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<div class="moz-cite-prefix">On 2/4/2018 11:05 PM, Michael Barnes
wrote:<br>
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<div>I was mentioning to a local ham I was interested in
putting APRS in my truck. (I'm still digesting all the
great comments from my previous query.) He said I will
need a minimum of 50 watts and the frequent transmissions
will pretty much eliminate the possibility of other 2
meter operations while I have the APRS running. This news
is certainly disconcerting. My original intention was to
use a Raspberry Pi with a TNC-Pi connected to a Kenwood HT
with an external antenna. Having to add another full sized
mobile radio and running that much power with all the
other radios in my truck might be an issue.<br>
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So, is the need for 50 watts realistic? </div>
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YES!!<br>
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<div>I do live in a rural area, but other 2 meter use seems to
get by quite well with handheld radios.<br>
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The difference is that......<br>
<br>
<u> ALL of a packet MUST be received PERFECTLY before ANYTHING can
be recovered from it.</u> <br>
<br>
Standard AX.25 packet has NO forward error correction that can fix
corrupted packets at the receiving end. You must deliver a
"full-smash" completely-quieted signal to the far end for successful
decodes. The kind of hissy, popping, fluttering,
not-fully-quieted signal that so many users of underpowered
hand-helds inflict on voice repeaters just won't work on packet
data. <br>
<br>
You MUST have enough excess signal level that even with random 15-20
dB drops in signal level (typical mobile flutter) the signal will
remain in full quieting. <br>
<br>
Roughly speaking, given the same transmit power and antenna gain,
the reliable range on packet will be about one-half to one-third
what you would get on voice under the same conditions. Consider the
range you would expect with a hand-held on simplex voice. Now cut
that by a half to two-thirds. <br>
<br>
[This assumption is based on typical mid-western flat-lands
conditions where digipeaters are seldom more than 50-150 feet above
ground level; i.e. a horizon-grazing non-line-of-sight path between
the mobile and the digipeater. In the real west, where digipeaters
are often on mountain tops <b>THOUSANDS </b>of feet above the
users, it is not uncommon for low-power mobiles to be copied 50-75
miles away. ]<br>
<br>
As far as desensing the voice radio: <br>
1) Disable smart beaconing with it's incessant rapid-fire
transmissions and beacon only once every three minutes or so. <br>
2) Use the highly-compressed "Mic-E" data format for your
transmissions. The Mic-E coding can burst out a complete position
report in less than half a second. The effect in the other radio is
just a fractional-second hiccup in reception that is quite easy to
live with. (Or for total isolation, try to do your voice operation
on UHF as much as possible.)<br>
3) Unobtrusive short-burst Mic-E operation puts a premium on radios
that can keyup and settle on frequency really fast. And then unkey
back to receive just as fast. Most hand-helds are really poor at
fast-turn-around RX-toTX and TX-to-RX operation. <br>
<br>
<hr size="2" width="100%">Stephen H. Smith wa8lmf (at) aol.com <br>
Skype: WA8LMF<br>
EchoLink: Node # 14400 [Think bottom of the 2-meter band]<br>
Home Page: <a class="moz-txt-link-freetext" href="http://wa8lmf.net">http://wa8lmf.net</a><br>
<br>
Live Off-The-Air APRS Activity Maps<br>
<a class="moz-txt-link-rfc2396E" href="http://wa8lmf.net/map"><http://wa8lmf.net/map></a><br>
<br>
Long-Range APRS on 30 Meters HF <br>
<a class="moz-txt-link-rfc2396E" href="http://wa8lmf.net/aprs/HF_APRS_Notes.htm"><http://wa8lmf.net/aprs/HF_APRS_Notes.htm></a><br>
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