: "Hank Oredson" <horedson@att.net>
Subject: 56k UHF-100W / 19.2K VHF -50W
Gary Coffman <ke4zv@bellsouth.net> wrote in message
news:WrT3N9E9pNAjdZum8bQg63ohRQBI@4ax.com...
> The correct answer is to install a sufficient number of unmanned relay
> stations sited so that the paths are bridged by hops of reasonable length
> and quality. That's what the Europeans are telling you that they've done,
> and it is what I've been telling you that the GRAPES network has done in
> our service area. (It is what AT&T did too in the days when their long
line
> networks were RF based.)
Please read what I write before responding.
The particular topic was bridging the gaps BETWEEN metro areas
using ham radio. The topic was not "Why we cannot do it."
It was "How can we do it." The answer is not a bunch of relay
sites. Cannot be done. Terrain, politics and cost prohibit it.
A nice geo satellite or three would work also (although this would
not meet your latency criteria), but is not feasible for the
obvious reasons.
I'm looking for things that CAN be done.
Heck, I'm not just "looking", I'm doing them.
--
... Hank
http://horedson.home.att.net
>.
------------------------------
Date: Sun, 3 Oct 1999 11:26:42 -0700
From: "Hank Oredson" <horedson@att.net>
Subject: 56k UHF-100W / 19.2K VHF -50W
Paul Keinanen <keinanen@sci.fi> wrote in message
news:QiX3NzEKB4KJjELYJ9lrMd5Eleuc@4ax.com...
> On Sat, 2 Oct 1999 21:16:44 GMT, nomail@pe1chl.demon.nl (Rob Janssen)
> wrote:
>
> >Hank Oredson <horedson@att.net> wrote:
> >
> >>> With these directional antennas you don't need 100W, because if it
does
> >>not
> >>> work with about 5W on UHF it will not reasonably work with 100W
either.
> >
> >>Two posters have said this same thing. It is contrary to my experience,
> >>on HF, VHF and UHF. Why 5W? Why not .5W or 5 mw? You see the
> >>point? There is no magic "cutoff power level" involved. You simply need
> >>enough power spread (ERP above receiver noise floor) to obtain the S/N
> >>you need FOR THE PATH INVOLVED. Longer paths need more,
> >>shorter paths need less. I do understand that you live where long paths
> >>may not be required to build a network. Others live in places where
> >>long paths (200 km and above) are in fact common. 20 db more ERP
> >>gets you 20 db better S/N on any path with any system on any band.
> >
> >100 Watts are not required to build a 200 km path that is (nearly)
> >line-of-sight. 5 Watts will do. Maybe 0.5W will, when enough antenna
gain
> >is available.
> >
> >Just calculate the path loss over such a distance and you will see that a
> >5W signal is *very much* above the noise with bandwidths typical for
> >today's packet radio network.
>
> The path loss for a true line-of-sight path can be easily calculated
> from L = 32 dB + 20log(f) + 20log(d), where f is in Mhz and d in km.
> On 2 m a 200km path would have a path loss of 121 dB. A NBFM receiver
> has a sensivity around -120 dBm, thus 1 mW transmitter power and
> omnidirectional antennas should suffice. However, the background noise
> levels on 2 m is still quite high, so maybe -110 dBm is required at
> the receiver input, thus 10 mW Tx power would be required :-).
I'm confused. Didn't you forget to add in the desired S/N and the
fade margin requirement? NBFM means 20 khz bandwidth? 9600 baud
maximum signalling rate? Three cases to consider: 1200 baud, 9600 baud,
and 56k (thus higher bandwidth, probably 100 khz. min).
Try it with a desired S/N of 35 db and 20 db fade margin.
(These two numbers were derived from the desired BER and link availability.)
(Use others if you like.)
Repeat for 70 cm.
We cannot run the 56k links on 2M in the US.
One useful assumption: antennas on either band have about the same
capture area, i.e. they are about the same physical size. Going higher
in frequency helps the BER because more of the signal is on-path
and less is scattered by off-path reflectors
--
... Hank
http://horedson.home.att.net
>.
------------------------------
Date: Mon, 04 Oct 1999 08:41:03 +0300
From: Paul Keinanen <keinanen@sci.fi>
Subject: 56k UHF-100W / 19.2K VHF -50W
On Sun, 3 Oct 1999 11:26:42 -0700, "Hank Oredson" <horedson@att.net>
wrote:
>
>Paul Keinanen <keinanen@sci.fi> wrote in message
>news:QiX3NzEKB4KJjELYJ9lrMd5Eleuc@4ax.com...
>> On Sat, 2 Oct 1999 21:16:44 GMT, nomail@pe1chl.demon.nl (Rob Janssen)
>> wrote:
>> The path loss for a true line-of-sight path can be easily calculated
>> from L = 32 dB + 20log(f) + 20log(d), where f is in Mhz and d in km.
>> On 2 m a 200km path would have a path loss of 121 dB. A NBFM receiver
>> has a sensivity around -120 dBm, thus 1 mW transmitter power and
>> omnidirectional antennas should suffice. However, the background noise
>> levels on 2 m is still quite high, so maybe -110 dBm is required at
>> the receiver input, thus 10 mW Tx power would be required :-).
>
>I'm confused. Didn't you forget to add in the desired S/N and the
>fade margin requirement?
I took the received signal power level from the BERR vs. signal power
graph in ARRL handbook which was taken from an article by K9NG in
August 1983 QEX. In those figures the S/N is factored in as the BERR.
The -121 dBm gave 20 dB quieting and -120 dBm gave 1E-2 BERR, which is
too low for AX.25, but might be sufficient for better protocols. At
-116 dBm, the BERR was 1E-5 for both 300 and 1200 bit/s, which
apparently has to do with the F2D modulation used. It is interesting
to note that for 9600 bit/s the BERR curve was almost identical (using
F1D) with 1E-5 at -114 dBm.
Why do you need a fade margin for a true line-of-sight path ? If you
need a fade margin, it is not a true line-of-sight path anymore :-).
To be continued in digest: hd_99_250B
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